Newly published macrofossil discoveries from 4 papers strongly suggest that modern warmth may still be about 3°C colder than nearly all of the last 10,000 years.
Image Source: Leunda et al. (2018)
1. Permafrost is still present today in a region that was permafrost-free throughout the Holocene (until the onset of the Little Ice Age)
According to data from a new study (Sannel et al., 2018), subarctic Northern Sweden has yet to rise out of the range of frigid, permafrost-friendly temperatures coeval with of the Little Ice Age (roughly 1400 to 1900 AD).
Permafrost still exists today in regions where there was no recorded permafrost during nearly all of the Holocene, when temperatures were too warm (3°C higher than today) for permafrost to form.
These findings strongly suggest that modern warmth is nothing unusual, unprecedented, or even remarkable. In fact, it may be said that today’s subarctic warmth may only be slightly warmer than the coldest centennial-scale period — the Little Ice Age — of the last 10,000 years.
“At all these sites the datings together with the results of the plant macrofossil analyses suggest that permafrost aggradation took place around 600–100 cal. a BP [the Little Ice Age]. … [A]t these sites there are no indications of permafrost inception prior to the Little Ice Age. … Warmer conditions during recent decades have resulted in extensive permafrost degradation in Fennoscandian palsas and peat plateaus (Sollid& Sørbel 1998; Zuidhoff& Kolstrup 2000; Luoto & Seppala 2003; Sannel & Kuhry 2011; Borge et al. 2017).”
“At present the permafrost is close to thawing with mean annual ground temperatures just below 0°C (Christiansen et al. 2010; Johansson et al. 2011; Sannel et al. 2016).”
“Throughout most of the Holocene, these northern peatlands have not experienced climatic conditions cold enough for permafrost to form.”
“During peatland initiation (c. 10 000–9600 cal. a BP) the climatic conditions in the region were favourable for plant productivity with around 3 °C warmer summer temperatures compared to today (Kullman & Oberg 2015), resulting in relatively rapid peat and net carbon accumulation rates during the early Holocene.”
Image Source: Sannel et al., 2018
2. Italian Alps tree lines ~460 meters higher than present indicate surface temperatures were 3.1°C warmer during the Mid-Holocene
The modern forest line position in an Italian Alps study area (Badino et al., 2018) is 2,360 meters above sea level (m asl).
Per the data indicated in the graph below, the forest line position averaged between 2,750 to 2,900 m asl during 7400 to 3600 cal yrs BP — about 460 m higher than today’s forest line.
With a temperature lapse rate of 0.67°C/100 m for the region documented in the paper, this indicates temperatures were about 3.1°C warmer than today [4.6 X 0.67°C] during the Mid-Holocene.
“Between 7400 and 3600 yrs cal BP, an higher-than-today [~2820 m asl vs. 2360 m asl today] forest line position persisted under favorable growing conditions (i.e. TJuly at ca. 12 °C).”
“Between ca. 8.4-4 ka cal BP, our site [Italian Alps] experienced a mean TJuly of ca. 12.4 °C, i.e. 3.1 °C warmer than today.”
“[O]ur lapse rate (0.67°C/100 m), [is] slightly steeper than the average of 0.6°C for temperate regions (Barry and Chorley, 1992) was provided at higher spatial resolution.”
Images Source: Badino et al., 2018
3. The Pyrenees (Spain) timber line exceeded today’s uppermost limit by 300-400 meters ~4,500 years ago, indicating Mid-Holocene surface temperatures were about 2°C higher
The temperature lapse rate for the Pyrenees region is 0.517°C/100 m according to a newly published paper.
“Estimation of near‐surface air temperature lapse rates over continental Spain and its mountain areas … The median NSLR [near‐surface (i.e., non‐free atmosphere) air temperature lapse rates] … Pyrenees −5.17 °C/km“ (Navarro-Serrano et al., 2018)
Leunda et al. (2018) have published a study that shows the Mid-Holocene (about 4,500 years ago) Pyrenees timber line was 300-400 meters higher than today’s uppermost limit.
This would suggest that Pyrenees temperatures were about 1.6 to 2.1°C warmer [3.5 X 0.517°C] than today during 4,650-4,200 calendar years before present.
“The tree line ecotone was located at the cave altitude from 5,700 to 4,650 cal year bp, when vegetation consisted of open Pinus uncinata Ramond ex DC and Betula spp. Woodlands and timberline were very close to the site. Subsequently, tree line slightly raised and timberline reached the ice cave altitude, exceeding its today’s uppermost limit by c. 300–400 m during more than four centuries (4,650 and 4,200 cal year bp) at the end of the Holocene Thermal Maximum.”
Image Source: Leunda et al. (2018)
4. Tree remnants found northern Sweden mountain sites 600 to 700 meters atop where the 21st century tree line ends imply the Early Holocene (9500 years ago) was 3-4°C warmer than today
The temperature lapse rate for the Swedish Lapland region is 0.6°C/100 m.
Kullman (2018) found tree debris (trunks, cones, roots, etc.) 600 to 700 meters atop modern tree line positions that were dated to about 9,500 years ago. Accounting for glacio-isostatic uplift, this tree line elevation implies surface air temperatures were 3.6°C higher than today during the Early Holocene.
“The present paper reports results from an extensive project aiming at improved understanding of postglacial subalpine/alpine vegetation, treeline, glacier and climate history in the Scandes of northern Sweden. The main methodology is analyses of mega fossil tree remnants, i.e. trunks, roots and cones, recently exposed at the fringe of receding glaciers and snow/ice patches. This approach has a spatial resolution and accuracy, which exceeds any other option for tree cover reconstruction in high-altitude mountain landscapes.”
“All recovered tree specimens originate from exceptionally high elevations, about 600-700 m atop of modern treeline positions.”
“Conservatively drawing on the latter figure and a summer temperature lapse rate of 0.6 °C per 100 m elevation (Laaksonen 1976), could a priori mean that, summer temperatures were at least 4.2 °C warmer than present around 9500 year before present. However, glacio-isostatic land uplift by at least 100 m since that time (Möller 1987; Påsse & Anderson 2005)implies that this figure has to be reduced to 3.6 °C higher than present-day levels, i.e. first decades of the 21st century. Evidently, this was the warmth peak of the Holocene, hitherto. This inference concurs with paleoclimatic reconstructions from Europe and Greenland (Korhola et al. 2002; Bigler et al. 2003; Paus 2013; Luoto et al. 2014; Väliranta et al. 2015).”