Holocene thermal maximum in the western Arctic (0–180°W)

Abstract

The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0-180 W; north of B60 N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.670.8 C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12-10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet. r ARTICLE IN PRESS

Rapid early Holocene deglaciation of the Laurentide ice sheet

“…3b & d), matching early Holocene paleoclimate reconstructions 10,11 . Further contributing to cooler temperatures is the influence of LIS elevation and albedo, with surface air temperature (SAT) up to 13.5 °C cooler over the ice sheet in the summer, of which 1-2 °C is due to the ice albedo effect (Fig.…”

“…To determine the LIS retreat rate, we compiled minimum limiting radiocarbon dates of deglaciation for Quebec, Labrador, eastern Nunavut 20 and Hudson Bay 21 , along with cosmogenic 10 Be ages from Labrador 22 and Quebec 14 (Fig. 1c, 2a & S1).…”

“…1a) warmed Northern Hemisphere summers by several degrees, presumably driving retreat of the Laurentide Ice Sheet (LIS) [7][8][9] . However, portions of the northern and eastern North Atlantic were colder than present [10][11][12] , possibly as a consequence of the suppression of Labrador Sea deep water production by LIS meltwater 13,14 . The gradual rise in sea level during the early Holocene [15][16][17] was punctuated by at least two increases in the rate of sea level rise, which were sourced from either the LIS or Antarctic Ice Sheet, indicating variations in the rate of ice sheet melting 14,18,19 .…”

Terrestrial biogeochemical feedbacks in the climate system

“…Almost half of the modern peatland began to form before 8,000 years ago 69 ; around 100 Pg of carbon were sequestered in the process, contributing to the observed drawdown of atmospheric CO 2 and the peak in atmospheric CH 4 in the early Holocene 70 . Increased peat-accumulation rates are registered during the warmer conditions of the early to mid-Holocene in the northern high latitudes 71 , whereas substantially reduced accumulation occurred during the cold intervals of the Younger Dryas and the Little Ice Age 72 . The most extensive lateral expansion of high-latitude peatlands occurred only after 5,000 years ago 73 , coinciding with the late-Holocene increase in CH 4 shown by ice-core records.…”

Vegetation changes in the Neotropical Gran Sabana (Venezuela) around the Younger Dryas chron

“…Stansell et al, 2005). This phase is known as the EHW in the Northern Hemisphere (Kaufman et al, 2004;Kaplan and Wolfe, 2006). In Lake Chonita, the appearance of diatoms and algae assemblages clearly indicates a shift to a more humid environment.…”