Episodic Neoglacial Cryosphere Expansion Reconstructed from 14C Ages of Ice-Entombed Plants on Svalbard

Abstract:

The Northern Hemisphere cryosphere’s response to the monotonic decline in summer insolation through the Holocene has been irregular expansion, interrupted by periods of retreat, culminating in the Little Ice Age, when most glaciers attained their maximum late Holocene dimensions. This non-linear response to near-linear forcing implies other factors modulate the radiative effects of the regular insolation decline. Understanding how the Earth system accomplishes this modulation is relevant to the development of reliable models for future climate change. Accurately dating the onset of persistent summer coolings across the Arctic is a first step toward developing this understanding. Here we report 52 precise radiocarbon dates on rooted plants emerging from beneath receding glaciers on Svalbard that define times when colder summers led to snowline depression and an expanded cryosphere. The earliest persistent depression of snowline documented by our dates occurred between 4.0 and 3.4 ka, but with little additional summer cooling until early in the first millennium AD. Episodes of subsequent summer cooling were centered on 240-340, 410-540 and 670-770 AD, followed by additional cooling between 1000 and 1230 AD, and between 1300 and 1470 AD. Cooling that occurred after 1470 AD, includes of the Little Ice Age when the Svalbard cryosphere reached its maximum Neoglacial dimensions. We suggest that in addition to insolation forcing, irregular reductions in the strength of the North Atlantic Current and expansions of Arctic Ocean sea ice were dominant factors that led to episodic snowline depression over Svalbard, but the extent to which these changes are linked to radiative forcing from volcanism or solar irradiance as opposed to unforced variability remains unclear. The widespread exposure of entombed plants dating from the first millennium AD suggests that Svalbard’s average summer temperatures of the past century now exceed those of any century since at least 700 AD, including medieval times.