A Warmer Past: Non-Hockey Stick Reconstructions
Collins et al., 2019 Over the past 2300 yrs, SST values range between 14.3°C and 12.2°C (Fig. 4a), and hence most of the record is warmer than today. The earlier half of the record is relatively warm and stable and displays a gradual warming from 13.2°C at 2300 cal yrs BP to 14°C at 1200 cal yrs BP. The largest feature of the record is the cooling transition from 14°C to 12.5°C between 1100 and 600 cal yrs BP. This is followed by warming to 13.5°C at 300 cal yrs BP and then cooling to 12.5°C at present day. Multi-centennial variability is more clearly highlighted in the filtered record and is most pronounced over the last 1200 years. The record exhibits relatively warm conditions during the periods 1200 – 950 cal yrs BP and 500 – 200 cal yrs BP and relatively cool conditions during the periods 950 – 500 cal yrs BP and 200 – present. … Southern Ocean cooling is expected to have further enhanced sea ice cover in the Southern Ocean (Park and Latif, 2008; Zhang et al., 2017a). This is in accordance with two records displaying increased sea ice in the western Ross Sea at a similar timing (between 1250 and 650 cal yrs BP) to the cooling (Mezgec et al., 2017). Late Holocene sea-ice increases are also observed to the west of the Ross Sea (Denis et al., 2010), to the west of the West Antarctic Peninsula (Etourneau et al., 2013) and in the Eastern Ross Sea (Mayewski et al., 2013). Associated ice-albedo and ice-insulation feedbacks (Renssen et al., 2005; Varma et al., 2012) may have contributed to the rapidity of the cooling and sea-ice expansion. … Solar variability would be a potential driver of the changes in ENSO and SAM coupling. Increased (decreased) TSI has been shown to promote La-Niñalike (El-Niño-like) conditions by enhancement of the trade winds (Mann et al., 2005). Similarly, the SHW are sensitive to the 11yr solar cycle (Haigh et al., 2005) and solar variability on centennial timescales (Varma et al., 2011) and thus solar variability might be expected to exert an influence on the SAM. Therefore, it is plausible that solar variability may have controlled the phasing of ENSO and the SAM, and this remains an interesting avenue for further climate modeling research.
Lee et al., 2019
Salvatteci et al., 2019 [O]tolith δ18O data from Peruvian catfish (Galeichthys peruvianus) excavated from archeological sites in northern Peru suggest SST ~4 °C warmer than present‐day SST (Andrus et al., 2002).
Liu et al., 2019 The modern vegetation around Tianchi Crater Lake is categorized as temperate mixed conifer-broad-leaved forest … The area experiences a typical cold temperate monsoon climatic regime with long cold winters and short cool summers. The [modern] annual average air temperature is 0–0.5 °C … The PFT-MAT-based reconstructed Pann record for Tianchi Crater Lake has a similar trend of variation to that of the pollen percentage diagram. The ranges of Pann [annual precipitation] and Tann [annual mean temperature] are 268–881 mm and 1.7–10.5°C, respectively [for the Holocene]. …[V]ariations in Tann exhibit […] a marked rise from the beginning of the early Holocene and a peak at 9000–8000 cal yr BP.
Klinge and Sauer, 2019 For the Tsambagarav Mountain (central Mongolian Altai), Herren et al. (2013) reconstructed the climatic development over the last 6 ka, based on an ice core. Because the maximal age obtained for the base of the glacier ice was approximately 6 ka, they concluded that warm conditions led to disappearance of most of the glaciers in the Altai Mountains during the early to mid-Holocene. This assumption was supported by Ganyushkin et al. (2018), who found fossil wood above the modern tree line in the Mungun-Taiga Mountain in northwest Mongolia, dating between 10.6 ka and 6.2 ka. They concluded that the tree line was 350 m higher than today at that time, indicating that summer temperatures were 2.0-2.5°C warmer than at present, and MAP was about twice as much as today, which led to a decrease of the glaciated area.
Axford et al., 2019 Deltasø chironomids indicate peak early Holocene summer temperatures at least 2.5-3°C warmer than modern and at least 3.5-4°C warmer than the pre-industrial last millennium. We infer based upon lake sediment organic and biogenic content that in response to declining temperatures, North Ice Cap reached its present-day size ~1850 AD, having been smaller than present through most of the preceding Holocene.
Jimenez-Moreno et al., 2019 The pollen record is also consistent with higher than modern summer temperatures. Previous studies from the area show that upper treeline would have been between 80 and 300 m higher than today. … Here, we find that low elevation thermophilous species, such as Quercus, Juniperus and Cercocarpus, reached maximum abundances between 8000 and 6700 cal yr BP in subzone EL-2a. This pattern probably points to the occurrence of these species at their highest Holocene elevations at this time and thus adds to evidence for the highest regional temperatures in the middle Holocene, possibly >2°C warmer than today (Andrews et al., 1975; Carrara et al., 1984; Elias, 1996). … A continuous cooling trend over the last few millennia is also recorded at Emerald Lake by a progressive decline in the pollen percentages of Quercus and other thermophilous species such as Juniperus and Cercocarpus. These species probably retreated to lower elevations under colder conditions. Decreasing summer insolation in the late Holocene (Neoglacial) led to cooler summers than earlier in the Holocene (Laskar et al., 2004; Alder and Hostetler, 2015), and most likely caused these vegetation trends.
Reißig et al., 2019 [T]he Tobago Basin core 235 subSSTMg/Ca record is highly variable and ranges from ~13-23°C, which is approximately three times as much as at Beata Ridge.. In Tobago Basin, the subSSTMg/Ca decrease by ~2°C from 30 ka BP (18°C) to the onset of HS1 (16°C). Within HS1, the subSSTMg/Ca increase continuously by 2°C, while at ~15.5 ka it rises abruptly by ~6°C up to maximum temperatures of 23°C. The abrupt subSST rise is delayed too the reconstructed SST rise at the beginning of HS1 by Bahr et al. (2018) (Fig. S7). Subsequently, subSSTMg/Ca scatters around 20°C until the beginning of the Bølling-Allerød (B/A). During the B/A and the YD the subSSTMg/Ca remains higher than ~19°C, abruptly increases up to ~22°C at mid YD, while steadily decreasing afterwards reaching modern values of ~15.5°C in the mid Holocene. Lowest subSSTMg/Ca of ~13°C are observed after ~7 ka BP. On average, the LGM subSSTMg/Ca are warmer by ~2.5°C than during the Holocene. … [T]he subsurface temperature variability is a robust climate signal in the tropical W Atlantic. Both records show an increase of ~5°C in subSSTMg/Ca from the LGM to the early YD and a subSSTMg/Ca decrease by ~7-8°C during the Holocene suggesting that both sediment cores are influenced by the same oceanographic changes. Notably, the mid Holocene subSSTMg/Ca in Tobago and Bonaire Basins remain cooler by ~1.5°C and ~3°C, respectively, than during the LGM. … At Tobago Basin and Bonaire Basin, the deglaciation is characterized by abrupt rises in subSSTMg/Ca by ~5.5°C at the end of HS1 and by ~6°C at the middle of the YD to peak values of up to ~23°C and ~22°C, respectively, accompanied by changes towards saline conditions (mean δ18Osw-ivf of ~2.25‰ and ~2‰, respectively (Fig. 3). These highly variable changes occur within less than 400 years. … In contrast to modern conditions Tobago Basin core 235 was influenced by a warm water mass between 30-10 ka BP, indicated by elevated subSSTMg/Ca (~2.5°C warmer than the modern conditions)
Yasuhara et al., 2019 Our reconstructions reveal a series of multi-centennial-scale abrupt warming events likely caused by upper NADW reduction coinciding with deglacial and Holocene stadial events. Notably, we discovered pervasive Holocene upper NADW variability in the western North Atlantic for at least the past 4000 yr and perhaps throughout the Holocene. Bottom-water temperature at ODP Site 1055 (1798 m) averaged ~5 °C during the last glacial period, which is slightly (by 1 °C) warmer than present-day and late Holocene values of ~4 °C. During the last deglaciation, ODP 1055 BWT shows several warming events supported by multiple data points (apart from noisy, singlepoint excursions). During the Holocene, ODP 1055 BWT shows ±1–1.5 °C multi-centennial-scale variability. Evidence for warmer-than-present lastglacial BWT at 1800 m water depth is consistent with low-resolution depth transect reconstructions (Dwyer et al., 2000).
Tonno et al., 2019 In North Europe, changes in early Holocene climate were rather intense, starting with low temperatures at the beginning of the period, followed by gradual warming, interrupted periodically by short cooling periods (Antonsson and Seppa¨ 2007). During the Holocene Thermal Maximum (HTM), the period from 8.0 to 4.0 cal ka BP, average temperatures in Northern Europe were approximately 2.5–3.5°C higher than today (Antonsson and Seppa¨ 2007; Heikkila¨ and Seppa¨ 2010; Ilvonen et al. 2016).
Pitulko et al., 2019 Our data show that from c. 10 600 BP, Zhokhov Island was situated on the margin of the shrinking Arctic coast (Anisimov et al. 2009); this is supported by the presence of a large quantity of driftwood that washed ashore at the Zhokhov site. The environmental situation was relatively favourable for human occupation: the climate was [5-6°C] warmer than today, and Zhokhov Island was covered by an Arctic tundra comprising sedge grass, shrubs and dwarf birch (Makeyev et al. 2003).
Feakins et al., 2019 At lake level [Lake Elsinore, southern California] mean annual temperature (MAT) averages 18°C, with summer average temperatures of 25°C, resulting in high potential evaporation rates and lake water loss of >1.4 m yr−1(Kirby et al., 2013). … [T]he reconstructed temperatures are reasonable with early Holocene values close to modern (18°C). … Temperatures >20°C are reported for six late glacial samples, including a high of 22°C at 29.4 ka and a maximum of 23°C at 26.8 ka which coincides with the pollen-inferred warm and dry period from 27.5–25.5 ka (Heusser et al., 2015) when the lake was shallow (∼3.2–4.5 m deep; Kirby et al., 2018). … The lowest temperature, 10°C, occurs at 23.5 ka and the deglacial warming from 14–12 ka is >10°C, agreeing with the pollen interpretations of ∼11°C warming at the glacial termination (Heusser et al., 2015).
Marret et al., 2019 The studied region is the only coastal region in Russia to have subtropical landscapes as well as humid to semi-arid landscapes (Petrooshina, 2003). Winter temperatures average 3–5°C in winter [today] up to 23–24°C in summer. … A possible maximum of warm conditions may have occurred between 3.0 and 2.5 cal. ka BP, as highlighted by the occurrence of O. israelianum. This species has not been seen in modern sediments from the Black Sea nor the Caspian Sea and mainly occurs in waters where winter SSTs are above 14.3°C and summer SSTs are more than 24.2° C … Establishment of present-day conditions may have happened within the last 1500 years, but the low-resolution sampling at the top of the core prevents us to exactly pinpoint this change. However, our dinocyst assemblage indicates cooler conditions [today] with the decrease of S. mirabilis.
Røthe et al., 2019 Our findings do not provide any confirmation that the glacier Sørfonna melted entirely away during the Holocene Thermal Maximum (HTM). Bjune et al. (2005) suggest summer temperatures were 1.5–2 °C warmer than present during this period. Combined with the evidence from the glaciers Nordfonna and Hardangerjøkulen, it is, however, likely that the glacier Sørfonnawas also significantly smaller than at present during this period. … Since 1650 cal. a BP, we infer that the glacier was larger than the 2002 CE glacier extent until 1910 CE when a GLOF [glacier outburst flood] occurred. Svartenutbreen has been retreating since 1910 CE, which led to the ice damming of the two historical GLOFs [glacier outburst floods] in the 1980s and 2002 CE separated by a glacier advance in the 1990s CE.
Colin et al., 2019 The Holocene subpolar North Atlantic climate is characterized by an early to mid-Holocene “thermal maximum” followed byprogressive cooling induced by decreased insolation forcing (related to orbital precession) (e.g. Marchal et al., 2002; Sarnthein et al., 2003). This climatic cooling reflects a major reorganization of atmospheric and ocean circulation in the North Atlantic (e.g. O’Brien et al., 1995; Came and Oppo 2007; Repschlager et al., 2017). … The time interval from 1 to 0.68 ka BP, which is marked by a strong eastward extension of the SPG, has been associated with the warm Medieval Climatic Anomaly and a subsequent intensification of the surface limb of the AMOC (Copard et al., 2012; Wanamaker et al., 2012; Ortega et al., 2015) (Fig. 4). The westward contraction of the weak SPG observed thereafter (between 0.68 and 0.2 ka BP) is coeval with the cold period of the Little Ice Age and may be linked to reduced AMOC intensity.
Steinman et al., 2019 The early Holocene d18O [hydroclimate] maximum in the Castor Lake record at 9630 (9110-10,100) yr BP is likely in part a result of higher summer insolation, which produced higher temperatures and greater evaporation during the warm season. Additionally, atmospheric circulation in the early Holocene was substantially different from the modern configuration (Bartlein et al., 2014), and precipitation amounts were likely lower, due to the presence of the residual Laurentide and Cordilleran Ice Sheets (Dyke, 2004), which affected air mass trajectories and the seasonal distribution and amount of precipitation on a hemispheric scale. … A chironomid based climate reconstruction from Windy Lake, south-central British Columbia, supports the assertion that greater summer insolation produced warmer summer temperatures at this time (Chase et al., 2008).
Lozhkin et al., 2019 Mixed Larix-Betula forest was established at the Tanon site by ∼6600 14C BP (∼7500 cal BP). This forest included Betula platyphylla, a species common in moderate zones of the Russian Far East (e.g., B. platyphylla-Larix forests of central Kamchatka). The importance of Betula in the Middle Holocene assemblage is unusual, as tree Betula is not a common element in the modern coastal forest. The abundance of B. platyphylla macrofossils particularly suggests warmer than present summers and an extended growing period. This inference is supported by a regional climate model that indicates a narrow coastal region where the growing season was longer and summer temperatures were 2-4 °C warmer than today. Variations in Betula pollen percentages at other sites in northern Priokhot’ye are suggestive that this Middle Holocene forest was widespread along the coast.
Novenko et al., 2019 [D]uring the Holocene Thermal Maximum when the mean annual temperatures were 2°С higher than those of the present day. Roughly 5.7–5.5 ka BP, the Holocene Thermal Maximum was followed by gradual climatic cooling that included several warming and cooling phases with temperature fluctuations ranging between 2 and 3°С. …The CFSNBR [Central Forest State Natural Biosphere Reserve] is situated roughly 360 km northwest of Moscow (the Tver region, 56º35’ N, 32º55’ E) in an ecological zone transitioning from taiga to broadleaf forests. The vegetation of the CFSNBR is primary southern taiga forests, and it has been undisturbed by any human activities for at least 86 years. The climate of the study area is temperate and moderately continental with a mean annual temperature of 4.1°C and annual precipitation of roughly 700 mm.
Rey et al., 2019 Our results imply that mixed Fagus sylvatica forests with Abies alba and Quercus may re‐expand rapidly in these areas, if climate conditions will remain within the range of the mid‐Holocene climatic variability (with summers c. +1–2°C warmer than today). … [T]he rise and fall of early farming societies was likely dependent on climate. Favourable climatic conditions (i.e. warm and dry summers) probably led to an increase in agricultural yields, the expansion of farming activities and resulting forest openings, whereas unfavourable climatic conditions (i.e. cold and wet summers) likely caused crop failures, abandonment of agricultural areas and forest succession. A better understanding of the environmental and societal factors controlling coeveal land-use dynamics as shown in this study would require new climate proxy data (e.g. temperature reconstruction from well dated and complete Holocene tree ring series). On the basis of our results and considering the ongoing spread of temperate forests in lowland Central Europe, we conclude that the existing beech forest ecosystems are resilient to anthropogenic disturbances under a changing climate.
Eda et al., 2019 Recent taxonomic composition and faunal distribution patterns support recognition of three biogeographic regions in Asia, Palaearctic (north), Indomalayan (south), and a transition zone between the two (Hoffmann 2001). In the division, the Yangtze River delta is located at the boundary of the Indomalayan region and transition zone. Pollen records suggest that, middle Holocene temperatures were ca. 2–4 °C warmer than today in the middle Yangtze River delta (Yi et al. 2003). Peters et al. (2016) indicated that the middle Yangtze River basin would delimit the northern most boundary for required habitat of (sub-) tropical red junglefowl during the Holocene thermal optimum. Furthermore, Xiang et al. (2014) reported that the wild distribution area of red junglefowl extended to northern China in the early Holocene, and domestic chicken farming began in the region.
Yuan et al., 2019 During the early Holocene (10.0–6.0 ka), the modern-type circulation system was not established, which resulted in strong water column stratification; and the higher sea surface temperature (SST) might be associated with the Holocene Thermal Maximum (HTM). The interval of 6.0 to 1.0/2.0 ka displayed a weaker stratification caused by the intrusion of the Yellow Sea Warm Current (YSWC) and the initiation of the circulation system. A decreasing SST trend was related to the formation of the cold eddy generated by the circulation system in the ECS. During 1.0/2.0 to 0 ka, temperatures were characterized by much weaker stratification and an abrupt decrease of SST caused by the enhanced circulation system and stronger cold eddy, respectively.
Lasher and Axford, 2019 More positive δ18O values are found between 900 and 1400 CE, indicating a period of warmth in South Greenland superimposed on late Holocene insolation-forced Neoglacial cooling, and thus not supporting a positive NAO anomaly during the MCA. Highly variable δ18O values record an unstable climate at the end of the MCA, preceding Norse abandonment of Greenland. The spatial pattern of paleoclimate in this region supports proposals that North Atlantic subpolar ocean currents modulated South Greenland’s climate over the past 3000 yr, particularly during the MCA. Terrestrial climate in the Labrador Sea and Baffin Bay regions may be spatially heterogeneous on centennial time scales due in part to the influence of the subpolar gyre.
Gebbie and Huybers, 2019 The ongoing deep Pacific is cooling, which revises Earth’s overall heat budget since 1750 downward by 35%. … In the deep Pacific, we find basin-wide cooling ranging from 0.02° to 0.08°C at depths between 1600 and 2800 m that is also statistically significant. The basic pattern of Atlantic warming and deep-Pacific cooling diagnosed from the observations is consistent with our model results, although the observations indicate stronger cooling trends in the Pacific. …. At depths below 2000 m, the Atlantic warms at an average rate of 0.1°C over the past century, whereas the deep Pacific cools by 0.02°C over the past century. … Finally, we note that OPT-0015 indicates that ocean heat content was larger during the Medieval Warm Period than at present, not because surface temperature was greater, but because the deep ocean had a longer time to adjust to surface anomalies. Over multicentennial time scales, changes in upper and deep ocean heat content have similar ranges, underscoring how the deep ocean ultimately plays a leading role in the planetary heat budget.
Caballero et al., 2019 Diatom-based transfer functions for salinity, precipitation and temperature were developed using a training set that included data from 40 sites along central Mexico. … Maximum last glacial cooling of ∼5°C is reconstructed, a relatively wet deglacial and a warmer (+3.5°C) early Holocene. … The early Holocene marked a change towards high lake salinities and the highest positive temperature anomalies (+3.5°C) during a peak in summer insolation.
Lasher and Axford, 2019 More positive δ18O values are found between 900 and 1400 CE, indicating a period of warmth in South Greenland superimposed on late Holocene insolation-forced Neoglacial cooling, and thus not supporting a positive NAO anomaly during the MCA. Highly variable δ18O values record an unstable climate at the end of the MCA, preceding Norse abandonment of Greenland. The spatial pattern of paleoclimate in this region supports proposals that North Atlantic subpolar ocean currents modulated South Greenland’s climate over the past 3000 yr, particularly during the MCA. Terrestrial climate in the Labrador Sea and Baffin Bay regions may be spatially heterogeneous on centennial time scales due in part to the influence of the subpolar gyre.
Svare, 2019 Seppä et al. (2008) set the summer temperature maximum in the northern European tree-line region to ca. 7500-6500 cal. yrs BP (ca. 1.5°C higher than present), similar to the Dovre area (Paus et al., 2011). Further, Bjune et al., (2005) found the HTM to last from ca. 8000 to 4000 cal. yrs BP in Western Norway, with temperatures reaching 12-13°C. … The early establishment of pine-forests in the area surrounding both study sites from ca. 9600 cal. yrs BP give evidence of local mean July temperatures of at least 11°C ca. 9600-8200 cal. yrs BP, 0.8-1.1°C warmer than present. From ca. 8200 cal. yrs BP until present day the July mean temperature has presumably been around 8-10°C.
Warming Since Mid/Late 20th Century?
Kutta and Hubbart, 2019 Between 1900 and 2016, climatic trends were characterized by significant reductions in the maximum temperatures (−0.78°C/century; p = 0.001), significant increases in minimum temperatures (0.44 °C/century; p = 0.017) [overall -0.34°C per century], and increased annual precipitation (25.4 mm/century) indicative of a wetter and more temperate WV climate. Despite increasing trends of growing degree days during the first (p ≤ 0.015) and second half of the period of record, the long-term trend indicated a decrease in GDD [warm growing degree days] of approximately 100 °C/days.
Byambaa et al., 2019 [T]he recent cool-moist period from 1985 to 2000 has been related to the Arctic Oscillation (this study, Robock, 1984, He et al., 2017). The recent cooling could have been caused by volcanic aerosols of the El Chichón eruption (VEI5, 1982) in Southern Mexico, which impacted atmospheric wind patterns, including a positive phase of the Arctic Oscillation (Robock, 1984). No large volcanic-induced cooling was observed at this time due to the simultaneous warming ocean temperature caused by El-Niño (Robock, 2002). Also, the positive AO competing with ElNiño could reinforce the anomalous westerlies in the midlatitudes (He et al., 2017). During this recent cool-moist period, ice mass accumulation of the glaciers in the Russian Altai Mountains was observed and Narozhniy and Zemtsov (2011) connected this phenomenon to annual precipitation increased by 8% – 10% especially in winter and spring (April-May) as a result of a strengthening of the zonal circulation over the Altai Mountains.
Eck et al., 2019 A majority (12/14) of the regions within the SAM [southern Appalachian Mountains] have experienced a long‐term decline in mean winter temperatures since 1910.
Gan et al., 2019 Daily Minimum temperature (Tmin) is an important variable in both global and regional climate changes, and its variability can greatly affect the ecological system. In the early 21st century, warming slowdown is seen over the North Hemisphere and North America is one of the major cooling centers. … In this study, we found that Tmin experienced an obvious decline in North America during warming slowdown period. Such Tmin decline is closely related to the Atlantic Multidecadal Oscillation (AMO), the correlation between the decadal components of Tmin and AMO reached 0.71 during 1950-2014.
Araźny et al., 2019 Air temperature in 1899–1914 during three expeditions was 1.8–4.6 °C lower than the modern period in winter (Oct–Apr). However, during the 1930/31 expedition it was 4.6 °C warmer than the years 1981–2010. Our results relate to what has been called the ‘1930s warming’, referred to by various authors in the literature as the ETCW or the ETCAW. … In individual months, the highest negative anomalies were identified in Calm Bay (hereafter CB) in January 1914 (− 7.4 °C) and in February 1900 (− 6.8 °C). In contrast, during the 1930/31 expedition, it was 4.6 °C warmer than the present day in CB [Calm Bay]. Such a high thermal anomaly was influenced by a warm autumn and winter, especially February 1931, when the average monthly temperature was 10.7 °C higher than in the modern period.
Zhang et al., 2019 In core 31003, the SST record shows a distinctly anti-phase relationship with that of core 38002 over the last millennium. For instance, from the MWP to LIA, SST values increased from ∼17.0 ± 0.3°C to ∼19.1 ± 0.6°C in the northern core 38002 but decreased from ∼24.3 ± 0.4°C to ∼23.5 ± 0.3°C in the southern coastal core 31003. Since ∼1850 AD, the SST record in core 31003 elevated within the range of 24.3 ± 0.4°C, similar to values during the MWP, but decreased gradually to ∼18.0°C in core 38002, in line with the SST trends at two additional locations from the YSWC [Yellow Sea Warm Current] pathway as reported by He et al. (2014).
Huang et al., 2019 The temperature effect of the Zhada δ18OTR series is further verified by consistency with nearby ice-core δ18O variability.
Huang et al., 2019 Climatic change is exhibiting significant effects on the ecosystem of the Tibetan Plateau (TP), a climate-sensitive area. In particular, winter frost, freezing events and snow avalanche frequently causing severe effects on ecosystem and social economy, however, few long-term winter temperature records or reconstructions hinder a better understanding on variations in winter temperature in the vast area of the TP. In this paper, we present a minimum winter (November–February) temperature reconstruction for the past 668 years based on a tree-ring network (12 new tree-ring chronologies) on the southeastern TP. The reconstruction exhibits decadal to inter-decadal temperature variability, with cold periods occurring in 1423–1508, 1592–1651, 1729–1768, 1798–1847, 1892–1927, and 1958–1981, and warm periods in 1340–1422, 1509–1570, 1652–1728, 1769–1797, 1848–1891, 1928–1957, and 1982–2007. … It also shows the possible effects of volcanic eruption and reducing solar activity on the winter temperature variability for the past six centuries on the southeastern TP.
Vermassen et al., 2019 A link between the physical oceanography of West Greenland and Atlantic SSTs has indeed been suggested previously: a positive phase of the AMO [Atlantic Multidecadal Oscillation] is related to an increase of warm Atlantic waters flowing towards and along the SE and W Greenland shelf (Drinkwater et al., 2014; Lloyd et al., 2011). … Despite differences in the timing and magnitude of the retreat of the different glaciers, they broadly share the same retreat history. High retreat rates occurred between the mid ‘30s and mid ‘40s (400-800m/yr), moderate retreat rates between 1965-1985 (~200 m/yr, except for Upernavik) and high retreat rates again after 2000 (>200 m/yr). … Since the meridional overturning circulation strength and associated heat transport is currently declining, (Frajka-Williams et al., 2017), this may lead to cooling bottom waters during the next decade in Upernavik Fjord and most likely also other fjords in West-Greenland.
Deng et al., 2019 Recent SST records based on longchain alkenones imply that the MCA [Medieval Climate Anomaly] was slightly warmer than the CWP [Current Warm Period] in the northern SCS [South China Sea] (Kong et al., 2017). … [I]t still should be noted that the SST record reconstructed from a Tridacna gigas Sr/Ca profile by Yan et al. (2015a) suggested that the annual average SST was approximately 0.89°C higher during the MCA [Medieval Climate Anomaly] than that of the CWP [Current Warm Period].
Zhang et al., 2019 Natural variability of Southern Ocean convection as a driver of observed climate trends … Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many historical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convection may have contributed strongly to the observed temperature and sea-ice trends.
Etourneau et al., 2019 Based on water stable isotopes calibrated to recent air temperatures [Abram et al., 2013, Mulvaney et al., 2012], the reconstructed mean annual SAT documents a 1.5 °C cooling over the Holocene occurring in two steps between 10,000 and 6000 years before present (BP), and 3500 and 500 years BP. The Holocene cooling was interrupted by a slightly warmer period. The first main cooling episode corresponds to a phase of major EAP [East Antarctic Peninsula] ice shelf retreat reported in the literature [Domack et al., 2005, Cofaigh et al., 2014, Johnson et al., 2011, Davies et al., 2012]. … The Larsen A ice shelf was probably destabilized at least as early as ~6300 years BP [Brachfeld et al., 2003], while evidence show that the Larsen B ice shelf experienced a continuous and significant shrinkage throughout the Holocene [Domack et al., 2005]. Hence, the EAP ice shelves underwent a major retreat mostly between ~8000 and 6000 years BP. … [T]he ice core-derived SAT were overall warmer throughout the Holocene than during the last two millennia and could have hence favored the EAP [East Antarctic Peninsula] ice shelf surface melting during the entire period. … The long-term SOT [subsurface ocean temperatures] increasing trend at the JPC-38 core site was punctuated by up to 1.5 °C warm events at the centennial scale.
Lack Of Anthropogenic/CO2 Signal In Sea Level Rise
Parker, 2019 Japan has strong quasi-20 and quasi-60 years low frequencies sea level fluctuations. These periodicities translate in specific length requirements of tide gauge records. 1894/1906 to present, there is no sea level acceleration in the 5 long-term stations. Those not affected by crustal movement (4 of 5) do not even show a rising trend. … In Japan tide gauges are abundant, recording the sea levels since the end of the 19th century. Here I analyze the long-term tide gauges of Japan: the tide gauges of Oshoro, Wajima, Hosojima and Tonoura, that are affected to a lesser extent by crustal movement, and of Aburatsubo, which is more affected by crustal movement. Hosojima has an acceleration 1894 to 2018 of +0.0016 mm/yr2. Wajima has an acceleration 1894 to 2018 of +0.0046 mm/yr2. Oshoro has an acceleration 1906 to 2018 of −0.0058 mm/yr2. Tonoura has an acceleration 1894 to 1984 of −0.0446 mm/yr2. Aburatsubo, has an acceleration 1894 to 2018 of −0.0066 mm/yr2. There is no sign of any sea level acceleration around Japan since the start of the 20th century. The different tide gauges show low frequency (>10 years) oscillations of periodicity quasi-20 and quasi-60 years. The latter periodicity is the strongest in four cases out of five. As the sea levels have been oscillating, but not accelerating, in the long-term-trend tide gauges of Japan since the start of the 20th century, the same as all the other long-term-trend tide gauges of the world, it is increasingly unacceptable to base coastal management on alarmist predictions that are not supported by measurements. … The Japan Meteorological Agency (2018) has shown that the relative rise in sea level on the coast of Japan has stabilized since the beginning of the 20th century and has not accelerated. The analysis presented here has further strengthened this result. … The relative sea level rise measured by a tide gauge has a sea and a land component. The relative sea level may rise, or fall, not only because the volume of the water is increasing, or reducing. It may also rise, or fall, because the tide gauge instrument is sinking, or uplifting. The sea component has important multi-decadal periodicities of quasi 60 years. Hence, not less than 60 years of data are needed to infer a rate of rise, and many more years, not less than 100 years, are needed to infer an acceleration.
Duvat, 2019 This review first confirms that over the past decades to century, atoll islands exhibited no widespread sign of physical destabilization by sea level rise. The global sample considered in this paper, which includes 30 atolls and 709 islands, reveals that atolls did not lose land area, and that 73.1% of islands were stable in land area, including most settled islands, while 15.5% of islands increased and 11.4% decreased in size. Atoll and island areal stability can therefore be considered as a global trend. … Importantly, islands located in ocean regions affected by rapid sea-level rise showed neither contraction nor marked shoreline retreat, which indicates that they may not be affected yet by the presumably negative, that is, erosive, impact of sea-level rise. … It is noteworthy that no island larger than 10 ha decreased in size, making this value a relevant threshold to define atoll island areal stability. … [A]mong the 27 islands having a land area lying between 100 and 200 ha (9 in French Polynesia, 6 in the Marshall Islands, 6 in Kiribati, 5 in Tuvalu and 1 in the Federated States of Micronesia), only 3 increased in area, while 24 were stable. … The great majority of Pacific islands showed positional stability, as illustrated by the Tuamotu atolls, where 85–100% of islands were stable, depending on atolls (Duvat & Pillet, 2017; Duvat, Salvat, et al., 2017). … Importantly, the reanalysis of available data on atoll island planform change indicates that over the past decades to century, no island larger than 10 ha and only 4 out of the 334 islands larger than 5 ha (i.e., 1.2%) underwent a reduction in size.
Zhai et al., 2019 To estimate the resilience influences on 15 islands in Florida Bay (Florida, U.S.), our study used indicators (areas of the 15 islands and their mangrove forests) by analyzing 61-yr high-resolution historical aerial photographs and a 27-yr time-series of Landsat images. … Comparative spatial analysis of the historical aerial images showed that the island area significantly increased from 1953 to 2014. For example, Joe Kemp Key had the largest area increase from 0.34 km2 to 0.37 km2. Moreover, the similar increased patterns of island area were found for annual total areas of the 15 islands from 1984 to 2011 by analysis of Landsat images. The total areas showed a significant increasing pattern with time. Therefore, results from the analysis of both aerial and satellite images revealed increases in island area, which indicate the island resilience to inundation caused by SLR. However, three islands […] decreased in area. … The long-term island area increases estimated by our analysis supported the resilience of Florida Bay islands to SLR inundation. Moreover, both the positive relationship between the increases of island area and mangrove expansion, and previous field studies in the Florida Bay and nearby Caribbean mangroves suggested the contribution of the mangrove expansion were at the expense of non-mangrove habitats.
Mörner, 2019 [T]he Late Holocene and present sea level changes are dominated by the horizontal redistribution of oceanic water masses primarily driven by planetary beat. The future changes in sea level are estimated at a maximum of + 20 cm by the year 2100.
Derrick, 2019 Sea levels in and around 1886 to 2018 … There has been no significant sea level rise in the harbour for the past 120 years, and what little there has been is about the height of a matchbox over a century. Along the northern beaches of Sydney, at Collaroy there has been no suggestion of any sea level rise there for the past 140 years. Casual observations from Bondi Beach 1875 to the present also suggest the same benign situation.
Parker and Ollier, 2019 Over the past decades, detailed surveys of the Pacific Ocean atoll islands show no sign of drowning because of accelerated sea-level rise. Data reveal that no atoll lost land area, 88.6% of islands were either stable or increased in area, and only 11.4% of islands contracted. The Pacific Atolls are not being inundated because the sea level is rising much less than was thought. The average relative rate of rise and acceleration of the 29 long-term-trend (LTT) tide gauges of Japan, Oceania and West Coast of North America, are both negative, −0.02139 mm yr−1 and −0.00007 mm yr−2 respectively. Since the start of the 1900s, the sea levels of the Pacific Ocean have been remarkably stable.
Dean et al., 2019 Results show RSL in Israel rose from ~0.8 ± 0.5 m at ~2750 a BP (Iron Age) to 0.0 ± 0.1 m by ~1850 a BP (Roman period) at 0.8 mm/a, and continued rising to 0.1 ± 0.1 m until ~1600 a BP (Byzantine Period). RSL then fell to ~0.3 ± 0.1 m by 0.5 mm/a until ~650 a BP (Late Arab period), before returning to present levels at a rate of 0.4 mm/a. The reassessed Israeli record supports centennial-scale RSL fluctuations during the last 3000 a BP, although the magnitude of the RSL fall during the last 2000 a BP is 50% less. The new Israel RSL record demonstrates correspondence with regional climate proxies.
Mörner, 2019 The sea level changes documented in the five equatorial sites investigated: A 20 cm drop in sea level in the mid 20th century (i.e. at the cooling phase after the 1930-1950 warm period in the Northern Hemisphere). Quite stable sea level conditions in the last 50-70 years (i.e. during the period when sea level was rising at a mean rate of 1.1±0.2 mm/yr in the northern Hemisphere).
Armstrong and Lazarus, 2019 [T]rends in recent rates of shoreline change along the U.S. Atlantic Coast reflect an especially puzzling increase in accretion, not erosion. Using U.S. Geological Survey shoreline records from 1830–2007 spanning more than 2,500 km of the U.S. Atlantic Coast, we calculate a mean rate of shoreline change, prior to 1960, of −55 cm/year (a negative rate denotes erosion). After 1960, the mean rate reverses to approximately +5 cm/year, indicating widespread apparent accretion despite steady (and, in some places, accelerated) sea‐level rise over the same period.
Sea Levels Multiple Meters Higher 4,000-7,000 Years Ago
Oliver and Terry, 2019 ~6000 cal yr B.P. old oysters can be found from between 3.8 ± 0.1 m to 2.5 ± 0.1 m above present day mean sea level. … Dead (fossil) oysters were collected from between 1 and 3 m above the centre of the live oyster band in a more sheltered cleft inside the notch. The oldest sample with an age of 5270–4950 cal yr B.P. was collected at an elevation of 3.01 ± 0.1 m above the apex of the notch. The ages decrease with elevation down to 920–710 cal yr B.P. at 1.03 m. … In all the sites, the 14C age of the dead oysters inside the notches increases with increasing elevation above present day MSL. Clearly, relative sea level was 2 to 3 m higher than present between 6000 and 3000 B.P. and has steadily fallen since. … There was a progressive warming from ~13,500 years ago to a peak at 6500 ± 200 years ago followed by a cooling of −2.6 °C to the present day. … Generally, there is a ~1 m wide live oyster band (with modern 14C ages) in the apex of the sea notch that corresponds to the present day MSL. 14C ages of dead oysters are systematically older higher up the sea notch and reach a maximum 14C cal yr B.P. age of 6513–6390 cal yr B.P. at an elevation of 2.5 ± 0.1 m above present day MSL in an exposed site at West Railay Beach. Consequently, relative sea levels must have been higher in the mid Holocene than they are now. … [A]t a more sheltered site inside a bay on Ko Pha Nak, the highest preserved oyster shell is at 3.2 ± 0.1 m above MSL and has a younger 14C calibrated age of 5845–5605 cal yr B.P. Furthermore, oysters from 3.8 ± 0.1 m above present day MSL, encrusted on a stalactite in a cave at West Railay Beach has a 14C calibrated age of 6176–6041 cal yr B.P.
Yamano et al., 2019 (SW Japan) Evidence from the core samples and fossil microatolls suggests sea level reached its present position before 5100 cal yr B.P., and a relative sea-level highstand of 1.1–1.2 m above the present sea level occurred from 5100 to 3600 cal yr B.P. This was followed by a gradual fall in relative sea level. The tectonically corrected sea-level curve indicates a stable sea level after 5100 cal yr BP., with a sea-level highstand of up to 0.4 m between 5100 and 3600 cal yr B.P.
Brooke et al., 2019 (Queensland) Indicator data for Queensland have been assessed for their accuracy and robustness by Lambeck et al. (2014), who identified a number of coastal and inner shelf island sites in the northeastern region, in which Cowley Beach is located (Fig. 1), where accurately dated in situ fossil coral, coral microatolls and sediment core samples provide robust sea-level records (Chappell, 1983; Chappell et al., 1983; Horton et al., 2007; Yu and Zhao, 2010; Zwartz, 1995; Fig. 3). Here, relative sea level reached a Holocene highstand between 6770 and 5520 yr BP approximately 1–2 m above the present level (Lewis et al., 2013; Fig. 3). Following the highstand, the data record a gradual fall in sea level to the present position (Perry and Smithers, 2011; Lambeck et al., 2014). … Local and regional records for the Holocene at far-field sites may also reflect the influence of climatic variations on sea level, such as shifts in the El Nino Southern Oscillation (ENSO), that can induce minor (<0.5 m) changes in sea level (Duke et al., 2017; Leonard et al., 2018; Sloss et al., 2018) on annual to multi-decadal, rather than millennial, timescales.
A Model-Defying Cryosphere, Polar Climate
Sapunov, 2019 This chapter aims at the consideration of world temperature dynamics and its prediction in the polar regions of the planet. The global warming started in the 17th century and has been progressing since then. The decline in average global temperature began in 1997. There exist various factors which affect the process, the abiotic ones being among the major in controlling the climate. The climate is also dependent on the interaction between abiotic, biotic, and social spheres. This system seems rather stable and not very much dependent on human activity. The effects of contemporary cooling are not expected to be significant for the mankind but are definitely important for the polar regions. In the Arctic, the temperature is increasing. The one in the Antarctic declines. The average global temperature thus becomes variable. … Arctic region is under control of methane (CH4) emission and anthropogenic pressure. Antarctic is almost free from anthropogenic pressure and thus develops as a significant source of global cold snap. We live in a relatively cold period. During 80% of the entire history of the Earth, Greenland and the Antarctic were free from ice (Sapunov, 2011; Howat, Negrete, & Smith, 2014). In XVII-XX centuries, global temperature increased and the ice in Greenland and the Arctic Ocean melted (Helm, Humbert, & Miller, 2014). According to the chain reaction principle, this led to the emission of greenhouse gases (pseudo green house effect) (Sapunov, 2011), such as CO2 and CH4, from melting permafrost, which, in turn, accelerated ice melting in the North (Semiletov, Makshtas, Akasofu, & Andreas, 2004). … Since 1997, a period of global cooling began. At the same time, big (several millennia) climatic cycle increased while more noticeable small one (several centuries) decreased. This was particularly noticeable in the southern hemisphere in a form of growth of glacial massif in the Antarctic. At the same time, the asymmetry of the Earth began to grow. In the North, both ice melting and the rise in temperature tend to decrease (Sapunov, 2011). These data must be taken into account for further forecasting of climatic trends.
Johnson et al., 2019 Here we present a new Holocene deglacial chronology from a site on the Lassiter Coast of the Antarctic Peninsula, which is situated in the western Weddell Sea sector. … [T]he ice sheet experienced a period of abrupt thinning over a short time interval (no more than 2700 years) in the mid-Holocene, resulting in lowering of its surface by at least 250 m. Any late Holocene change in ice sheet thickness — such as re-advance, postulated by several modelling studies — must lie below the present ice sheet surface. The substantial difference in exposure ages derived from 10Be and 14C dating for the same samples additionally implies ubiquitous 10Be inheritance acquired during ice-free periods prior to the last deglaciation, an interpretation that is consistent with our glacial-geomorphological field observations for former cold-based ice cover. The results of this study provide evidence for an episode of abrupt ice sheet surface lowering in the mid-Holocene, similar in rate, timing and magnitude to at least two other locations in Antarctica.
England et al., 2019 Over the last half century, the Arctic sea ice cover has declined dramatically. Current estimates suggest that, for the Arctic as a whole, nearly half of the observed loss of summer sea ice cover is not due to anthropogenic forcing, but due to internal variability. Using the forty members of the Community Earth System Model Large Ensemble (CESM-LE), our analysis provides the first regional assessment of the role of internal variability on the observed sea ice loss. The CESM-LE is one of the best available model for such an analysis, as it performs better than other CMIP5 models for many metrics of importance. Our study reveals that the local contribution of internal variability has a large range, and strongly depends on the month and region in question. We find that the pattern of internal variability is highly non-uniform over the Arctic, with internal variability accounting for less than 10% of late summer (August-September) East Siberian Sea sea ice loss but more than 60% of the Kara Sea sea ice loss. In contrast, spring (April-May) sea ice loss, notably in the Barents Sea, has so far been dominated by internal variability. … In contrast with the results for the late summer, we find that in April-May internal variability is responsible for the vast majority (> 75%) of recent observed sea ice changes. … Given that sea ice concentrations in the spring are highly correlated with sea ice conditions in the late winter in this region, our findings are consistent with previous studies which show the strong contribution of internal variability on decadal timescales, driven by ocean heat transport, to winter sea ice loss (Smedsrud et al. 2013; Yeager et al. 2015).
Khazendar et al., 2019 Jakobshavn Isbrae has been the single largest source of mass loss from the Greenland Ice Sheet over the last 20 years. During that time, it has been retreating, accelerating and thinning. Here we use airborne altimetry and satellite imagery to show that since 2016 Jakobshavn has been re-advancing, slowing and thickening. We link these changes to concurrent cooling of ocean waters in Disko Bay that spill over into Ilulissat Icefjord. Ocean temperatures in the bay’s upper 250 m have cooled to levels not seen since the mid 1980s. Observations and modelling trace the origins of this cooling to anomalous wintertime heat loss in the boundary current that circulates around the southern half of Greenland. … Over the past several years, ocean temperatures have cooled on the continental shelf in the vicinity of Jakobshavn Isbrae . We find that ocean temperatures in Disko Bay below about 150 m cooled by nearly 2 °C between 2014 and 2016. … The data then show cooling in the first half of 2016 of a normal magnitude (~2 °C) acting on water at already below-average temperatures cooling it to 1 °C, which is ~2–2.5 °C colder than the 2009–2015 values. The mooring data also show that temperatures remain significantly below average through summer 2017. … Since the late 1990s, Jakobshavn developed Greenland’s largest cumulative ice discharge anomaly, contributing the equivalent of ~0.9mm [9/100ths of a cm] to global mean sea-level rise between 2000 and 2010. … To explain the cooling observed in Davis Strait and in Disko Bay in 2015 and 2016, we first note that anomalous wintertime heat loss lowered ocean temperatures across the entire North Atlantic subpolar gyre since 2011 by about 0.6 °C on average in the top 300m of the water column (Supplementary Fig. 13). In the northern Irminger Sea where Atlantic Water first enters the East Greenland Current, ECCO shows that average temperatures have cooled by 0.75 °C over the same time period with the greatest cooling occurring during the winter of 2015. This 0.75 °C cooling of waters far upstream in the Irminger Sea explains part of the 2 °C cooling observed in Davis Strait and in Disko Bay. … Most prominently, the sharp drop in ocean temperatures in 2016 and 2017 by 2 °C relative to the peak temperature in 2014 corresponds to the slowing and dramatic thickening of the glacier in 2017 and 2018. The higher melting in 2014 simulated in our plume model is not reflected in flow acceleration and thinning, which we cannot explain.
(press release) In the 2000s, Jakobshavn Isbrae was the fastest flowing ice stream on the island, travelling at 17km a year. But now it’s all changed. Jakobshavn is travelling much more slowly, and its trunk has even begun to thicken and lengthen. Where previously this was dropping in height by 20m a year, it’s now thickening by 20m a year. “It’s a complete reversal in behaviour and it wasn’t predicted,” said Dr Anna Hogg from Leeds University and the UK Centre for Polar Observation and Modelling (CPOM).
Holmlund and Holmlund, 2019 The strongest melt in the mass change curve […] occurs between the 1930s and 1960s, with the beginning of this negative trend occurring in the early 1920s. The imagery from 1922 by Odencrants support this likely start of a melting trend, as almost no snow is present on top of Storglaciären, and other glaciers show tendencies of retreat. Between 1920 and 1970, 76% of the mass loss seen from 1910 to 2015 occurred while only constituting 48% of those years. This considerable melt, compared to the subsequent years, is also reflected in the retreat of the terminus, where it retreated approximately 370 m between 1929 and 1959 (Karlén 1973). The mass change of Storglaciären stabilized in the following years, even containing periods of increases in mass, and its mass was almost the same in 2001 as in 1970. … Various weather parameters from Tarfala Research Station, a kilometre north east of Storglaciären, exist since 1945, measuring average summer and winter temperatures of +5.5°C and −8.9°C, respectively … From 1900 to 1920, the mean June–August temperature in Karesuando was 9.88°C, while it rose to 11.63°C for the period 1930–1950. … [G]enerally lower ELA between 1880 and the 1930s, with occasional high values, and the ELA reaches comparable lows during the late 1970s and 1990s, corresponding with positive mass balance. The curve thus suggests similar conditions in the late 1800s to, for example, the 1990s.
Sadatzki et al., 2019 The biomarker signals of enhanced sea ice cover during late GI/early GS are similar to those observed in sediment traps and surface sediments on the modern proximal East Greenland margin (25, 26). Here, the sea ice season extends from late autumn to spring such that ice algae growth is enhanced and open-water phytoplankton growth is reduced due to only short periods of ice-free conditions during summer (25–27). In turn, lowered IP25 with contemporaneously increased brassicasterol and dinosterol values, leading to minimum PBIP25 and PDIP25 values of ~0.2 [ a reduced variable sea ice cover], reflects periods of maximum open-ocean conditions in our record (Fig. 2). These maximum open-ocean conditions in the southern Norwegian Sea appear to have persisted only during peak GI, and similar conditions have been reconstructed for the warm early Holocene based on a PIP25 record from a nearby core site (15). … Previously published PIP25 values of Arctic and subarctic surface sediments reveal a robust correlation with the modern sea ice concentrations during spring/summer, and PIP25 values of <0.1, 0.1 to 0.5, 0.5 to 0.75, and >0.75 are classified to indicate ice-free conditions, a reduced variable sea ice cover, a seasonal sea ice cover, and an extended to perennial sea ice cover, respectively. … Accordingly, increased benthic δ18O during GI indicates a 2° to 5°C cooling of deep waters <1200 m, which was likely linked to active convective deep-water formation in the Nordic Seas, similar to modern conditions (30).
Berben et al., 2019 The early Holocene (ca. 9500 – 5800 cal yr BP) … Relatively low IP25 concentrations with increased brassicasterol abundances indicate reduced seasonal (spring) sea ice cover and longer (warmer) summers with open water conditions suitable for phytoplankton production. The occurrence of reduced sea ice cover and longer summers is consistent with increased planktic foraminiferal concentrations (reported here and Carstens et al., 1997) and with longer ice-free seasons and a retreated ice margin in the northern Barents Sea (Duplessy et al., 2001) as well as increased phytoplankton production in the northern Fram Strait (Müller et al., 2009). Reduced spring sea ice cover also indicates the HTM recorded at the sea surface between ca. 9300 and 6500 cal yr BP, which probably results from maximum summer insolation at 78° N. … Our proposed sea ice scenario suggests that water masses south of the study area were ice free, which agrees with open water conditions observed in the western Barents Sea (Berben et al., 2014) and the West Svalbard margin (Müller et al., 2012) during the early Holocene. … For the West Svalbard margin, Werner et al. (2013) associated high planktic foraminiferal fluxes ca. 8000 cal yr BP to ice-free or seasonally fluctuating sea ice margin conditions. … The PBIP25 index shows the lowest values of the record (0.16 – 0.40) suggesting a period characterized by low or variable seasonal sea ice cover and influenced substantially by open water conditions (Müller et al., 2011). … The late Holocene (ca. 2200 – 0 cal yr BP) is characterized by the highest abundances of IP25 (0.35 µg/g OC) and relatively low (but stable) brassicasterol (12.5 µg/g OC) (Figure 7A-B).). Consistent with the opposing trends in the IP25 and brassicasterol records, the PBIP25 [sea ice proxy] values reach their highest value (0.87) of the record at ca. 0 cal yr BP. An increase in PBIP25 suggests a further extension in sea ice cover, reflecting Arctic Front conditions (Müller et al., 2011), most similar to modern conditions.
Koch et al., 2019 We report on an accumulation of mummified southern elephant seals (Mirounga leonina) from Inexpressible Island on the Victoria Land Coast (VLC), western Ross Sea, Antarctica. This accumulation is unusual, as elephant seals typically breed and molt on sub-Antarctic islands further north and do not currently occupy the VLC. Prior ancient DNA analyses revealed that these seals were part of a large, Antarctic breeding population that crashed ~1,000 yr ago. Radiocarbon dates for Inexpressible Island mummies range from 380 to 3,270 yr before present. This wide distribution of elephant seal remains is surprising, as the species typically breeds and molts on sub-Antarctic islands at lower latitudes. The closest extant breeding colony to VLC is on Macquarie Island (~54.5°S), ~2,400 km to the north. … The presence of southern elephant seals, geomorphic evidence for wave-generated beaches, and diatom data from nearshore cores all indicate that, for much of the Holocene, open water was seasonally present on VLC beaches north and south of Terra Nova Bay (Hall et al. 2006, Mezgec et al. 2017). Together, these lines of evidence suggest that land-fast and multiyear sea ice has become much more pronounced in coastal settings over the last millennium.
Harning et al., 2019 The NIS [North Iceland Shelf] represents one of the few global examples where paleo-IP25 abundance in marine cores has been calibrated against observational and documentary records (Massé et al., 2008; Andrews et al., 2009b). As a result, the variability of IP25 has been routinely applied to marine sediment around Iceland as a robust indicator for seasonal sea ice (Massé et al., 2008; Andrews et al., 2009b; Sicre et al., 2013; Cabedo-Sanz et al., 2016a). Similar to these previous studies, IP25 concentrations in B997-316 GGC increase abruptly during the 13th century, and with the exception of the period 1450-1650 CE, remain elevated until the 19th century when concentrations begin to diminish … By applying temperature calibrations to our down core TEX86 record, our data reveal rapid and abrupt temperature variability on the NIS during the last millennium (Fig. 5). If the existing annual SST (Kim et al., 2010) and annual subT TEX86 calibrations developed for polar regions (Kim et al., 2012) are applied, the GDGT distributions suggest that subT fluctuated up to 5°C over the course of decades. These observations are considerably higher than expected, especially given that they are comparable to the magnitude of SST changes observed in other NIS proxy records over the entire Holocene (e.g., Andersen et al., 2004; Bendle & Rosell-Melé, 2007; Jiang et al., 2015; Kristjánsdóttir et al., 2016). … A variety of model and data-based studies have demonstrated that the LIA was triggered by a combination of sustained stratospheric volcanic sulfate injection (Zhong et al., 2010; Miller et al., 2012; Sicre et al., 2013; Slawinska & Robock, 2018), low total solar irradiance (Shindell et al., 2001) and changes in the North Atlantic Oscillation, one of the major modes of internal climate variability in the North Atlantic (Trouet et al., 2009). On the NIS, these radiative forcings directly impact the ocean surface, as manifested in the immediate and abrupt increase in seasonal sea ice, reduced northward heat transport and suppression of SSTs (Miller et al., 2012).
Szpak et al., 2019 Given that the earliest ringed seals (c. 4000 yr BP) have the lowest d13C values of any of the sites analyzed suggests that at this time the CAA [Canadian Arctic Archipelago (4000-800 yr BP)] experienced the lowest amount of sympagic productivity reaching higher trophic levels over the period studied. This scenario is consistent with the extensive study of naturally stranded Holocene bowhead whales by Dyke et al. (1996) who found that from 5000 to 3000 14C yr BP bowheads had a wider geographic distribution than at present, moving into the channels of the central CAA. This range extension would only be possible if sea ice did not exclude the whales from this region, which is the case today.
Moore et al., 2019 The north and east slopes of Mount Rainier, Washington, are host to three of the largest glaciers in the contiguous United States: Carbon Glacier, Winthrop Glacier, and Emmons Glacier. Each has an extensive blanket of supraglacial debris on its terminus, but recent work indicates that each has responded to late twentieth- and early twenty-first-century climate changes in a different way. While Carbon Glacier has thinned and retreated since 1970, Winthrop Glacier has remained steady and Emmons Glacier has thickened and advanced.
Schröder et al., 2019 We demonstrate the impact of our model changes on the timing of mean melt and freeze onset (2005–2014) between CICE-best and CICE-default in Fig. 9. In CICE-best, the melt onset day is later (0–4 days in the central Arctic, up to 10 days in the Fram Strait) and the freeze onset is earlier (4–12 days in most areas), resulting in a shorter melting season. The simulated mean length of the melting season over the Arctic Basin reduces from 107 days (CICEdefault) to 100 days (CICE-best). This is an improvement with respect to the observed value of 94 days. The observed of 94 days is based on a mean value of 88 days for the period 1979 to 2012 and accounting for the trend of 3.7 days decades−1(Stroeve et al., 2014). The impact of the model changes is remarkable given that we apply the same 2 m air temperature data (NCEP-2) as atmospheric forcing.
Yokoyama et al., 2019 Compilation of previously reported sea level indicators from Sri Lanka, Southeastern India and the Maldives, together with predicted sea level obtained from a glacio-hydro-isostatic adjustment model (GIA), suggest that 3–4 m of global sea level equivalent ice sheet melting occurred during the Mid Holocene due to the retreat of the Antarctic and/or Greenland ice sheets. Previous works suggests late Holocene (ca. 4 ka) climate anomalies in both the low and high latitudes. We suggest the low latitude climate anomaly, transmitted via atmosphere to the high latitude during the late Holocene, seems to have induced changes in polar ice sheets. … Cumulative evidence suggest that climate anomalies are reported in late Holocene at around 4 ka (Walker et al., 2012). Widespread collapse of the Ross Ice shelf occurred around the same time (Yokoyama et al., 2016b).
Ding et al., 2019 The relative contribution and physical drivers of internal variability in recent Arctic sea ice loss remain open questions, leaving up for debate whether global climate models used for climate projection lack sufficient sensitivity in the Arctic to climate forcing. Here, through analysis of large ensembles of fully coupled climate model simulations with historical radiative forcing, we present an important internal mechanism arising from low-frequency Arctic atmospheric variability in models that can cause substantial summer sea ice melting in addition to that due to anthropogenic forcing. This simulated internal variability shows a strong similarity to the observed Arctic atmospheric change in the past 37 years. Through a fingerprint pattern matching method, we estimate that this internal variability contributes to about 40–50% of observed multi-decadal decline in Arctic sea ice.
Zhou et al., 2019 The Arctic sea ice is becoming thin and young, whereas the sea ice extent in Antarctica has slightly increased over the last four decades. Parkinson et al. (2012) used satellite passive–microwave data and found a substantial increasing trend (17100 ± 2300 km2 year−1) of sea ice extent over Antarctica from 1978 to 2010. Turner et al. (2016) determined an increasing trend by 195 × 103 km2 per decade for the total Antarctic sea ice extent from 1979 to 2013. Then, the satellite-derived sea ice extent during 1979 to 2015 was studied by Jena et al. (2018) who declared that the increasing trend of sea ice extent in the Indian Ocean was about 2.4 ± 1.2% per decade. Thus, understanding the changes in sea ice in the Antarctic sea ice region (ASIR) is essential to global climate research. … The albedo, an important factor that affects the radiation balance of the earth–atmosphere system, has frequently been used for research on global climate change. Given the high albedo of snow and ice surfaces, most of the solar radiation on the surface of snow and ice in the ASIR are reflected back to the atmosphere. The albedo of unfrozen ocean is between 5% and 20% and is affected by solar zenith angle. Snow/ice albedo, which is strongly dependent on incident solar irradiance, snow grain size, and soot content, ranges from 50% to 90%, and fresh snow albedo reaches 90%. However, substantial incident solar radiation is absorbed by the Antarctic sea ice during summer; thus, the physical state of the snow/ice surface changes rapidly, such that the melting of snow and ice leads to dramatic changes in snow/ice surface albedo. … These results demonstrated that the climate of the ASIR [the entire Antarctic Sea Ice Region, the (1) Weddell Sea (WS), (2) Indian Ocean, (3) Pacific Ocean (PO), (4) Ross Sea, and (5) Bellingshausen-Amundsen Sea (BS)] exhibits a cooling trend during summer [1982-2015], except for the BS. … Consistent with the trend of SAL [surface albedo], the slope values of SIC [sea ice concentration] were mostly positive, except for the BS (Table 4), which further demonstrated that the climate of the ASIR exhibits a cooling trend in recent decades. … The average SAL (Table 3), SIC (Table 4), and SST (Table 5) for the total ASIR were 46.75%, 65.39%, and −2.44 °C during summer.
Qian et al., 2019 Differing from the decreasing sea-ice concentration (SIC) in Arc
tic, the sea-ice cover surrounding Antarctica has experienced an expansion in area over the past decades, particularly up to 2015 (Zwally et al., 2002; Comiso et al., 2008; Turner et al., 2009; Holland and Kwok, 2012; National Academies of Sciences, Engineering, and Medicine, 2017). Many papers have confirmed that the increasing sea-ice is the sum of opposing regional trends surrounding Antarctica such as negative SIC trends in a small portion around the Bellingshausen and Amundsen Seas while positive SIC trends with two large centers in the Weddell and Ross Seas (Comiso et al., 2011; Parkinson and Cavalieri, 2012; Holland, 2014; Turner et al., 2015). The explanations for the opposite sea-ice trends in the two hemispheres, as well as the total and regional sea ice trends and extreme sea ice events surrounding Antarctica are all hot topics. These studies are important to understanding of the global and hemispheric energy budget.
Castruccio et al., 2019 Most climate model simulations forced by the past evolution of external forcing underestimate this decline (Day et al. 2012; Stroeve et al. 2012), and its significant acceleration since the late 1990s (Comiso et al. 2008; Ogi and Rigor 2013) is mostly not captured (Rampal et al. 2011). This suggests a strong role for natural variability in Arctic climate (Stroeve et al. 2007; Swart et al. 2015; Ding et al. 2017) provided that the simulated sensitivity of the Arctic sea ice to the external radiative forcings is approximately correct. Studies using climate models estimate that 50%–60% of recent Arctic sea ice changes are attributable to anthropogenic global warming, with the remainder resulting from internal variability in the climate system (Kay et al. 2011; Stroeve et al. 2012). … Using a set of perturbed climate model experiments, we provide evidence that atmospheric teleconnections associated with the Atlantic multidecadal variability (AMV) can drive low-frequency Arctic sea ice fluctuations. … Positive AMV anomalies induce a decrease in the frequency of winter polar anticyclones, which is reflected both in the sea level pressure as a weakening of the Beaufort Sea high and in the surface temperature as warm anomalies in response to increased low-cloud cover. Positive AMV anomalies are also shown to favor an increased prevalence of an Arctic dipole–like sea level pressure pattern in late winter/early spring. The resulting anomalous winds drive anomalous ice motions (dynamic effect).
Scott et al., 2019 Understanding the drivers of surface melting in West Antarctica is crucial for understanding future ice loss and global sea level rise. This study identifies atmospheric drivers of surface melt on West Antarctic ice shelves and ice sheet margins and relationships with tropical Pacific and high-latitude climate forcing using multidecadal reanalysis and satellite datasets. Physical drivers of ice melt are diagnosed by comparing satellite-observed melt patterns to anomalies of reanalysis near-surface air temperature, winds, and satellite-derived cloud cover, radiative fluxes, and sea ice concentration based on an Antarctic summer synoptic climatology spanning 1979–2017. Summer warming in West Antarctica is favored by Amundsen Sea (AS) blocking activity and a negative phase of the southern annular mode (SAM), which both correlate with El Niño conditions in the tropical Pacific Ocean. Extensive melt events on the Ross–Amundsen sector of the West Antarctic Ice Sheet (WAIS) are linked to persistent, intense AS blocking anticyclones, which force intrusions of marine air over the ice sheet. Surface melting is primarily driven by enhanced downwelling longwave radiation from clouds and a warm, moist atmosphere and by turbulent mixing of sensible heat to the surface by föhn winds. Since the late 1990s, concurrent with ocean-driven WAIS mass loss, summer surface melt occurrence has increased from the Amundsen Sea Embayment to the eastern Ross Ice Shelf. We link this change to increasing anticyclonic advection of marine air into West Antarctica, amplified by increasing air–sea fluxes associated with declining sea ice concentration in the coastal Ross–Amundsen Seas.
Zhang et al., 2019 [T]he ice loss in 2007 has been largely attributed to wind and air temperature anomalies in the western Arctic (Stroeve et al. 2008; Zhang et al. 2008), increased Bering Strait inflow (Woodgate et al. 2010) and the reduced cloudiness (Kay et al. 2008). A similar condition occurred in 2012 due to an intense early August cyclone combined with preconditioning of the thin sea ice (Simmonds and Rudeva 2012; Parkinson and Comiso 2013; Zhang et al. 2013). In addition, there is a strong connection between the Arctic sea ice in September and the cyclone frequency in the preceding late spring and early summer (Screen et al. 2011; Mills and Walsh 2014). Several studies have also investigated the role of the local winds in the Bering Strait transport (Danielson et al. 2014) and the ice cover in the Bering, Chukchi and Beaufort seas (Frey et al. 2015). Danielson et al. (2014) have argued that the local winds associated with the Aleutian Low play a primary role in the heat transport through Bering Strait. Frey et al. (2015) suggested that recent rapid changes of sea ice cover in the western Arctic region are mainly related to the local winds. … During positive NPO [North Pacific Oscillation] years, the total downward solar radiation in the southern Beaufort sea and northwestern region of North American tends to decrease due to the moist warm air and cloud effect associated with the increased cyclone activity. However, due to the loss of sea ice, the southern Beaufort sea tends to gain more solar radiation, which can accelerate the melting of the ice in spring and summer. In addition, there is an increase in the downward longwave radiation in the southern Beaufort sea and the northwestern region of North American due to the enhanced advection of warm air. Therefore, the net surface heat flux in the Beaufort sea tends to increase in positive NPO years, and the increased solar radiation persists in the following months of July and August. This result suggests that the ice loss in spring fosters a stronger ice-albedo feedback in the following summer.
Wang et al., 2019 SW [surface wind] plays an important role in the modulating SIC trends in two ways: by transporting moist and warm air that melts sea ice in peripheral seas (typically evident in the Barents Sea) and by exporting sea ice out of the Arctic Ocean via passages into the Greenland and Barents Seas, including the Fram Strait, the passage between Svalbard and Franz Josef Land (S-FJL), and the passage between Franz Josef Land and Severnaya Zemlya (FJL-SZ). … The consecutive occurrence of record low sea ice extents (during the summer) over the past decade was partly influenced by SW forcing (Kwok, 2009; Zhang et al., 2013). … [T]he 2012 minimum sea ice extent was linked to the activity of a cyclone (Zhang et al., 2013), which occurred in August and brought a large section of ice to the SCESS, whereupon it melted away. The primary source of energy for ice melting in this case was from solar heating due to the largely reduced sea ice extent and surface albedo in the Pacific sector of the Arctic Ocean (Perovich et al., 2008).
[Neither CO2 or anthropogenic forcing is mentioned anywhere in the paper as a factor in driving Arctic sea ice cover changes.]
Stewart et al., 2019 Although surface waters have been considered a potential driver of ice shelf basal melting for some time, the observations presented here provide detailed evidence of this process. These data suggest that solar-heated surface water contributes substantially to the basal mass balance of the RIS, and that surface water plays a larger role in the mass balance of ice shelves than previously assumed. In the north-western Ross Sea, the impact of surface water can be attributed to two processes; localized solar heating of the surface ocean during summer, and transport of this energy into the cavity by a seasonal inflow. Surface heating seems to be closely linked to the consistent wind-driven expansion of the Ross Sea Polynya in spring. During this period, sustained southerly winds (guided by the Transantarctic Mountains) preferentially export sea ice from the western ice front. As air temperatures and insolation increase throughout November and December, the polynya expands rapidly, as illustrated by the sea-ice distribution during this period. This process increases the absorption of solar energy in the surface layer, and removes the latent heat sink presented by sea ice, aiding rapid heating of the surface layer.
Schomacker et al., 2019 In summary, the lake sediment record from Kløverbladvatna reveals the environmental history from 9768–9500 cal. yr BP to the present, and our RSL data from Palanderbukta extends it back to c. 10.7 cal. kyr BP. The early Holocene was characterized by shallow marine conditions with accumulation of the clay-silt facies with outsized clasts in the Kløverbladvatna basin and a rapid regression as documented by the RSL curve from Palanderbukta. … In Kløverbladvatna, we find evidence of glacial meltwater inflow across the threshold at the culmination of the AD 1938 surge of Etonbreen, but not earlier. This suggests that the glacier reached its late Holocene maximum immediately after the Little Ice Age.
Vermassen et al., 2019 A link between the physical oceanography of West Greenland and Atlantic SSTs has indeed been suggested previously: a positive phase of the AMO [Atlantic Multidecadal Oscillation] is related to an increase of warm Atlantic waters flowing towards and along the SE and W Greenland shelf (Drinkwater et al., 2014; Lloyd et al., 2011). Our data indeed supports that the AMO influences bottom water temperature variability along the West Greenland shelf and shows that this influence is strong within Upernavik Fjord. … Despite differences in the timing and magnitude of the retreat of the different glaciers, they broadly share the same retreat history. High retreat rates occurred between the mid ‘30s and mid ‘40s (400-800m/yr), moderate retreat rates between 1965-1985 (~200 m/yr, except for Upernavik) and high retreat rates again after 2000 (>200 m/yr). … [O]ur study shows that while warming of ocean waters in Upernavik fjord likely contributed to the retreat phases during the 1930s and early 2000s, ocean warming is not a prerequisite for retreat of Upernavik Isstrøm. … This is important since it implies that the future potential oceanic forcing of Upernavik Isstrøm will depend on changes related to circulation in the North Atlantic (i.e. the AMO). Since the meridional overturning circulation strength and associated heat transport is currently declining, (Frajka-Williams et al., 2017), this may lead to cooling bottom waters during the next decade in Upernavik Fjord and most likely also other fjords in West-Greenland.