Part 2. Natural Mechanisms Of Weather, Climate Change
Solar Influence On Climate
ENSO, NAO, AMO, PDO Climate Influence
Modern Climate In Phase With Natural Variability
Cloud/Aerosol Climate Influence
Volcanic/Tectonic Climate Influence
The CO2 Greenhouse Effect – Climate Driver?
Solar Influence On Climate
Rajesh and Tiwari, 2018 The major harmonics centred at ~ 63 ± 5, 22 ± 2, and 10 ± 1 years are similar to solar periodicities and hence may represent solar forcing, while the components peaking at around 7.6, 6.3, 5.2, 4.7, and 4.2 years apparently falls in the frequency bands of El-Nino-Southern Oscillations linked to the oceanic internal processes. Our analyses also suggest evidence for the amplitude modulation of ~ 9–11 and ~ 21–22 year solar cycles, respectively, by 104 and 163 years in northern and southern hemispheric SST data [during 1850 to 2014]. The absence of the above periodic oscillations in CO2 fails to suggest its role on observed inter-hemispheric SST difference. The cross-plot analysis also revealed strong influence of solar activity on linear trend of NH- and SH-SST [Northern/Southern Hemisphere Sea Surface Temperature] in addition to small contribution from CO2. Our study concludes that (1) the long-term trends in northern and southern hemispheric SST variability show considerable synchronicity with cyclic warming and cooling phases and (2) the difference in cyclic forcing and non-linear modulations stemming from solar variability as a possible source of hemispheric SST differences. … The trend components of NH-SST and SH-SST show strong relationship with TSI [Total Solar Irradiance] trend variations and poor in relation with global CO2 trend.
Lubin et al., 2018 Over the past decade there has been increasing realization and concern that the steady and high solar luminosity of the past century may transition to greater variability later this century (Abreu et al. 2008; Feulner & Rahmstorf 2010; Lockwood 2010). Specifically, the Sun may descend into a period of low magnetic activity analogous to the historical Maunder minimum (MM; circa 1640–1715; Eddy 1976). A resulting decrease in total solar irradiance (TSI) impacting the terrestrial lower atmosphere energy budget is linked to changes in high-latitude circulation patterns that strongly influence the climate of Europe and the Atlantic sector of the Arctic and subArctic (Song et al. 2010; Meehl et al. 2013), and may also influence Antarctic climate (Orsi et al. 2012). Studies have also shown the importance of stratospheric response to a grand minimum (e.g., Gray et al. 2010; Bolduc et al. 2015; Maycock et al. 2015). Over a solar cycle and certainly in response to a future grand minimum, irradiance variability at middle ultraviolet (UV) wavelengths that drive oxygen photolysis and ozone chemistry is much larger that that of the TSI. Resulting changes to stratospheric ozone abundance alter the stratosphere–troposphere temperature gradient and feed back to tropospheric planetary wave refraction, further altering climatically relevant circulation patterns (Maycock et al. 2015). With this realization that both direct radiative and indirect stratospheric influences affect terrestrial climate under a solar grand minimum, it is important to understand how UV irradiance would respond to such a large and prolonged change in solar magnetic activity.
Zherebtsov et al., 2018 Based on a complex analysis of hydrometeorological data, it has been shown that changes in the temperature of the troposphere and the World Ocean reflect a response both to individual helio-geophysical perturbations and to long-term changes (1854–2015) of solar and geomagnetic activity. It is established that the climatic response to the influence of solar and geomagnetic activity is characterized by considerable spatio-temporal heterogeneity, is of a regional nature, and depends on the general circulation of the atmosphere. The largest contribution of solar activity to the global climate changes was observed in the period 1910–1943. … For the last 1000 years, the world climate experienced changes that quite closely corresponded to variations in SA [solar activity]: in the 11th–13th centuries, when SA was high, there was a warm period (the “medieval climatic optimum”), and two distinct temperature drops in the small ice age (16th–17th centuries) correspond to the Maunder and Spörer minima. A general rise in the level of SA [solar activity] occurred after the completion of the Maunder minimum [1700s], and the world climate became warmer during most of this period. … It is shown that solar activity contributed significantly to the global climate change, mainly during the first warming in the 20th century (1910–1943). This period is characterized by a significant positive trend in the level of geomagnetic activity that was maximal over the entire considered time interval (1868–2015) and coincided with enhanced meridional heat transfer in the North Atlantic.
Song et al., 2018 [A] general warm to cold climate trend from the mid-Holocene to the present, which can be divided into two different stages: a warmer stage between 6842 and 1297 cal yr BP and a colder stage from 1297 cal yr BP to the present. … The general cooling trend may represent a response to decreasing solar insolation; however, the relative dryness or wetness of the climate may have been co-determined by westerlies and the East Asian summer monsoon (EASM). The climate had a teleconnection with the North Atlantic region, resulting from changes in solar activity.
Cionco et al., 2018 Here we argue that both the in situ mean-daily insolation and the LIG [latitudinal insolation gradients] metrics are important for a fuller and more comprehensive study of how the changes of the external insolation forcing may trigger, sustain and modulate the local, regional and hemispheric scales of climate on decadal, multidecadal to centennial timescales. LIG [latitudinal insolation gradients] which, in turn, can be closely associated with the modulation of LTG or the so-called equator-to-pole temperature gradient (Lindzen 1994; Soon and Legates 2013) that in turn represents a clear negative feedback on the broad, hemispheric scale. Local in situ mean-daily insolation clearly emulates the role imagined by Milankovic´ but has been recently re-proposed and shown, for example, by Soon (2009) to play a key role for the Arctic-mediated modulation of the multidecadal to centennial scale climate variations observed using both available instrumental thermometer, rain-gauges and paleoclimatic proxies records.
Shi et al., 2018 The results show that during periods of strong solar activity, the solar shortwave heating anomaly from the climatology in the tropical upper stratosphere triggers a local warm anomaly and strong westerly winds in mid-latitudes, which strengthens the upward propagation of planetary wave 1 but prevents that of wave 2. … The Sun is the most important source of energy in the Earth’s climate system and variations in the intensity of solar radiation influence both the weather and climate (Chen et al., 2015; Rind, 2002, 2008; Shang et al., 2013; Wang et al., 2015; Zhao et al., 2012). Gray et al. (2010) showed that there are two main mechanisms, bottom-up mechanism and top-down mechanism, by which solar activity affects the Earth’s climate. The top-down mechanism is connected to solar ultraviolet radiation. Solar ultraviolet radiation is mainly absorbed by ozone in the tropical stratosphere, which changes the meridional temperature gradient and wind field in the atmosphere. This further affects the propagation of stratospheric planetary waves in the winter hemisphere (Balachandran and Rind, 1995). Therefore, the solar radiation change can affect the interaction between the stratospheric circulation and the planetary waves (Haigh, 1996, 1999; Kodera and Kuroda, 2002; Shindell et al., 1999, 2006).
Moreno, 2018 A number of revealed common key changes in the main assemblages’ composition has been attributed to larger scale climatic shifts, particularly as regards the transitions firstly from the Medieval Climatic Anomaly (MCA) to the Little Ice Age (LIA), and next from the LIA to the Current Warm Period (CWP) in the Iberian Peninsula, as well as to major temperature–precipitation excursions throughout the LIA and directly correlated with sustained negative phases of the NAO index in periods of lowest SA [solar activity], known as Grand Solar Minima. It is also found, throughout the application of spectral and cross wavelet methods, that in the time span analyzed (from the 1300s to present), the signals of solar forcing in both foraminiferal and x paleoclimatic records were intermittent, with the regional climate modulated by the solar secular Gleissberg cycle, especially after AD 1700–1750, following the Maunder Minimum (1645–1715).
Booth, 2018 The TCR [transient climate response] to doubled CO2 is less than 2K (1.93 ± 0.26K). Only 1.1K of HadCRUT4 warming is expected between 2000 and 2100AD. ∼35% of the warming during 1980–2001 was from solar variability, by 2 different analyses. Temperature is nearly 3 times as sensitive to solar radiation as to CO2 radiation. A model for ocean warming estimates equilibrium sensitivity as 15% greater than TCR.
Wang et al., 2018 The reconstructed temperature series showed a linear increasing trend with a rise 0.11 °C from 1880 to 2012. … The multi-taper method reveals several significant periodicities in our reconstruction over the past 160 years, suggesting possible linkages with the El Niño-Southern Oscillation, lunar gravity, Pacific Decadal Oscillation (PDO) and solar activity. Correlation analysis between the reconstruction and Southern Oscillation Index (SOI), lunar geocentric declination, PDO and sunspot number further demonstrates that the temperature variations in Weichang region are negatively correlated with SOI and positively correlated with lunar gravity, PDO and solar activity in the long term.
Li et al., 2018 [O]ur synthesis suggests that insolation plays a major role that has been responsible for the increased SSTs in ETIO [Eastern Tropical Indian Ocean] since the early Holocene, while other mechanisms remain effective in determining the timing of our reconstructed SST variations. In particular, our SST pattern shares less similarity with that of coral Sr/Ca SST and is decoupled from the coral IOD events in the mid-Holocene. We interpret that our reconstructed ETIO [Eastern Tropical Indian Ocean] SSTs are driven dominantly by the solar forcing, but are also affected by other internal climate mechanisms such as the local shifts in AM-controlled upwelling and precipitation, episodic reductions in the flow of warm western Pacific surface water into the Indian Ocean due to increased precipitation over the Indonesian archipelago, and long-term ENSO or IOD-like climate change.
Di Rita et al., 2018 Here we focus on the central and western Mediterranean. We show that recurrent forest declines from the Gulf of Gaeta (central Tyrrhenian Sea) reveal a 1860-yr periodicity, consistent with a ca. 1800-yr climate fluctuation induced by large-scale changes in climate modes, linked to solar activity and/or AMOC intensity. … Soon et al. , through a cross-wavelet analysis applied to three main solar activity proxy time series (nitrate concentration, 10Be and 14C) show a strongly modulated and time-dependent signal for a ~1800-yr climate cycle (1885 years), together with a climate cycle of 1500-yr. The authors suggest that these cyclicities may correspond to fundamental solar modes influencing the global climate and producing an internal threshold response of the global THC [Thermohaline Circulation] to solar forcing.
Zhang and Feng, 2018 Based on pollen data from 30 sequences reviewed here, we reconstruct the spatial and temporal variations in temperature and in aridity that occurred during the Holocene in the Altai Mountains and the surrounding areas (i.e., the examined area). The synthesized regionally-averaged temperature-index curves from low-elevation regions show that the climate was consistently warming from ~12,000 to ~9000 cal. yr BP and has experienced a gradual cooling trend since ~9000 cal. yr BP. It means that the Holocene temperature trend in low-elevation regions of the examined area has sensitively responded to variations in the total solar irradiance. … The regionally averaged temperature curve (Fig. 34d) for the entire examined area (i.e., the Altai Mountains and the surrounding area) is relatively well parallel to the summer solar irradiance curve at 60°N (Fig. 34e; Berger and Loutre, 1991), implying that the temperature in the Eurasian interior including the examined area may have been generally sensitively responded to the solar irradiance during the Holocene (Moossen et al., 2015).
Kasatkina et al., 2018 The tree growth response in polar region (Kola Peninsula and Finnish Lapland) to the most powerful (VEI>4) volcanic eruptions is assessed over a period of 1445-2005. The analysis was based on the Loparskaya (1445- 2005) and Finnish supra-long (~ 7500 years) tree-ring chronologies. These chronologies were developed from Pinus sylvestris L. … The application of wavelet analysis for the Loparskaya tree ring chronology allowed to identify the existence of the main cycles of solar activity (11, 20-25, and ~ 100 years). … Moreover, the wavelet analysis revealed a strong coherence in the 8-13 and 20-30 year bands, indicating a possible link between solar activity and climate change on a regional scale.
Li et al., 2018 Here we investigate the possible modulation of the total energy flux input from the solar wind into the Earth’s magnetosphere on the global tropical cyclone activity during 1963–2012. From a global perspective, the accumulated cyclone energy increases gradually since 1963 and starts to decrease after 1994. Compare to the previously frequently used parameters, e.g., the sunspot number, the total solar irradiation, the solar F10.7 irradiation, the tropical sea surface temperature, and the south oscillation index, the total solar wind energy flux input exhibits a better correlation with the global tropical cyclone activity. Furthermore, the tropical cyclones seem to be stronger with more intense geomagnetic activities. A plausible modulation mechanism is thus proposed to link the terrestrial weather phenomenon to the seemingly-unrelated solar wind energy input. … Global tropical cyclone activity is modulated by solar wind energy flux.
Tan et al., 2018 Some decadal scale wet and dry intervals were also identified. The abnormal drought during 1160–1245 AD might have accelerated Dali kingdom’s demise at 1253 AD. Power spectrum analysis indicated significant 373-, 187-, 22-, 12- and 11- yr cycles in our stalagmite record, suggesting the impact of solar activity. Increased monsoon precipitation on southeastern TP was observed in solar activity minima during the last millennium. We further synthesized an integrated precipitation record for southwestern China and discussed spatial patterns of precipitation over China during the last two millennia. The comparisons confirm a “dry southern and wet northern” pattern in monsoonal China during the Medieval Warm Period and a “wet southern and dry northern” pattern during the Little Ice Age and Dark Age Cold Period. Solar activity, the strength of westerly jet and summer monsoon, as well as the SST of tropical Indo-Pacific might play important roles on the rainfall spatial patterns over monsoonal China during the last 2000 years.
Aldahan et al., 2018 Climatic changes during the Holocene are characterized by relatively small variability in temperature with rather different regional responses globally. Periods of reduced solar activity comparable to the Maunder Minimum (1645 to 1715 AD) and short-lived (days to months) solar storms are likely to occur throughout the Holocene time. The radioactive isotope Be-10 has been a crucial source of information related to the past activity of the Sun and cosmic rays and the consequent impact on the Earth’s climate. … Here we show that the Be-10 signal preserves rather significant imprints of the natural production pathway that still reflects climatic changes. The materials used here include saw dust samples taken from the NEEM ice core (North Greenland, 77.45°N, 51.06°W) and cover the period 6500-4500 BP (b2k time scale). This Holocene period is characterized by climatic transition from a relatively warmer to colder conditions and the end of the Holocene climate optimum. The results show Be-10 concentrations range at 0.5-3x10e4 atoms/g with a general decreasing trend towards recent times. Within this trend there are periods of high and low Be-10 values spanning variable time intervals. These fluctuations in the Be-10 values suggest both cyclic and noncyclic events that are largely related to changes in solar activity and/or flux of cosmic rays. The imprint of these events in the Holocene climate (translated in the temperature record) is oscillations in the climate from relatively warmer to colder periods. Understanding the Be-10 behavior in the ice archives during the Holocene will further the knowledge about effects of the atmosphere and land surface processes on past proxy records of solar activity and cosmic rays.
Maruyama, 2018 A significant part of temperature variation could be the result of a solar wind interaction with the Earth’s atmosphere and a subsequent modulation of the North Atratic Ocillation (NAO) [Boberg et al., 2003]. … A decadal variation that correlates positively with 11-year solar activity cycle of tropical lower stratospheric ozone and temperature has previously been identified.
Zhong et al., 2018 The response and potential amplify mechanism of earth climate at certain regions to the solar activity are one of the most important scientific issues in the modern climate research. This study reviews the advancements of satellite observation of solar spectrum and the reconstruction of dataset based on that. Then the key processes of the solar ultraviolet impacting the winter climate in East Asia are summarized. Generally, it includes the direct impacts of solar ultraviolet on the distribution of stratospheric ozone, the temperature and circulation. Then the signal is transported downward and poleward by the interaction of stratosphere and troposphere, and modulates the phase of the Arctic Oscillation (AO), which further impact the circulation and the winter climate of East Asia. The current studies show that the enhancement (reduce) of the solar ultraviolet tends to arouse the positive (negative) phase of AO, and then cause the warm (cold) winter in East Asia. Finally, the paper points out that quantitative study of these impacts based on the full coupled earth system model will be one key of breakthrough in this area.
Chang, 2018 The solar magnetic field plays a central role in the field of solar research, both theoretically and practically. Sunspots are an important observational constraint since they are considered a discernable tracer of emerged magnetic flux tubes, providing the longest running records of solar magnetic activity … [W]e find that as the Sun modulates the amount of observed galactic cosmic ray influx, the solar North-South asymmetry seems to contribute to the relationship between the solar variability and terrestrial climate change.
Collet and Schuh, 2018 By the 1270s the era of high solar irradiance responsible for the Medieval Climate Anomaly had ended and as the Wolf Solar Minimum started to bite, the transition began to the cooler global temperatures and altered and less stable atmospheric circulation patterns that characterised the LIA (Campbell 2016, 2–6, 198–208).
Zamelczyk et al., 2018 We compared paleo-data with modern planktic foraminiferal fauna distributions and the carbonate chemistry of the surface ocean. The results showed that cold sea surface conditions prevailed at ~400–800AD and ~1400–1950AD are associated with the local expression of the Dark Ages Cold Period and Little Ice Age, respectively. Warm sea surface conditions occurred at ~21–400AD, ~800–1400AD and from ~1950AD until present and are linked to the second half of the Roman Warm Period, Medieval Warm Period and recent warming, respectively. On the centennial to multi-centennial time scale, sea surface conditions seem to be governed by the inflow of Atlantic water masses (subsurface and surface) and the presence of sea-ice and the variability of sea-ice margin (near surface water masses). However, the close correlation of sea surface temperature recorded by planktic foraminifera with total solar irradiance implies that solar activity could have exerted a dominant influence on the sea surface conditions on the decadal to multidecadal time scale.
Roy, 2018 This chapter focused on the detected robust solar signal on climate. It presented some observational results that identified solar signature on sea level pressure (SLP), sea surface temperature (SST) and annual mean air temperature. A technique of Multiple Linear Regression (MLR) methods was discussed, and a detected significant signal on SLP around Aleutian Low (AL) was analysed. A solar signal was observed around AL and Pacific High also using other techniques. In terms of SST, the region of tropical Pacific was addressed. One study noted an in-phase relationship between the Sun and tropical Pacific SST, and another study even observed a phase locking between those. A widely debated study that used the method of solar maximum compositing on tropical Pacific SST was presented discussing the methodology. …The recent study by White and Liu (2008), using the method of singular value decomposition (SVD) and compositing of nine solar cycles (period 1900–2000), even detected the phase locking of harmonics of the ENSO time series and the solar cycle.
Itoh et al., 2018 Evaluating the magnitude of natural climate variations is important because it can greatly affect future climate policies. As an example, we examine the influence of changes in solar activity (solar wind in particular) on surface temperatures (Ts) and major teleconnection patterns such as the Arctic Oscillation and Pacific Decadal Oscillation. We compared correlation maps (spatial distribution of correlation coefficient) for a combination of Ts and a geomagnetic index (aa, an indicator of solar wind strength) and a combination of Ts and the teleconnection patterns. The phase of the quasi-biennial oscillation of the equatorial zonal wind and magnitude of sunspot number were considered. As a result, we found that the influence of the solar wind is as strong as that of the teleconnection patterns and hence, the former appears to affect the climate via the latter. It was also found that both the solar wind and ultraviolet change should be considered to explain the influence of solar activity variability, i.e., a multi-pathway scheme is necessary.
Moreno et al., 2018 The second, an AD 1654–2010 benthic foraminiferal record from the Caminha salt marsh, located in the lower estuary of the Minho River.The series were analysed together for the common period to outline how both palaeoclimatic proxies respond to the most likely natural environmental drivers of temporal variability, solar forcing included. Singular spectral analysis revealed a common significant multidecadal periodicity agreeing with recognized long-term changes in solar activity, i.e. the Lower Gleissberg cycle (50–80 years). The application of wavelet analysis allowed the detection of high coherence at this time scale (centred at c. 64 years) between marsh foraminifera and both total solar irradiance and the North Atlantic Oscillation index. This relationship persists throughout the c. AD 1730–1875 period.
Kushnir and Stein, 2018 The lake and river flood levels dropped between the late 10th and the middle 11th centuries AD and during much of the 14th century AD. During these times, historical documents describe unprecedented severe and persistent cold and dry winters in the northern regions of the Middle East, between Mesopotamia to historical Iran. The simultaneous cold and dry winters in the northern Near East and the droughts in the Levant and Egypt are consistent with timing of the two, medieval grand solar event: the Oort and Wolf minima. We argue and provide evidence that these solar events influence the state of the North Atlantic Oscillation and the intensity of the Siberian High, on the one hand and the frequency and intensity of El Niño, on the other hand, thus simultaneously afflicting the East Mediterranean with a severe hydroclimatic state that affected the history of the region.
Kelsey, 2018 Here I show statistically-significant correlations between independent, total solar irradiance (TSI) reconstructions from 10Be Antarctic ice-core data during the late Holocene and a normalised, chronologically-anchored model of superimposed 209-yr and 133-yr cycles. These two constituent cycles are evident in astronomical data of solar and lunar declinations, Earth-Sun and Earth-Moon distances, and consequently top-of-the-atmosphere (TOA) TSI, insolation, and gravitational data. The associated physical models of insolation, TSI, and gravitation covering the last 5.5 ky also show pronounced centennial- and millennial-scale oscillations. Apart from the known influence of the Moon on tidal sedimentation patterns, this evidence suggests that lunar gravitation also modulates the cosmogenic nuclide record through these 133-yr and 209-yr cyclical patterns. Because the 133-yr cycle is strongly featured in solar declination data, modelling of insolation shows patterns of sudden and extreme insolation change, especially at high latitudes. Multiple lines of evidence suggest that the Sun and Moon produce tidally-forced climate signals of Earth’s ocean and atmosphere at multiple time scales, where precession, perihelion, perigee, lunation, and nutation are key components. Consequently, the 1500-yr quasi-periodicity can be seen as a high-frequency expression of the same physical forces that produce the Milankovitch precessional and obliquity cycles.
Wang et al., 2018 Furthermore, the summer solar radiation is a significant influence factor that drove the climate conditions in south‐west China on a millennial scale. Interplays of glacial boundary conditions (sea‐level change, sea‐surface temperature, and ice‐sheet extent) and position of ITCZ likely influenced the Holocene climate variations directly or indirectly.
Miao et al., 2018 In this study, we investigate the influence of low-frequency solar forcing on the East Asian winter monsoon (EAWM) by analyzing a four-member ensemble of 600-year simulations performed with HadCM3 (Hadley Centre Coupled Model, version 3). We find that the EAWM [East Asian winter monsoon] is strengthened when total solar irradiance (TSI) increases on the multidecadal time scale. The model results indicate that positive TSI anomalies can result in the weakening of Atlantic meridional overturning circulation, causing negative sea surface temperature (SST) anomalies in the North Atlantic. Especially for the subtropical North Atlantic, the negative SST anomalies can excite an anomalous Rossby wave train that moves from the subtropical North Atlantic to the Greenland Sea and finally to Siberia. In this process, the positive sea-ice feedback over the Greenland Sea further enhances the Rossby wave. The wave train can reach the Siberian region, and strengthen the Siberian high. As a result, low-level East Asian winter circulation is strengthened and the surface air temperature in East Asia decreases. Overall, when solar forcing is stronger on the multidecadal time scale, the EAWM is typically stronger than normal. Finally, a similar linkage can be observed between the EAWM and solar forcing during the period 1850–1970.
Egorova et al., 2018 There is no consensus on the amplitude of the historical solar forcing. The estimated magnitude of the total solar irradiance difference between Maunder minimum and present time ranges from 0.1 to 6 W/m2 making uncertain the simulation of the past and future climate. One reason for this disagreement is the applied evolution of the quiet Sun brightness in the solar irradiance reconstruction models. This work addresses the role of the quiet Sun model choice and updated solar magnetic activity proxies on the solar forcing reconstruction. … A new reconstruction of the TSI and SSI covering the period 6000 BCE – 2015 CE is presented. The model simulates solar irradiance variability during the satellite era well. The TSI change between the Maunder and recent minima ranges between 3.7 and 4.5 W/m2 depending on the applied solar modulation potential. The implementation of a new quietest Sun model reduces, by approximately a factor of two, the relative solar forcing compared to the largest previous estimation, while the application of updated solar modulation potential increases the forcing difference between Maunder minimum and the present by 25-40 %.
Hughes et al., 2018 A varve thickness sequence is compared to sunspot observations from 1610-1995 CE. Maunder and Dalton minima are clearly expressed in a varve power spectrogram; varve signal amplification beginning ca. 1950s CE coincides with increasing activity evident in a sunspot spectrogram, features that are only vaguely discernible in the raw time-series plots. Spectral relationships at sunspot periodicities are consistent with direct solar forcing of varve thickness, independent of any effect solar activity might otherwise have on climate. Simulations based on a meltwater model indicate that direct forcing could result from amplified ultraviolet (UV) emission during solar maxima, combined with lower UV albedo of glacial ice. The plausible forcing mechanism bolsters epistemology for concluding a cause-effect relationship: solar variability likely contributed directly to inter-decadal patterns in Iceberg Lake varve thicknesses.
Allan et al., 2018 Results were compared to reconstructed sunspot number data to determine whether solar signal is presents in PN speleothem. The occurrence of significant solar periodicities (i.e., cycles of Gleissberg, de Vries, unnamed 500 years, Eddy and Hallstatt) supports for an impact of solar forcing on PN speleothem trace element contents. Moreover, several intervals of significant rapid winter change were detected during the Holocene at 10.3, 9.3-9.5, around 8.2, 6.4-6.2, 4.7-4.5, and around 2.7 ka BP. Those intervals are similar to the cold winter events evidenced in different natural paleoclimate archives, suggesting common climate forcing mechanisms related to changes in solar irradiance.
Ramos-Román et al., 2018 According to this, the results from the Padul-15-05 Holocene record suggest that the regional climate variability during the early and middle Holocene was partially due to external forcing (i.e. solar irradiance) and variability during the late Holocene (since ~ 4.7 cal kyr BP) was dominated by the effect of internal forcing (atmospheric-oceanic dynamic) -established since the NAO system influencing the western Mediterranean region- enhanced since ~ 5 cal kyr BP (Debret et al., 2007; 2009). Fletcher et al. (2013) described a shift in the millennial-scale periodicity since around ~ 6 cal kyr BP related with the establishment of the actual climate system in the western Mediterranean region. The similarities between the millennial-scale oscillations observed in the Padul-15-05 record with the total solar irradiance anomaly (TSI) and cooling events in the North Atlantic region (e.g. Bond et al., 2001; Steinhilber et al., 2009; Fig. 8) support the solar-atmospheric-oceanic link in the Atlantic-western Mediterranean region previously suggested (Debret et al., 2009). Our results are consistent with similar cyclical patterns detected throughout the North Atlantic records and related with solar activity also describing ~2500 and 1000 yr periodicities during the early Holocene (Debret et al., 2007; 2009). A similar periodicity of about 2300 yr is recognized in the ∆14C residual series from the Greenland Ice Sheet record (Mayewski et al., 1997). This periodicity has also been evidenced in sea surface temperatures (SST) reconstructions in the Aegean Sea in the NE Mediterranean related with glacier advance and suggesting a solar modulation (Rohling et al., 2002). The ~ 1000 yr periodicity is also established as a signal of solar activity in many other records in the Mediterranean and the North Atlantic region (e.g. Debret; 2007; 2009 and references therein). … Beige shadings highlight decreases in Mediterranean forest and coldest events related with decreases in total solar irradiance and decreases in SST.
Oliva et al., 2018 Cold period during 1645–1706 (Maunder solar minimum). Cold period during 1810–1838 (Dalton solar minimum). Warm period during the mid-20th and 21st centuries (modern solar maximum). LIA [Little Ice Age] was characterized by a cold phase having lower annual and summer temperatures relative to the long-term mean, consistent with the solar minima. … The record shows rapid cooling since the start of the Spörer Minimum, which intensified during the Maunder Minimum (with the lowest estimated temperature being 2 °C lower than the recent average). A later increase in the temperature and another slight cooling probably coincided with the Dalton Minimum. Particularly cold winters occurred during the MCA (from 1090 to 1179), during the LIA onset (1350) and from the late 15th to early 16th centuries. Winter temperatures would have been approximately 0.5 °C lower during the LIA (1500–1900) than during the 20th century. … [T]he Maunder Minimum coincided with a cold period from 1645 to 1706, and the Dalton Minimum (1796–1830) is correlated with a cold stage spanning the years from 1810 to 1838. Four warm periods (1626–1637, 1800–1809, 1845–1859, and 1986–2012) coincided with periods of increased solar activity. … The gradual increase in temperature during the second half of the 19th century resulted in significant glacier retreat, with rates of receding [in the second half of the 19th century] similar to those recorded during the last decades of the 20th century and in the early 21st century (Chueca et al., 2008). … The colder climate of the LIA was accompanied by severe droughts, floods, and cold/heat waves that showed significant spatio-temporal variation across the Iberian mountains. … The 20th century did not show unprecedented warmth over the last 800 years.
Maley et al., 2018 Chase et al. (2010) showed that solar forcing modulated by variations in the Earth’s geomagnetic shield is “a potentially important factor driving climate at suborbital timescales in both the northern and southern tropics.” In Africa particularly, “the minimum of the geomagnetic dipole moment was linked to a relatively humid phase (at 8000–7000 cal yr BP); the sharp increase in dipole strength at ca. 3800 cal yr BP is concurrent with the beginning of a drought phase, and the maximum dipole moment with a relatively arid phase (at 2500–2000 cal yr BP)” (Chase et al., 2010, p.42, fig. 3 and 6). … Moreover, in the framework of the climatic teleconnections occurring between the Atlantic and the Pacific Oceans, Emile-Geay et al. (2007) and Gray et al. (2010) estimated that solar influences could have impacted the climatic circulation above the Pacific Ocean, as ENSO and ITCZ activity, and hence could have played a role in these climatic teleconnections, as described during the late Holocene. As the effect of these Sun-Earth interactions on global climates remains a matter of debate, it appears that the recent increase in lightning activity in Central Africa, mainly in the Congo Basin, could result from some kind of solar influence, as was also the case for forest fragmentation between ca. 2500 and 2000 cal yr BP.
Lockwood et al., 2018 During the Dalton minimum [1797-1825] these reconstructions predict an average Ap that is roughly half of that during the modern maximum [1938-2000] but the number of storm-like days (with <Ap>=1dy > Apo) falls radically by an order of magnitude. … For the Maunder minimum, the mean Ap is lower than for the modern grand maximum by a factor of about 5 and the reconstructions predict no storm-like days would have been detected. Given the strong correlation between annual means of AE and Ap (r = 0.98), it is not surprising that the reconstructed AE index behaves in a somewhat similar way to Ap, with average values relative to the modern maximum that are roughly halved for the Dalton minimum and a fifth for the Maunder minimum. … The number of strong substorm-like hours p.a. in the Dalton minimum [1797-1825] is predicted to have been 140 compared to 512 in the modern grand maximum [1938-2000]. … In the Maunder minimum this to falls 28 per annum (i.e. this predicts a total of 1,680 substorm-like hours during the 60 years of the Maunder minimum compared to 30,720 for the 60 years of the modern grand maximum). … Looking to the future, the weakening of Earth’s magnetic moment means that the terrestrial disturbance levels during a future repeats of the solar Dalton and Maunder minima will be weaker and we here quantify this effect for the first time.
Ukhvatkina et al., 2018 It is well known that cold and warm periods of the climate are correlated with intensive solar activity (e.g., the Medieval Warm Period), while decreases in temperature occur during periods of low solar activity (e.g., the Little Ice Age; Lean and Rind, 1999; Bond et al., 2001). … . Long cold periods from 1643 to 1667 and from 1675 to 1690 that were revealed for another territory (Lyu et al., 2016; Wilson et al., 2016) coincided with the Maunder Minimum (1645–1715), an interval of decreased solar irradiance (Bard et al., 2000). The coldest year in this study (1662) was revealed in this period too. The Dalton minimum period centered in 1810 is also notable. … We suppose that a 9-year cycle may be related to solar activity, as, first of all, many authors showed influence of solar activity on the climate variability (Bond et al., 2001; Lean and Rind, 1999; Lean, 2000; Mann et al., 2009; Zhu et al., 2016). Secondly, the significant correlation between of the August–December minimum temperature reconstruction and TSI [total solar irradiance] can be regarded as additional evidence of this assumption. Finally, there is a coincidence of the reconstructed cold periods with the Maunder Minimum (1645–1715) and the Dalton minimum period centered in 1810. The solar activity influence in the region is traditionally associated with an indirect effect on the circulation of the atmosphere (Erlykin et al., 2009; Fedorov et al., 2015). In the second half of the 20th century the solar radiation intensity changes contributed to more intensive warming of the equatorial part of the Pacific Ocean and more active inflow of warm air masses to the north (Fedorov et al., 2015). … Close periodicity is revealed in long-term climate reconstructions and is linked to the quasi-200-year solar activity cycle in other studies (Raspopov et al., 2008, 2009). Raspopov et al. (2008) showed that in tree-ring-based reconstructions the cycle varies from 180 to 230 years. Moreover, the high correlation between the minimum temperature reconstructions and TSI, and also the revealed link between the reconstructed temperatures and solar activity minima, lead us to suppose that the solar activity may be the driver of the 200-year cycle. Such climate cycling, linked not only to temperature but also to precipitation, is revealed for the territories of Asia, North America, Australia, the Arctic, and the Antarctic (Raspopov et al., 2008).
Knizova et al., 2018 Weng (2005) has shown that the intensity of the seasonal forcing, modulated by the 11-year solar activity, is likely an important factor causing different dominant timescales in regional sea-surface temperatures. Even a small change in the solar constant may result in a regime change in the response with various dominant time scales. The large-term climatological study of Scafetta (2014) reveals solar signatures within surface temperature records taking into account the non-linearity of the systems since these systems are related through complex and non-linear processes. Various atmospheric parameters are in some periods positively and in others negatively correlated with solar activity. The study shows that using only one solar index does not capture all the complexity of the solar influences on the atmosphere.
Ma et al., 2018 Solar activity has the profound influence to geodynamics processes, and the Sun directly or indirectly affects some terrestrial phenomena on the Earth. Some studies showed variation of solar activity closely relates to global and regional climate change (Rasmus, 2006; Miyahara et al., 2008; Mendoza & Velasco, 2009; Ogurtsov et al., 2013; Dergachev et al., 2016). After analyzing the solar variation, global and regional sea-surface temperature, Weng (2005) concluded that inter-annual and centennial climate change signals were not purely internal, but also external because of the existence of the solar activity cycle. Kilcik et al. (2008) made use of surface air temperature, pressure and tropospheric absorbing aerosol data as climate parameters and solar flare index data as solar activity indicator, to study effect of solar activity on the surface air temperature of Turkey. With Indian temperature series of more than one-hundred years, Aslam (2014) investigated the influence of solar activity on regional climate. Results indicated that the solar variation may still be contributing to ongoing climate change.
Qin et al., 2018 Three quasi-oscillations with cycles of 31–22, 22–18, and 12–8 years may reflect the joint influence of PDO, southern oscillation, and solar activity on climate variation in the Qinling Mountains. … he third cycle of 12–8 years exhibited 18 distinct cold-hot events, which were approximately equivalent to the changes of solar activity and sunspot activity and corresponded to the 11-year cycle of drought in northwestern China (Cai and Liu. 2007). Nevertheless, tree growth may also be affected by solar activity through the influence on temperature variations, since solar activity has been inferred from tree-ring data in many regions worldwide (Murphy 1990; Damon et al. 1998; Rigozo et al. 2007; Wang and Zhang 2011; Duan and Zhang 2014). These three cycles indicate the June July temperature of the Mt. Taibai timberline in the Qinling Mountains is most likely affected by large-scale atmosphere-ocean interactions and solar activity, as suggested in other tree-ring records in northern China (Liu et al. 2005b, 2011; Bao et al. 2012; Liu et al. 2013).
Bhushan et al., 2018 The study observed that the periods of drift-ice events inferred based on the increase in the Hematite Stain Grains (HSG) correlates reasonable well with the low concentration of detritial proxies implying reduced precipitation induced runoff in the lake catchment (weak ISM). The present study thus indicates that the short-term ISM [Indian Summer Monsoon] variability in the central Himalaya were coupled with the northern latitude climatic events and solar forcing played a major role in modulating the Holocene millennial to centennial scale climatic fluctuations.
Zhang et al., 2018 The climate proxies and quasi-period of Lugu Lake indicate the ASWM [Asian Southwest Monsoon] intensified with an increase by LSI [low-latitude solar insolation] and solar activity during the early Holocene. … During the late Holocene, LSI [low-latitude solar insolation] and ASWM [Asian Southwest Monsoon] gradually decreased, and the climatic quasi-period signals recorded the progressive southward of ITCZ precipitation and solar activity. It exhibited an apparent synchrony with a large amount of climatic records from ASWM region. Moreover, the signals of human activities are not significant in driving periodic regularity, but only in the records of climate proxies. These suggest that LSI [low-latitude solar insolation] and solar activity dominated the climate change of ASWM region over the Holocene.
Zaffar et al., 2018 Various methods have been used to secure the certainty of significant relations among the sunspot cycles and some of the terrestrial climate parameters such as temperature, rainfall, and ENSO. This study investigates the behavior of ENSO cycles and mean monthly sunspot cycles. Sunspot cycles range from 1755 to 2016 whereas, ENSO cycles range from 1866 to 2012. … The results of this study confirm that during the period 1980–2000, ENSO cycles were very active. Simultaneously, ENSO was active for the periods 1982–1983, 1986–1987, 1991–1993, 1994–1995, and 1997–1998; these periods include two strongest periods of the century viz., 1982–1983 and 1997–1998. Sunspot cycles and ENSO cycles both were found to be persistent. Self-similar fractal dimensions exhibited a better persistency and a better correlation as compared to self-affine fractal dimension. This research is a part of a larger research project investigating the correlation of sunspot cycles and ENSO cycles, and the influence of ENSO cycles on variations of the local climatic parameters which in turn depends on solar activity changes. … The influence of the earth climatic condition of oscillations of solar activity is measurable only in the long-run duration. The solar cycles (solar activity) and ENSO episode are correlated with each other. Theory describes the relationship between sunspots and ENSO phenomena is premature, but now is established by a collection of evidence that the solar cycle moderates wind field in the stratosphere and troposphere.
White et al., 2018 Our data, together with published work, indicate both a long-term trend in ENSO strength due to June insolation [solar] forcing and high-amplitude decadalcentennial fluctuations; both behaviors are shown in models. The best-supported mechanism for insolation-driven dampening of ENSO is weakening of the upwelling feedback by insolation-forced warming/deepening of thermocline source waters. … Another potential source of decadal-centennial forcing is total solar irradiance, which varied more in the early Holocene than the mid- to late Holocene (Marchitto et al., 2010). Changing solar irradiance is theoretically capable of affecting ENSO via ocean dynamical cooling (Emile-Geay et al., 2007), and is correlated with centennial-scale variations in early Holocene ENSO (Marchitto et al., 2010). Overall, the apparent increase in decadal-centennial variability in early Holocene ENSO strength shown in coral/mollusk records [Cobb et al., 2013; Emile-Geay et al., 2016] is likely an accurate representation of ENSO’s behavior in response to a range of forcings. However, these short-term fluctuations cannot be taken as evidence for the lack of a long-term insolation-forced trend. … Overall, model results are consistent with Holocene proxy data in showing a long-term trend in ENSO strength due to insolation forcing, superimposed on short-term fluctuations in ENSO strength.
Siddiqui et al., 2018 The lower atmospheric forcing effects on the ionosphere are particularly evident during extreme meteorological events known as sudden stratospheric warmings (SSWs). During SSWs, the polar stratosphere and ionosphere, two distant atmospheric regions, are coupled through the SSW‐induced modulation of atmospheric migrating and nonmigrating tides. The changes in the migrating semidiurnal solar and lunar tides are the major source of ionospheric variabilities during SSWs. … Further, we examine the influence of solar flux conditions and the phases of quasi‐biennial oscillation (QBO) on the lunar tide and find that the QBO phases and solar flux conditions modulate the EEJ lunar tidal response during SSWs in a similar way as they modulate the wintertime Arctic polar vortex. This work provides first evidence of modulation of the EEJ lunar tide due to QBO.
Mazzarella and Scafetta, 2018 According to the IPCC (2013), solar forcing is extremely small and cannot induce the estimated 1.0–1.5 °C since the LIA. However, the solar radiative forcing is quite uncertain because from 1700 to 2000 the proposed historical total solar irradiance reconstructions vary greatly from a minimum of 0.5 W/m2 to a maximum of about 6 W/m2 (cf..: Hoyt and Schatten 1993; Wang et al. 2005; Shapiro et al. 2011). Moreover, it is believed that the sun can influence the climate also via a magnetically induced cosmic ray flux modulation (e.g.: Kirkby 2007) or via heliospheric oscillation related to planetary resonances (e.g.: Scafetta 2013, 2014b; Scafetta et al. 2016, and others). Since solar and climate records correlate quite significantly throughout the Holocene (cf: Kerr 2001; Steinhilber et al. 2012; Scafetta 2012, 20104b), the results shown herein may be quite realistic, although the exact physical mechanisms linking astronomical forcings to climate change are still poorly understood.
Seifert and Lemke, 2018 Global temperature evolution did not proceed directly along the nominal 30th sine half-wave line, because the fifth climate driving mechanisms, the 62-year SPO cycle, modifies the evolution line with consistent warm peaks. In the previous paper, Holocene part 7, we proved the consistency of this SPO cycle by demonstrating previous 62-year temperature peaks on a multi-millennial scale. This exact timing of warm peaks cannot be of tropospheric origin, because, using a Stocker quote: “The internal atmosphere-ocean climate system is unable to produce a forcing with a well-defined periodicity” (Stocker and Mysak, 1992). For this reason, this climate forcing is caused by solar – planetary oscillations (Scafetta, 2013). For a 60+ year cycle of external forcing, the SPO cycle is the best candidate. We add the abstract conclusion made in the latest AMO study of (Murphy, 2017): “We conclude that there is an essential role for external forcing in driving the observed AMO”. … Only the PaKern Recognition analysis is capable to explain each single temperature peak of the Holocene. The underperformance of other models and simulations can easily be explained. Fundamental causes are: 1. Their omission of decadal and centennial cosmic Earth orbital variations, and 2. The omission of solar motion variations. Instead, models and simulations exclusively center on internal atmosphereocean system variables, combined to some extremely long Milankovitch features, 20 – 40 kyr in length, with which centennial and single millennium features cannot be explained. Therefore, underperformance must be the logical result (ScafeKa, 2013). We emphasize again, that the present fourth flat temperature plateau, since 2004 AD, will continue until 2046 AD, the end of the recent 62-year SPO cycle.
Payomrat et al., 2018 During the third segment (1870–2001), the maximum temperature pattern seemed to be constant compared to the changing rate (+0.004 °C/decade). … The short fourth segment, which occurred from 2002 to 2013, showed a deceasing trend at a rate of -0.12 °C/decade. … The mean temperature from the first cool decades (1788–1829) in the Tmax reconstruction is the lowest among all four of the cool periods, with a mean maximum temperature of 29.82 °C. This condition may result from a negative climate forcing phase. Negative solar forcing and volcanic forcing (which is also known as volcanic-solar downturn) during 1791–1820 has been reported in volcanic forcing reconstructions based on ice core index analyses … Two negative climate forcing events coincidentally occurred during the same period, namely, the Dalton minimum (which featured low solar activity due to a low sunspot count) from 1800–1820 AD (Shapiro et al., 2011) and the eruptions of two volcanoes in 1809 and 1815 (Gao et al., 2008); these events caused a significant decrease in global temperature.
Roy, 2018 Solar cyclic variability can modulate winter Arctic climate … This study investigates the role of the eleven-year solar cycle on the Arctic climate during 1979–2016. It reveals that during those years, when the winter solar sunspot number (SSN) falls below 1.35 standard deviations (or mean value), the Arctic warming extends from the lower troposphere to high up in the upper stratosphere and vice versa when SSN is above. … Compositing also detects an opposite solar signature on Eurasian snow-cover, which is a cooling during Minimum years, while warming in maximum. It is hypothesized that the reduction of ice in the Arctic and a growth in Eurasia, in recent winters, may in part, be a result of the current weaker solar cycle. … Studies suggest that 50–60% of that ice loss is likely caused by externally forced anthropogenic emissions, with the rest caused by natural climate variability.
Guo et al., 2018 The temperature variations inferred from the records correlate well with changes in the solar irradiance and Northern Hemispheric temperature, which suggests a possible link between solar forcing and climate variabilities over the last 2000 years on the southern Tibetan Plateau. … The warm and dry period indicated by the high percentage of Artemisia and low percentage of Cyperaceae spanning the MWP [Medieval Warm Period] in Yamzhog Yumco Lake appeared to chronologically correspond with a strong period of solar radiation (Stuiver, 1998; Fig. 8A), as well as a warm period in the reconstructions of the Northern Hemispheric temperature (Mann and Jones, 2003; Fig. 8B). This study hypothesized that the relatively high solar radiation may have been the reason for the warmth and drought in the southern Tibetan Plateau during the MWP [Medieval Warm Period]. The relatively low solar radiation during the following LIA [Little Ice Age] may have lowered the temperature in the southern Tibetan Plateau, and the lowered temperature may have further increased moisture by suppressing evaporation. … It was determined that solar irradiance possibly played the most important role in influencing the climatic variabilities over the southern Tibetan Plateau on a multi-centennial timescale.
Li et al, 2018 The pollen-based reconstructions generally show an early Holocene climatic optimum with both abundant monsoonal rainfall and warm thermal conditions, and a declining pattern of both PANN and TANN values in the middle to late Holocene. The main driving forces behind the Holocene climatic changes in the LYR area are likely summer solar insolation associated with tropical or subtropical macro-scale climatic circulations such as the Intertropical Convergence Zone (ITCZ), Western Pacific Subtropical High (WPSH), and El Niño/Southern Oscillation (ENSO).
Ma et al., 2018 Solar activity has the profound influence to geodynamics processes, and the Sun directly or indirectly affects some terrestrial phenomena on the Earth. Some studies showed variation of solar activity closely relates to global and regional climate change (Rasmus, 2006; Miyahara et al., 2008; Mendoza & Velasco, 2009; Ogurtsov et al., 2013; Dergachev et al., 2016). After analyzing the solar variation, global and regional sea-surface temperature, Weng (2005) concluded that inter-annual and centennial climate change signals were not purely internal, but also external because of the existence of the solar activity cycle. … More and more people attach importance to studies about long-term solar variation (Usoskin & Mursula, 2003; Yin et al., 2007; Ma, 2007, 2009). However direct observations of solar activity in the past four centuries are insufficient to calculate the long-term solar variation. Some proxies including 14C, 10Be and geomagnetic variations can reflect the solar activity. Therefore solar activity in the past can be reconstructed with these proxies. In this work, rectified continuous wavelet transform reveals quasi ~500-year cycle signals existing in the reconstructed solar activity series. … Pollen record reflects the dynamics of vertical vegetation zones and temperature change. Using a high-resolution pollen record from a maar annually laminated lake in East Asia, Xu et al. (2014) revealed quasi ~500-year periodic cold-warm fluctuations over the past 5350 years.
Zhang et al., 2018 The evidence of solar forcing of the summer temperature variability from the site on centennial timescales where key solar periodicities (at 855±40, 465±40, 315±40 and 165±40 yr) are revealed. By using a band-pass filter, coherent fluctuations were found in the strength of Asian summer monsoon, Northern Hemisphere high latitude climate and high elevation mid-latitude (26 °N) terrestrial temperatures with solar sunspot cycles since about 7.6 ka. … Changes of solar irradiance can directly influence the continent surface temperature variation and ocean-atmospheric circulations (Gray et al. 2010; Shindell et al. 2001; Wang and Dickinson 2013). When TSI is reduced, the downward-propagating effects were triggered by changes in the top of the atmosphere. This leads to a cooling of the stratosphere and the Northern Hemisphere generally experiences cooler climates (Kaufmann et al. 2011; Shindell et al. 1999; Wang and Dickinson 2013). In addition, sensitive atmospheric responses around the North Atlantic region to reduced TSI could reduce North Atlantic Deep Water (NADW) intensity, cool the ocean surface temperature and trigger the southward migration of the mid-latitude westerlies and the mean position of the ITCZ (Shindell et al. 2001).
Sjolte et al., 2018 The temperature response to the long-term solar minima is a cooling across Greenland, Iceland and western Europe during solar minima. This cooling pattern corresponds well to the suggested cooling during the Little Ice Age in proxy records from Greenland (Stuiver et al., 1997), Iceland (Moffa-Sanchez et al., 2014) and Europe (Luterbacher et al., 2004). A NAO-type response to long-term solar forcing would give opposing temperature responses in Greenland and Europe, which is not the case. We find no consistent relation between our reconstructed NAO and solar forcing. Instead we would like to stress the importance of the connection between solar activity and the secondary circulation patterns, which likely captures the main response to solar forcing on decadal to centennial time scales.
Degroot, 2018 Scholars in many disciplines have used diverse methods and sources to establish that, between the 15th and 18th centuries, a “Little Ice Age” considerably cooled Earth’s climate. In four particularly chilly periods—the Spörer Minimum, Grindelwald Fluctuation, Maunder Minimum, and Dalton Minimum—falling temperatures both caused and reflected changes in atmospheric circulation that altered regional patterns of precipitation. Many scholars have argued that weather in these cold periods provoked or worsened regional food shortages, famines, rebellions, wars, and outbreaks of epidemic disease, in ways that may have contributed to mass mortality across the early modern world.
Bednarz et al., 2018 It is now well understood that changes in the incoming ultraviolet (UV) radiation associated with the 11-year solar cycle influence temperatures and ozone concentrations across much of the stratosphere (e.g.: Penner and Chang, 1978; Brasseur and Simon, 1981; Haigh, 1994; Randel et al., 2009; Ramaswamy et al., 2001; Keckhut et al., 2005; Soukharev and Hood, 2006; Mitchell et al., 2015b; Maycock et al., 2016). In addition to being a major driver of decadal variability within the stratosphere, these effects can initiate a dynamical response that propagates down into the troposphere (e.g.: Kuroda andKodera, 2002; Kodera and Kuroda, 2002), thereby affecting surface climate variability (e.g. Thieblémont et al., 2015).
Tang et al., 2018 The observed changes in the global O3‐CPM correlate well with the changes in solar activity during 2002–2016 with correlation coefficient of 0.92, and the global solar response of O3‐CPM is (20.18 ± 2.24)%/100 solar flux units in mesopause. Then, the latitudinal distribution of O3‐CPM and its solar cycle dependence are presented for 16 latitude bins. The latitudinal correlation analysis shows that the O3‐CPM is significantly correlated to the solar cycle at or above the 95% confidence level for each latitude bin from 84°S to 70°N, and the correlation coefficients are remarkably higher in the southern hemisphere than for corresponding latitudes in the northern hemisphere. … The present analysis has demonstrated that the global interannual variation of O3‐CPM, which is in accordance with 11‐year solar cycle, is significantly correlated to solar radiation, [O] density, and temperature and is not correlated to the [H] density in mesopause. These significant correlations are presented, but the main driver is solar activity.
Cionco et al., 2018 We present a new set of solar radiation forcing that now incorporated not only the gravitational perturbation of the Sun-Earth-Moon geometrical orbits but also the intrinsic solar magnetic modulation of the total solar irradiance (TSI). This new dataset, covering the past 2000 years as well as a forward projection for about 100 years based on recent result by Velasco-Herrera et al. (2015), should provide a realistic basis to examine and evaluate the role of external solar forcing on Earth climate on decadal, multidecadal to multicentennial timescales.
Crosta et al., 2018 Changes in seasonal insolation south of 60°S were suggested to be the main forcing mechanism whereby the strong decrease in spring insolation over the last 6 kyr (~30 W·m−2) would have allowed spring sea ice to melt later in the year while the increase in summer insolation (~15 W·m−2) would have allowed ice free surface water to warm and delay autumn sea-ice formation (Pike et al., 2009). Our simulations confirm that changes in seasonal insolation [surface incident solar radiation] had a profound impact on surface and subsurface ocean temperatures which, in turn, may have impacted on glacial ice discharge. The simulated Holocene response of summer subsurface temperature in the three-member ensemble shows an increase in the 0–200 m water depth since ~5 kyr BP, with episodic warm anomalies reaching down to 500 m during the 5–2 kyr BP period. This warming can be interpreted as a direct response to the increase in summer insolation south of 60°S which is penetrating at depth.
Prestes et al., 2018 In each tree-ring growth series, periods between 2 and 7 years were found, possibly related to the El Niño/La Niña phenomena, and a ∼ 23-year period was found, which may be related to temperature variation. These novel results might represent the tree-ring growth response to local climate conditions during its lifetime, and to nonlinear coupling between the Sun and the local climate variability responsible to the regional climate variations. … Tree-ring data have been used to reconstruct the climate (e.g., Case and MacDonald, 1995; Jacoby et al., 2003; D’Arrigo et al., 2001; Salzer and Kipfmueller, 2005; Shao et al., 2005; Therrell et al., 2006; Lorensi and Prestes, 2016), and there is evidence of solar influence on these data in timescales of decades to centuries. In addition, there is evidence of solar cycles in living and fossil tree-ring time series (Mori, 1981; Ammons et al., 1983; Nordemann et al., 2005; Raspopov et al., 2011; Prestes et al., 2011, 2014; Dorotovic et al., 2014; Perone et al., 2016). … In summary, these results indicate that the variability in tree-ring growth of Araucaria angustifolia is closely related to the variation of temperature and precipitation. This fact is possibly due to the nonlinear effects of Sun variability, and to the El Niño–Southern Oscillation in the climate system over South Brazil.
Daspattnayak et al., 2018 This study has undertaken the data during the Year 1952 to 2012 AD for the investigation on the correlation amongst the rainfall, sunspots in north-western part of India (specifically in the city of Jaipur). … During maximum and minimum solar activity, rainfall at Udaipur has been found to be more than average rainfall which indicated that solar activity helped in precipitation of clouds in the monsoon seasons after condensation and nucleation. … In order to understand the systematic trend of rain and its attribution with SSN [sunspot number] in clearer manner, the histograms of average SSN [sunspot number] and rain is plotted in two parts of figure 5.7. Here we found clear trends of maximum rainfall during maximum and minimum of SSN [sunspot number].
McCrann et al., 2018 The effect of the Sun’s activity on Earth’s climate has been identified since the 1800s. However, there are still many unknowns regarding the mechanisms connecting the Earth’s climate to the variation in solar irradiance. Climate modelling that implements the solar sciences is a novel approach that accounts for the considerable effect that natural factors have on the climate, especially at regional level. This paper discusses the noticeable effect that planet oscillations have on the Sun’s activity, which gives a very good correlation with the observed patterns in global surface temperatures, rainfall records and sea levels. In agreement with many studies that have identified a 60-year cycle in the variation of Earth’s temperature, it is expected that surface temperatures will reach a trough of the cycle around 2030-2040. Furthermore, considering the influence of the Solar Inertial Motion, a solar slowdown is predicted for Solar Cycles 24 and 25, which will create a weak grand minimum. It is anticipated that this weak grand minimum will be reflected in a dampening effect of global temperatures, and a subsequent moderation in the rate of sea level rise. … Current predictions on Solar activity show that we are in a low sunspot cycle, which is similar to that of the 1900 Minimum, and subsequent cycles are predicted to have even lower Solar activity, and therefore a drop in global temperatures is expected –. [B]ased on the effects of solar activity, it is logical to predict that a reduction in global average temperatures might manifest in a consequent slowing of the rate of sea level rise. … [M]any studies have reported that lower than average European temperatures were recorded during periods of low solar activity –. Such periods of low solar activity are the Maunder minimum (1645-1715), Dalton minimum (1800-1820), 1900 minimum (1880-1900), and a slight decrease between 1940 and 1970.
Kelsey, 2018 This research found a statistically-significant, strong positive correlation between solar insolation reconstruction derived from Antarctic 10Be ice-core data and a normalised, chronologically-anchored model of superimposed astronomical cycles that emulates the ~1500-yr climate cycle. … Supported by multiple lines of evidence, the results of this thesis suggest that the Sun and Moon act together through gravitation and insolation to produce millennial-, centennial-and decadal- scale climate signals through tidal forcing of Earth’s atmosphere and ocean. Key mechanisms and components are precession, perihelion, perigee, lunation, and nutation (wobble of Earth’s axis). Key inferences from these results are that astronomical forcing influences radiocarbon chronological variability, such as marine reservoir values, variability and time lag in radiocarbon data, and also suggest that the 209-yr SdV cycle is caused by combined solar and lunar forcing rather than previously inferred solar variability.
Solar Modulation Of Galactic Cosmic Rays/Cloud Cover Changes
Govil et al., 2018 The spectral analysis of the sedimentological parameters reveals the significant periodicities (>95% significance) centering at ∼1067, ∼907, and ∼824 years. The long-term trends in the data suggest the possible fluctuation of Antarctic ice-sheet superimposed on global climatic fluctuations due to solar activity. … The curiosity of climate scientists arises on the mechanism of reaction of the climate system in response to the changes in solar forcing. There are two possible mechanisms proposed which work through the atmospheric processes. The first mechanism includes the action of the ozone layer by increasing more UV radiations with increased solar activity. It must have raised the temperature in the stratosphere which produces stronger winds in lower stratosphere and troposphere. These strong winds in the troposphere result in the relocation of pressure cells and storm tracks which ultimately disturbs the climate system (Schindell et al., 1999; Crosta et al., 2007). The second proposed mechanism considers the cosmic rays and cloud cover responsible for amplifying the climate forcing (Svensmark, 2000). High solar activity is believed to be responsible for less cooling of the lower atmosphere due to reduced cloud cover (Marsh and Svensmar, 2000). Conversely, low solar activity is believed to provide additional cooling of the lower atmosphere. These two feedback mechanisms responsible for the climatic forcing due to solar activity may work alone or in conjugation and are also superposed to other climate forcing as well as variability of internal cycling (Rind, 2002). Further, the periodic increase in solar activity results in increased temperature in the lower atmosphere which causes melting of ice-sheets in the Antarctic region. It may further provide the periodicity in freshwater discharge in the Schirmacher lakes and hence regulates the depositional environment of the studies lake.
Frigo et al., 2018 In this work, we investigate the relationship between the ∼ 11-year and ∼ 22-year cycles that are related to solar activity and GCRs [galactic cosmic rays] and the annual average temperature recorded between 1936 and 2014 at two weather stations, both located near a latitude of 26◦ S but at different longitudes. … Sunspot data and the solar modulation potential for cosmic rays were used as proxies for the solar activity and the GCRs, respectively. Our investigation of the influence of decadal and bidecadal cycles in temperature data was carried out using the wavelet transform coherence (WTC) spectrum. The results indicate that periodicities of 11 years may have continuously modulated the climate at TOR [Torres, Brazil] via a nonlinear mechanism … . The obtained results offer indirect mathematical evidence that solar activity and GCR variations contributed to climatic changes in southern Brazil during the last century. The contribution of other mechanisms also related to solar activity cannot be excluded.
Wilson and Sidorenkov, 2018 The fact that the periods of eight out of nine of the most prominent peaks in the lunar alignment spectrum (highlighted column 3 of Table 2) closely match those in the spectra of ϕm [solar modulation potentional] and Tm [maximum daily temperature], strongly supports the contention that all three of these phenomena are closely related to one another. … principal component analyses of the 10Be and 14C records show that, on multi-decadal to centennial time scales, the radionuclide production signal accounts for 76% of the total variance in the data [18,19]. This would imply that there is a causal link between Tm [maximum daily temperature] and near-Earth GCR flux, with a factor related to the latter driving the former. … An implicit assumption that is used by those who reject GCR [galactic cosmic rays]-cloud models is that the GCR flux hitting the Earth needs to produce changes in the total amount of cloud cover over the majority of the globe in order to significantly affect the world mean temperature. However, this assumption ignores the possibility that regional changes in the amount of cloud cover could influence the rate at which the Earth’s climate system warms or cools. Of course, for this to be true there would have to be observational evidence that shows that the GCR flux can affect the level of cloud cover on a regional scale. Support for this hypothesis is provided  who claim that existing multi-decadal ground-based datasets for clouds show that there is a weak but significant correlation between the amounts of regional cloud cover and the overall level of GCR fluxes. In addition, Larken et al.  find that there is a strong and robust positive correlation between statistically significant variations in the short-term (daily) GCR ray flux and the most rapid decreases in cloud cover over the mid-latitudes (30° – 60° N/S). Moreover, Larken et al.  find that there is a direct causal link between the observed cloud changes and changes in the sea level atmospheric temperature, over similar time periods.
Hence, the solar connection between Tm and ϕm can be summarized using a heuristic luni-solar model like that shown in Figure 6. Firstly, the model proposes that there must be some, as yet, unknown factor associated with the level of solar activity on the Sun (e.g. possibly the overall level GCR hitting the Earth) that is producing long-term systematic changes in the amount and/or type of regional cloud cover. Secondly, the model proposes that the resulting changes in regional cloud cover lead to variations in the temperature differences between the tropics and the poles which, in turn, result in changes to the peak strength of the zonal tropical winds. Thirdly, the model further proposes that it is the long-term changes in the amount and/or type of regional cloud cover, combined with the variations in the temperature differences between the tropics and the poles that lead to the long-term changes in the poleward energy and momentum flux. And finally, the model proposes that it is this flux which governs the rate at which the Earth warms and cools, and hence, determines the long-term changes in the world mean temperature.
Vieira et al., 2018 Galactic cosmic rays (GCRs) are the main source of ionizing radiation in the lower troposphere, in which secondary products can penetrate the ground and underground layers. GCRs affect the physical–chemical properties of the terrestrial atmosphere, as well as the biosphere. GCRs are modulated by solar activity and latitudinal geomagnetic field distribution.
Tyasto et al., 2018 Variations of charged particles of galactic cosmic rays (GCRs), which are caused by variations in the Earth’s magnetic field, are one of most significant among the variety of phenomena that influence the near-Earth medium and, consequently, the Earth’s climate and weather. Being the main sources of atmospheric ionization, they influence the atmosphere transparency and play the key role in formation of clouds, thunderstorms, and lightnings (Dorman, 2009).
Palcsu et al., 2018 The relationship between the atmospheric concentration of cosmogenic isotopes, the change of solar activity and hence secondary neutron flux has already been proven. The temporal atmospheric variation of the most studied cosmogenic isotopes shows a significant anti-correlation with solar cycles. However, since artificial tritium input to the atmosphere due to nuclear-weapon tests masked the expected variations of tritium production rate by three orders of magnitude, the natural variation of tritium in meteoric precipitation has not previously been detected. For the first time, we provide clear evidence of the positive correlation between the tritium concentration of meteoric precipitation and neutron flux modulated by solar magnetic activity. We found trends in tritium time series for numerous locations worldwide which are similar to the variation of secondary neutron flux and sun spot numbers. This variability appears to have similar periodicities to that of solar cycle. Frequency analysis, cross correlation analysis, continuous and cross wavelet analysis provide mathematical evidence that the correlation between solar cycle and meteoric tritium does exist. Our results demonstrate that the response of tritium variation in precipitation to the solar cycle can be used to help us understand its role in the water cycle.
Veretenenko et al., 2018 Influence of galactic cosmic rays (GCRs) on cloud formation is suggested to be an important part of the mechanism of solar activity influence on weather and climate. A high positive correlation between low cloud amount and GCR fluxes was observed in the 1980s–1990s; however, in the early 2000s, it was violated. In this work, we consider a nature of long-term correlation links between cloud cover at middle latitudes and GCRs, as well as possible reasons for this correlation reversal. It was shown that the GCR-cloud links observed on the decadal time scale are indirect and caused by GCR effects on cyclonic activity which depend on epochs of the large-scale atmospheric circulation. The reversal of GCR-cloud correlation in the 2000s seems to be due to a sharp weakening of the Arctic and Antarctic stratospheric polar vortices, which results in the change of the troposphere-stratosphere coupling and, then, of GCR contribution to the development of extratropical cyclogenesis.
Correlation links observed between lower atmosphere characteristics and phenomena related to solar activity may weaken, disappear and even change sign depending on time period. So, a violation of the cloud-GCR link in the 2000s is not an extraordinary event. Herman and Goldberg  suggested that a reason for temporal variability of solar-atmospheric links may be long-term processes of the Sun which do not influence sunspot numbers and/or some changes of atmospheric conditions. Veretenenko and Ogurtsov [42, 43] showed that temporal behavior of correlation links between surface pressure at extratropical latitudes and sunspot numbers is characterized by a roughly 60-year periodicity caused by changes in the epochs of the large-scale atmospheric circulation. The reversals of the correlation signs were found in the end of the nineteenth century, in the early 1920s, the 1950s and the early 1980s coinciding with climatic regime shifts at middle latitudes , as well as with the transitions between cold and warm epochs in the Arctic . So, a violation of the cloud-GCR link in the 2000s seems not to be unexpected and may be associated with the next change of the circulation epochs resulting in the change of GCR contribution to extratropical cyclonic activity and, then, to cloud field formation.
Indeed, cloudiness changes can strongly modulate fluxes of both incoming short-wave solar radiation and outgoing long-wave radiation of the Earth and the atmosphere and, thus, influence significantly the radiative-thermal balance of the atmosphere. High-level clouds contribute to the warming of the atmosphere, whereas low-level clouds contribute, as a rule, to its cooling. A net influx of radiation coming to the Earth’s surface under cloudy conditions depends on latitude, season and underlying surface. According to the data obtained from spaceborne experiments [Nimbus 7 Earth Radiation Budget experiment (N7ERB) and Earth Radiation Budget Experiment (ERBE)], when averaged over the globe, cloudiness reduces the input of solar radiation by 44.5–54.3 W·m−2 (depending on the season) and the emission of long-wave radiation to space by 23.6–34.7 W·m−2 [1, 2]. As a result, cloudiness decreases the global radiative heating of the atmosphere by 17.3–26.8 W·m−2.
Padovani et al., 2018 The presence of small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The abundance of hydrogen atoms measured in the densest regions of molecular clouds can be only explained by the dissociation of H2 due to cosmic rays. … Our findings show that a careful description of molecular hydrogen dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.
Surface Solar Radiation Influence On Climate
Pfeifroth et al., 2018 Solar radiation is the main driver of the Earth’s climate. Measuring solar radiation and analysing its interaction with clouds are essential for the understanding of the climate system. … This multi-parameter analysis focuses on Europe and covers the time period from 1992 to 2015. A high correlation between these three variables has been found over Europe. An overall consistency of the climate data records reveals an increase of surface solar radiation and a decrease in top-of-atmosphere reflected radiation. In addition, those trends are confirmed by negative trends in cloud cover. This consistency documents the high quality and stability of the CM SAF climate data records, which are mostly derived independently from each other. The results of this study indicate that one of the main reasons for the positive trend in surface solar radiation since the 1990’s is a decrease in cloud coverage even if an aerosol contribution cannot be completely ruled out.
Kambezidis, 2018 The solar radiation climate of Athens: Variations and tendencies in the period 1992–2017, the brightening era … [T]he trend in SSR [surface solar radiation] over the period 1900–2012 (see Table 5 in Kazadzis et al., 2018) is +0.40%/decade [+1.5 Wm-2/decade], almost equal to the one found in this study for the period 1992–2017. Indeed, Bais et al. (2013) reported a positive trend in SSR of +0.33%/year after 1990 and a slowdown in its trend after 2000. On the other hand, the trend of +0.41%/decade of this study is equivalent to +0.15 Wm−2/year (or +1.50 Wm−2/decade), a finding to be compared with results from other similar studies. Boers et al. (2017) found a yearly-averaged SSR [surface solar radiation] trend for all-sky conditions of +1.81 Wm−2/decade over the Netherlands in the period 1966–2015, close to our trend of +1.50 Wm−2/decade. A higher value in the SSR trend over Europe (period 1983–2010) of +2.00 Wm−2/decade for all-sky conditions was found by Sanchez-Lorenzo et al. (2017) using satellite data. … Indeed, the slight global brightening observed over Europe (Hatzianastassiou et al., 2012) was associated with a decrease of 5–10% (in absolute terms) in cloud cover, in accordance with the results of Kambezidis et al. (2016).
Pfeifroth et al., 2018 The incoming solar radiation is the essential climate variable that determines the Earth’s energy cycle and climate. In this study, these new climate data records are compared to surface measurements in Europe during the period 1983–2015. The results show an overall brightening period since the 1980s onward (comprised between 1.9 and 2.4 W/m2/decade), with substantial decadal and spatial variability. The strongest brightening is found in eastern Europe in spring. … We conclude that the major part of the observed trends in surface solar radiation in Europe is caused by changes in clouds and that remaining differences between the satellite- and the station-based data might be connected to changes in the direct aerosol effect and in snow cover.
Feng and Wang, 2018 Surface Incident solar radiation (Rs), which is also often referred to as the downward solar irradiance, is a key parameter in many climate and ecological processes, such as evapotranspiration, canopy photosynthesis, net primary production, crop growth management, and so on. Long-term Rs datasets with global coverage and reasonable accuracy have a great value these days. Globally-distributed ground observations of Rs began in 1958, and provide solid evidence for global dimming and brightening.
Pan et al., 2018 Introduction: Solar radiation incidence at the surface plays a fundamental and determinant role in the climate and life on our planet . Surface solar radiation is a major component of the surface energy balance and governs many diverse surface processes, such as evaporation and associated hydrological components, plant photosynthesis, and the diurnal and seasonal courses of surface temperatures. Negative trends in the downwelling of surface solar radiation are collectively called “dimming”, whereas positive trends are called “brightening” . Any change in the amount of solar radiation profoundly affects the temperature field, atmospheric and oceanic general circulation, and the hydrological cycle . Widespread reduction in the annual average surface solar radiation, from the 1960s to the 1980s, has been reported by many researchers at the global and regional scales, including those from America, Europe, and China [2,4]. Subsequently, the term “brightening” was coined to emphasize the fact that global solar radiation is no longer declining at many sites since the late 1980s . Long, et al.  found that solar dimming has reversed at an increasing trend of 6 W m−2 per decade in the continental United States from 1995–2007. Wild  showed that the globally averaged trends in the 1980s typically reversed from dimming to brightening, while this study reports trends of 2.2–6.6 W m−2 per decade from the 1980s to the 2000s.
Myers et al., 2018 Between 2013 and 2015, the northeast Pacific Ocean experienced the warmest surface temperature anomalies in the modern observational record. This “marine heatwave” marked a shift of Pacific decadal variability to its warm phase and was linked to significant impacts on marine species as well as exceptionally arid conditions in western North America. Here we show that the subtropical signature of this warming, off Baja California, was associated with a record deficit in the spatial coverage of co‐located marine boundary layer clouds. This deficit coincided with a large increase in downwelling solar radiation that dominated the anomalous energy budget of the upper ocean, resulting in record‐breaking warm sea surface temperature anomalies. Our observation‐based analysis suggests that a positive cloud‐surface temperature feedback was key to the extreme intensity of the heatwave. The results demonstrate the extent to which boundary layer clouds can contribute to regional variations in climate.
Hu et al., 2018 During the global warming hiatus period, the transfer of latent heat energy from the ocean to atmosphere increases and the total downward radiative energy flux to the surface decreases due to a reduction of solar absorption caused primarily by an increase of clouds. … Observational evidence indicates the global-mean surface temperature (GMST) has experienced a relatively rapid warming from the early-1980s to the early-2000s, but has nearly stalled since, producing what is known as the ‘global warming hiatus’ period (Easterling and Wehner 2009; Knight et al. 2009; Liebmann et al. 2010; Solomon et al. 2010; Cowtan and Way 2014; Trenberth 2015). … Another important contributor to the rapid warming is the increase of shortwave (SW) irradiance received by the surface, referred to as the “global brightening” (Wild et al. 2005; Ohmura 2009; Wild 2009, 2012). The increase in shortwave irradiance is closely related to the decrease in cloud cover in the early 1980s (Eastman and Warren 2013). … Internal climate variability is also thought to substantially contribute to the rapid warming pace: some studies argue the Atlantic Multidecadal Oscillation (AMO) associated with the thermohaline circulation amplified the surface warming rate as it was in its warming phase during the last two decades of the twentieth century (Wu et al. 2007, 2011; Semenov et al. 2010; Delsole et al. 2011); while others argue the positive phase of the Pacific Decadal Oscillation (PDO) and associated Interdecadal Pacific Oscillation (IPO) accelerated the warming rate during the late twentieth century (Trenberth and Fasullo 2013; Kosaka and Xie 2016; Meehl et al. 2016).
Liu et al., 2018 The northern TP (NTP) experienced more warming than the southern TP (STP) in all seasons from 1982 to 1998, while the pattern was reversed in the period from 1998 to 2015. Water vapour was found to be the main driving force for the trend in Tmean and Tmin by influencing downward long wave radiation. Sunshine duration was the main driving force behind the trend in Tmax [maximum temperature] and DTR through a change in downward shortwave radiation that altered the energy source of daytime temperature. Water vapour was the major driving force for temperature change over the NTP, while over the STP, sunshine duration dominated the temperature trend. [CO2 concentration changes are not mentioned in the paper as a factor in the 1982-2015 temperature changes.]
ENSO, NAO, AMO, PDO Climate Influence
Hahn et al., 2018 North Atlantic Natural Variability Modulates Emergence of Widespread Greenland Melt in a Warming Climate … Record‐breaking melt over Greenland in recent decades is linked not only to climate change but also to natural variability, including persistent atmospheric high‐pressure conditions in the negative phase of the North Atlantic Oscillation and warm North Atlantic Ocean temperatures during the positive phase of the Atlantic Multidecadal Oscillation. However, the relative importance of natural variability for Greenland melt under varying degrees of greenhouse forcing is still unclear. Using reanalysis data and a large ensemble of climate model simulations, we find that a negative North Atlantic Oscillation and positive Atlantic Multidecadal Oscillation consistently promote heightened summer melt under various forcing conditions. Moreover, timing of widespread 21st century Greenland melt varies considerably between ensemble members due to different phasing of these modes of natural variability. These results indicate the importance of natural modes of variability across a range of external forcing conditions for interannual melt variability and the emergence of widespread Greenland melt. … North Atlantic warming in high melt years is driven by the negative NAO rather than the AMO (Hurrell & Deser, 2010). Downward (upward) turbulent heat flux anomalies over warmer (colder) ocean regions during high melt seasons further suggest that NAO-related wind and heat fluxes predominantly force ocean temperatures.
(press release) The study, published in Geophysical Research Letters, found that when the NAO stays in its negative phase (meaning that air pressure is high over Greenland) it can trigger extreme ice melt in Greenland during the summer season. Likewise, the AMO, which alters sea surface temperatures in the North Atlantic, can cause major melting events when it is in its warm phase, raising the temperature of the region as a whole.
Drinkwater et al., 2018 Following rapid cooling in the 1960s, much of the North Atlantic Ocean was characterized by a cold period during the 1970s and 1980s. This cold period was part of the multidecadal variability in sea surface temperatures known as the Atlantic Multidecadal Oscillation or AMO, which has a period of ∼60–80 years. During this cold period, below average air and sea temperatures predominated, increased ice cover was observed in those northern regions with seasonal sea ice, and evidence was found of reduced Atlantic inflow into the Northeast Atlantic Ocean. The ecological responses included a reduction in primary production and geographic shifts in zooplankton species. Also, there was a general southward expansion of arctic and boreal fish species and a retreat of the temperate species. Major fish stocks such as Atlantic cod off Greenland and Labrador/northern Newfoundland, as well as the Norwegian spring-spawning herring, collapsed commercially. These collapses were partly driven by climate-induced declines in growth rates and recruitment survival, as well as fishing. In contrast, in the more southern range of Atlantic cod, such as the North Sea, the opposite response occurred as the cool conditions led to improved growth rates and higher abundance. Long-term measurements in the English Channel documented the replacement of several warm-water species with more northern cold-water species. Benthic and nearshore species also underwent distributional shifts and changing abundances. Comparisons with the responses to the warm periods suggest that following the cold period of the 1970s and 1980s, the ecosystem in the 1990s and 2000s returned to conditions akin to what they were in the previous warm period of the 1930s–1950s. However, there were some notable exceptions, such as the continued low abundance of Atlantic cod off West Greenland and Labrador/northern Newfoundland.
Paolo et al., 2018 Satellite observations over the past two decades have revealed increasing loss of grounded ice in West Antarctica, associated with floating ice shelves that have been thinning. Thinning reduces an ice shelf’s ability to restrain grounded-ice discharge, yet our understanding of the climate processes that drive mass changes is limited. Here, we use ice-shelf height data from four satellite altimeter missions (1994–2017) to show a direct link between ice-shelf height variability in the Antarctic Pacific sector and changes in regional atmospheric circulation driven by the El Niño/Southern Oscillation. This link is strongest from the Dotson to Ross ice shelves and weaker elsewhere. During intense El Niño years, height increase by accumulation exceeds the height decrease by basal melting, but net ice-shelf mass declines as basal ice loss exceeds ice gain by lower-density snow.
Di Rita et al., 2018 When the timing of these patterns is compared with the climate proxy data available from the same core (planktonic foraminifera assemblages and oxygen stable isotope record) and with the NAO (North Atlantic Oscillation) index, it clearly appears that the main driver for the forest fluctuations is climate, which may even overshadow the effects of human activity. We have found a clear correspondence between phases with negative NAO index and forest declines. In particular, around 4200 cal BP, a drop in AP (Arboreal Pollen) confirms the clearance recorded in many sites in Italy south of 43N. Around 2800 cal BP, a vegetation change towards open conditions is found at a time when the NAO index clearly shows negative values. Between 800 and 1000 AD, a remarkable forest decline, coeval with a decrease in the frequencies of both Castanea and Olea, matches a shift in the oxygen isotope record towards positive values, indicating cooler temperatures, and a negative NAO. Between 1400-1850 AD, in the time period chronologically corresponding to the LIA (Little Ice Age), the Gaeta record shows a clear decline of the forest cover, particularly evident after 1550 AD, once again in correspondence with negative NAO index. … A previous study on this core (Margaritelli et al., 2016) provided a detailed reconstruction of the main climate oscillations over the last 4.5 ka, identifying nine time intervals associated with archaeological/cultural periods (top of Eneolithic ca. 2410 BC, Early Bronze Age ca. 2410 BC ca. 1900 BC, Middle Bronze Age Iron Age ca.1900-500 BC, Roman Period ca. 500 BC – 550 AD, Dark Age ca. 550-860 AD, Medieval Climate Anomaly ca. 860-1250 AD, Little Ice Age ca. 1250-1850 AD, Industrial Period ca. 1850-1950 AD, Modern Warm Period ca. 1950 AD – present day). The good correspondence between climate oscillations and archaeological intervals underlines the role exerted by climate change in determining rises and declines of civilizations. Within these time intervals, planktonic foraminifera and oxygen stable isotope data have allowed us to detect a series of past climate changes on decadal to millennial time scale, linked to dynamics of ocean-atmospheric coupling or to solar activity, such as the 4.2 ka event, four Roman solar minima, the Medieval Cold Period and the Maunder event.
Mallory et al., 2018 The AO [Arctic Oscillation] has positive and negative phases that infuence broad weather patterns across the northern hemisphere (Thompson et al. 2000). For example, during the positive phase of the AO, atmospheric pressure over the Arctic is lower than average, which tends to result in warmer and wetter winters in northern regions as warmer air is able to move further north (Thompson et al. 2000; Aanes et al. 2002). … From 1988 to 1996, the summer intensity of the AO was largely in the positive phase, with a mean value of 0.207 (± 0.135 SE), and this was a period of population stability or growth for each of the three herds that we examined here. In contrast, from 1997 to 2016 the summer AO has remained largely in the negative phase [cooling], with a mean value of − 0.154 (± 0.077 SE), and over this period the Bathurst, Beverly, and Qamanirjuaq herds declined in abundance. … Our results suggest that during periods of positive AO intensity, warmer temperatures on the summer range result in improved growing conditions for vascular plants that benefts foraging caribou. Conversely, negative summer AO intensity is associated with cooler temperatures with associated shorter growing seasons and increased precipitation on the Beverly summer range. … We found that positive intensities of the Arctic Oscillation (AO) in the summer were associated with warmer temperatures, improved growing conditions for vegetation, and better body condition of caribou. Over this same period, the body condition of female caribou was positively related to fecundity. We further identified that population trajectories of caribou herds followed the direction of the AO: herds increased under positive AO intensity, and decreased under negative AO intensity.
Perner et al., 2018 [W]e find evidence of distinct late Holocene millennial-scale phases of enhanced El Niño/La Niña development, which appear synchronous with northern hemispheric climatic variability. Phases of dominant El Niño-like states occur parallel to North Atlantic cold phases: the ‘2800 years BP cooling event’, the ‘Dark Ages’ and the ‘Little Ice Age’, whereas the ‘Roman Warm Period’ and the ‘Medieval Climate Anomaly’ parallel periods of a predominant La Niña-like state. Our findings provide further evidence of coherent interhemispheric climatic and oceanic conditions during the mid to late Holocene, suggesting ENSO as a potential mediator.
Mohammadi and Goudarzi , 2018 Sensitivity of solar radiation (H), wind speed (V) and precipitation (P) to ENSO events in California is studied. There are high relationships of El Niño and La Niña events with variations of H [solar radiation], P [wind speed] and V [precipitation] in California.
Valdés-Pineda et al., 2018 We conclude that a significant multi-decadal precipitation cycle between 40 and 60 years is evident at the rain gauges located in the subtropical and extratropical regions of Chile. This low-frequency variability seems to be largely linked to PDO and AMO modulation.
Ahn et al., 2018 These findings suggest that the variability at this site is remotely driven by processes such as those causing the Pacific Decadal Oscillation, rather than locally driven by processes such as increased or decreased vertical mixing of nutrients. … [I]t was shown that similar-sampling-frequency analyses of modern observations at this location reveal SST variability that is dominated by the PDO.
Stolpe et al., 2018 Multidecadal internal climate variability centered in the North Atlantic is evident in sea surface temperatures and is assumed to be related to variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). In this study, the extent to which variations in the AMOC may also alter hemispheric and global air temperature trends and ocean heat content during the past century is examined. … AMOC strength influences the air-sea heat flux into the high-latitude ocean, where a strengthening of the AMOC leads to decreased storage of heat in the Atlantic and a larger fraction of the heat taken up by the global ocean accumulates in the top 300 m compared to the case of a weakening AMOC. The spread in the amount of heat stored in the global ocean below 300 m is similar across the CESM members as in a set of CMIP5 models, confirming the AMOC as a “control knob” on deep-ocean heat storage. By influencing the ocean heat uptake efficiency and by shifting the pattern of heat uptake, global air temperatures are significantly altered on a multidecadal time scale by AMOC variability.
Bollasina and Messori, 2018 It is shown that the NAO generates a significant climate response over East Asia during both the dry and wet seasons, whose spatial pattern is highly dependent on the phase of the NAO’s life cycle. Temperature and precipitation anomalies develop concurrently with the NAO mature phase, and reach maximum amplitude 5–10 days later. These are shown to be systematically related to mid and high-latitude teleconnections across the Eurasian continent via eastward-propagating quasi-stationary Rossby waves instigated over the Atlantic and terminating in the northeastern Pacific. These findings underscore the importance of rapidly evolving dynamical processes in governing the NAO’s downstream impacts and teleconnections with East Asia.
Qin et al., 2018 Central China result from anomaly patterns in the large-scale atmospheric circulation in the mid-latitude Northern Hemisphere associated with the PDO [Pacific Decadal Oscillation]. Specifically, during the negative phase of the PDO (1945–1976 and 2003–2014) […] produces southerly advection of warm and moist air into North Central China, leading to increased precipitation there. These results reinforce the notion that PDO has a large impact on SON [September–November] rainfall over North Central China on decadal timescales.
Huang et al., 2018 A period of weak chemical weathering, related to cold and dry climatic conditions, occurred during the Little Ice Age (LIA), whereas more intense chemical weathering, reflecting warm and humid climatic conditions, was recorded during the Medieval Warm Period (MWP). Besides, an intensification of chemical weathering in Poyang Lake during the late Holocene agrees well with strong ENSO activity, suggesting that moisture variations in central China may be predominantly driven by ENSO variability. … Rao et al. (2016b) demonstrated that a humid late-Holocene in central China and an arid late-Holocene in southern and northern China were significantly related to strong ENSO activity. Thus, it seems that ENSO forcing may be likely dominant factor controlling moisture variations in central China.
Li et al., 2018 The Arctic sea ice cover has been rapidly declining in the last two decades, concurrent with a shift in the Atlantic Multi-decadal Oscillation (AMO) to its warm phase around 1996/97. … We suggest that the cold AMO phase is important to regulate the atmospheric response to AASIC [Atlantic sector of the Arctic sea ice cover] decline and our study provides insight to the ongoing debate on the connection between the Arctic sea ice and the AO.
Liu et al., 2018 In the late 1990s, the decadal variation of summer precipitation over eastern China was probably associated with the shift of the Pacific Decadal Oscillation (PDO) from positive to negative phase. The PDO is well known as the leading mode of decadal variability of Pacific SST (Mantua et al. 1997). Previous studies have revealed the impacts of the PDO on summer precipitation in China (Zhu and Yang 2003; Chan and Zhou 2005; Zhu et al. 2011; Qian and Zhou 2014; Yu et al. 2015). … The abrupt decrease in North China precipitation in the 1960s was proposed to be connected to the cooling of extra-tropical North Atlantic Ocean based on an AGCM simulation (Liu and Chiang 2012). Si and Ding (2016) revealed that AMO might lead to the decadal variability of East China summer precipitation by causing negative (positive) precipitation anomalies over the Yangtze River valley (HuangheHuaihe River valley) through a stationary circumglobal baroclinic wave train. … AMO and PDO have been regarded as the two principal drivers for the interdecadal variability of summer precipitation over East China.
Murphy et al., 2018 The [continuous 305-year (1711–2016) monthly rainfall series, Ireland] series has remarkably wet winters during the 1730s, concurrent with a period of strong westerly airflow, glacial advance throughout Scandinavia and near unprecedented warmth in the Central England Temperature record – all consistent with a strongly positive phase of the North Atlantic Oscillation.
Ramos Buarque and Salas y Melia, 2018 The relationship between the Surface Mass Balance (SMB) of the Greenland Ice Sheet (GrIS) and the North Atlantic Oscillation (NAO) is examined … Accumulation in South Greenland is significantly correlated with the positive (negative) phase of the NAO in a warm (cold) climate.
Valdés-Pineda We conclude that a significant multi-decadal precipitation cycle between 40 and 60 years is evident at the rain gauges located in the subtropical and extratropical regions of Chile. This low-frequency variability seems to be largely linked to PDO and AMO modulation.
Zhu et al., 2018 Overall, the variation of drought in the Daxing’an Mountains and its relationship with surrounding areas may be affected by the Pacific or Atlantic oscillations (e.g., ENSO, PDO, AMO, NAO and SNAO), which can affect the Asian monsoon, change the local temperature and precipitation, and lead to drought.
Modern Climate In Phase With Natural Variability
Ault et al., 2018 The western United States was affected by several megadroughts during the last 1200 years, most prominently during the Medieval Climate Anomaly (MCA; 800 to 1300 CE). A null hypothesis is developed to test the possibility that, given a sufficiently long period of time, these events are inevitable and occur purely as a consequence of internal climate variability. The null distribution of this hypothesis is populated by a linear inverse model (LIM) constructed from global sea surface temperature anomalies and self-calibrated Palmer drought severity index data for North America. Despite being trained only on seasonal data from the late twentieth century, the LIM produces megadroughts that are comparable in their duration, spatial scale, and magnitude to the most severe events of the last 12 centuries. The null hypothesis therefore cannot be rejected with much confidence when considering these features of megadrought, meaning that similar events are possible today, even without any changes to boundary conditions. In contrast, the observed clustering of megadroughts in the MCA, as well as the change in mean hydroclimate between the MCA and the 1500–2000 period, are more likely to have been caused by either external forcing or by internal climate variability not well sampled during the latter half of the twentieth century.
Brickman et al., 2018 In 2012, 2014, and 2015 anomalous warm events were observed in the subsurface waters in the Scotian Shelf region of eastern Canada. Monthly output from a high resolution numerical ocean model simulation of the North Atlantic ocean for the period 1990-2015 is used to investigate this phenomenon. … The observed warming trend can be attributed to an increase in the frequency of creation of warm anomalies during the last decade. Strong anomalous events are commonly observed in the data and model, and thus should be considered as part of the natural variability of the coupled atmosphere-ocean system.
Kendon et al., 2018 Natural variability appears to dominate current observed trends (including an increase in the intensity of heavy summer rainfall over the last 30 years) … [T]he attribution of rainfall trends to human influence on local and regional scales is not yet possible (Sarojini et al., 2016).
Dobrovolný et al., 2018 The new MJJ precipitation reconstruction is restricted to inter-annual and inter-decadal variability, which is in line with our understanding of natural precipitation variability. Reconstruction reveals two long periods of low precipitation variability, in the 13th–14th centuries and 1630s–1850s. It also demonstrates that precipitation anomalies of larger amplitude and longer duration occurred in the earlier part of the last millennium than those found in the instrumental period. Negative trends in soil moisture content and gradual changes in annual precipitation distribution leading to higher extremity of precipitation regime may be responsible for the lower sensitivity of oaks to precipitation after the 1980s. The new reconstruction does not indicate any exceptional recent decline in MJJ precipitation.
Chen et al., 2018 Good agreements between drought records from western and eastern Central Asia suggest that the PDSI records retain common drought signals and captures the regional dry/wet periods of Central Asia. Moreover, the wavelet analysis indicates the existence of centennial (100–150 years), decadal (50–60, 24.4 and 11.4 years) and interannual (8.0 and 2.0-3.5 years) cycles, which may be linked with climate forcings, such as solar activity and ENSO.
Chen and Tung, 2018 During an accelerating phase from the mid-1990s to the early 2000s, the AMOC stored about half of excess heat globally, contributing to the global-warming slowdown. By contrast, since mooring observations began in 2004, the AMOC and oceanic heat uptake have weakened. Our results, based on several independent indices, show that AMOC changes since the 1940s are best explained by multidecadal variability, rather than an anthropogenically forced trend. Leading indicators in the subpolar North Atlantic today suggest that the current AMOC decline is ending. We expect a prolonged AMOC minimum, probably lasting about two decades. If prior patterns hold, the resulting low levels of oceanic heat uptake will manifest as a period of rapid global surface warming.
Ruprich-Robert et al., 2018 Heat waves are primarily driven by internal atmospheric variability (Schubert et al. 2011, Dole et al. 2011), but their frequency of occurrence and severity can be modulated by atmospheric boundary forcing. Soil moisture deficits have been shown to play an important role in intensifying heat wave severity (Huang and Van den Dool 1993, Fischer et al. 2007, Jia et al. 2016, Donat et al. 2016). … Radiative forcing variations, such as those driven by anthropogenic emissions, can also modulate the occurrence of heat waves (e.g., Hansen et al. 2012). Previous studies, based on Coupled Global Climate Models (CGCMs) integrated under different anthropogenic forcing scenarios, concluded that over the US, the number of heat waves would increase during the 21st century (Meehl and Tebaldi 2004, Diffenbough et al. 2005, Lau and Nath 2012). However, this increasing trend may be modulated by the impacts on land of low frequency sea surface temperature (SST) variability (e.g., Schubert et al. 2016, Seager and Ting 2017), such as that associated with the internally-driven component of the Pacific Decadal Oscillation (PDO; Newman et al. 2016) or the Atlantic Multidecadal Variability (AMV; Schlesinger and Ramankutty 1994, Knight et al. 2005). These low frequency SST variations may explain why there has not been any long-term trend of heat waves detected over the US during the 20th century, despite the increase of radiative forcing (Kunkel et al. 1999, Easterling et al. 2000).
Depietri and McPhearson, 2018 The trends based on the NOAA meteorological data show that changes in the length of the heat wave events equal or beyond 3 days of duration are not significant. The mean maximum temperature of the heat wave is also close to stable over the 140-year period of study with no significant increase. … Results obtained from the in-depth analysis of the NYT articles, corresponding to the dates of longer lasting heat wave events (i.e., equal or more than 6 days in duration), show that the number of deaths and people affected in New York City significantly declined. … The change in coping strategies mentioned in the newspapers articles and divided before and after the 1960s illustrates how the advent of air conditioning can be most likely contributed to the significant reduction in mortality due to extreme heat. … Also not significant are the trends in extreme precipitation (beyond 1.75 in. and beyond 3.5 in.) with significant inter-annual and interdecadal variability.
Cloud/Aerosol Climate Influence
Zhang et al., 2018 [W]e conducted a statistical analysis to examine overall relationships between surface winds, SST [sea surface temperature], and sea ice in the CBS [Chukchi and Beaufort Seas, Arctic Ocean], using the newly developed CBHAR data set. The result shows a significant negative correlation between the surface winds and SIC [sea ice concentration], further confirming that increased wind speeds are closely associated with the reduction in SIC [sea ice concentration] (Stegall and Zhang 2012) […] during September and October from 1979−2009. … A scatter plot of mean SIC [sea ice concentration] and wind speed anomalies, as well as the variation in wind speed anomalies […] demonstrat[e] a clear inverse linear relationship between surface wind speed and SIC [sea ice concentration] anomalies, with a correlation coefficient of −0.94 at a 99% level of significance using the t-test (Snedecor and Cochran 1989). This statistically suggests that surface wind speeds generally increase as SIC [sea ice concentration] decreases. … Taken together, the negative correlation between winds and SST [sea surface temperatures] over the OW and LIC areas can be attributed to reduced shortwave radiation due to increased cloudiness, increased upward sensible and latent heat fluxes, and strong cold advection from sea ice towards the north when strong winds are present, or vice versa when weak winds occur. [Neither CO2 concentration or anthropogenic forcing is mentioned anywhere in the paper as radiative factors affecting sea surface temperatures or sea ice concentrations during 1979-2009.]
Tomicic et al., 2018 Secondary aerosol particles, which are formed by nucleation processes in the atmosphere, play an important role in atmospheric chemistry and in the Earth’s climate system. They affect the Earth’s radiation balance by scattering solar radiation back to space and can also act as cloud condensation nuclei (CCN) and thereby affect the amount of cloud and its radiative properties. Clouds have a net cooling effect on the Earth’s radiation budget of about −27.7 W m−2 (Hartmann, 1993). Thus, a small change in cloud properties can have significant effect on the climate system. Results by Merikanto et al. (2009) and Yu and Luo (2009) have shown that a significant fraction (ranging between 31 and 70 %) of cloud-forming aerosol particles in the atmosphere are secondary particles that originate from nucleation. Therefore, understanding nucleation is crucial in order to fully understand the atmospheric and climatic effects of aerosols.
Perovich, 2018 The surface radiation budget of the Arctic Ocean plays a central role in summer ice melt and is governed by clouds and surface albedo. … Longwave and shortwave radiation are primary drivers in the surface heat budget during summer melt (Persson et al., 2002). The surface radiative balance consists of contributions from incoming shortwave radiation, reflected shortwave radiation, incoming longwave radiation, and outgoing longwave radiation. Clouds have a major impact on both incoming longwave and shortwave radiative fluxes. … Future impacts on net radiative balances will depend on both ice and cloud conditions. As the sea ice cover evolves towards more first year ice, greater melt pond coverage, and more open water, the area-averaged albedo will be less than the break-even albedo for much of the summer. This implies less melting under cloudy conditions than sunny. However, the net radiative balance will still likely be less under sunny skies at the beginning of the melt season in May and early June.
Wu et al., 2018 Summer temperatures (MJT) at Xingyun Lake in the late glacial were low, increased during the early Holocene, were highest during the middle Holocene, and then decreased during the late Holocene. The range of inferred values [for the Holocene] was 21.0°- 26.5°C. The pollen inferred temperature derived from surface samples (21.2°C), is close to the modern instrumental July temperature in Kunming (22°C), supporting the reliability of reconstructions from down-core pollen assemblages. [Modern temperatures are 1.0°C above the coldest of the last 14,000 years, and 4.5°C cooler than the warmest temperatures of the last 14,000 years.] … The radiative effect of clouds has attracted increasing attention; for example, it was found that decreasing cloud cover drives the recent loss of mass from the Greenland ice sheet by enhancing the melt-albedo feedback (Hofer et al., 2017). Thus, enhanced albedo effect from increasing cloud cover in southwest China during the early Holocene could have caused a reduction in summer temperature. … From 1960 to 2005, total cloud cover decreased over southwest China, including Yunnan Province (Zhang et al., 2011b) … [as] summer temperature increased [1961-2007] (Liu et al., 2010). This negative relationship between cloud cover and summer temperature was also found in India during the period 1931-2002 (Roy and Balling, 2005).
Volcanic/Tectonic Climate Influence
Viterito, 2018 The resulting correlation between the HGFA [high geothermal flux areas] frequencies and the lagged global temperatures is 0.777, a statistically significant outcome that explains 60.3% of the variance in global temperatures. By contrast, an unlagged pairing of CO2 concentrations (ppm) with global temperatures yields a (lower) correlation of 0.735 (Figure 4) . More importantly, multiple regression analysis reveals that mid-ocean seismicity is a significant predictor of global temperatures (p0.05) but CO2 is not(p>0.05) (Table 1). … Using HGFA seismic frequencies as the sole predictor of global temperatures going forward, there is a 95% probability that global temperatures in 2019 will decline by 0.47° C ± 0.21° C from their 2016 peak. In other words, there is a 95% probability that 2019 temperatures will drop to levels not seen since the mid-1990s.
Birkel et al., 2018 We find that cool intervals across the North Atlantic coincide with two distinct episodes of explosive volcanic activity (1880s–1920s and 1960s–1990s), where key eruptions include 1883 Krakatau, 1902 Santa María, 1912 Novarupta, 1963 Agung, 1982 El Chichón, and 1991 Pinatubo. Cool SST patterns develop in association with an increased prevalence of North Atlantic Oscillation (NAO)+ atmospheric patterns caused by stratospheric aerosol loading and a steepened poleward temperature gradient. NAO+ patterns promote wind-driven advection, evaporative cooling, and increased albedo from enhanced Saharan dust transport and anthropogenic aerosols. SSTs across the subpolar gyre are regulated by strength of low pressure near Iceland and the associated wind-driven advection of cold surface water from the Labrador Sea. This is contrary to an interpretation that subpolar SSTs are driven by changes in ocean overturning circulation. We also find that North Pacific and global mean SST declines can be readily associated with the same volcanic triggers that affect the North Atlantic. Thus, external forcing from volcanic aerosols appears to underpin multi-decade SST variability observed in the historical record.