GLACIERS IN ARCTIC CANADA RECEDING AT UNPRECEDENTED MELT RATES REVEAL EVIDENCE DEMONSTRATING CONTEMPORARY SUMMERS WARMEST IN >40,000 YEARS

Although modern warming has resulted in glacier recession across the Arctic, the instrumental record is too short to evaluate whether the rate of cryosphere retreat is unprecedented on millennial timescales. A longer melt-season temperature perspective is provided by changes in ice-cap dimensions through the Holocene, the most direct Arctic summer paleotemperature proxy. Across Arctic Canada many ice caps on gentle terrain are frozen to their beds, hence act as preservation, rather than erosive agents. As these ice caps recede they expose entombed plants in growth position, which are rapidly removed by meltwater in most settings. The radiocarbon ages of plants protected from meltwater erosion document when ice last expanded across the collection site, and remained over the site until the year of plant collection. The ages also define the last time century-scale summer temperatures were as warm as the past century. Radiocarbon dates on in situ plants exposed the year of their collection at the margins of 30 different receding ice caps in Arctic Canada are at or beyond the limit of radiocarbon dating, implying that contemporary summer temperatures are warmer than any century in more than 40,000 years. To test this assertion, we collected rock surfaces adjacent to plants dated >40 ka at nine different ice caps, and measured the inventories of in situ ¹⁴C produced by cosmic ray interactions with quartz in the rock samples. Modeling the evolution of in situ ¹⁴C as ice cover thinned after ~15 ka confirms that Holocene exposure is unlikely at all but two sites and that continuous ice cover through the Holocene is plausible at all nine sites. We also measured the vertical melt rate of one ice cap; over the past 13 years the surface lowered 30 m, a melt rate in excess of 2 m a^-1. This rate contrasts with deglaciation ice-lowering rates of ~40 cm a^-1 for the Laurentide Ice Sheet over central Canada and <10 cm a^-1 for early Holocene melt rates of small ice caps in the Eastern Canadian Arctic. Collectively, these data indicate that contemporary warming is now causing some ice caps in Arctic Canada to shrink to dimensions smaller than at any time at least 40,000 years, and at melt rates an order of magnitude higher than the early Holocene, when summer insolation was 9% higher than at present, confirming the exceptional nature of contemporary summer warmth.