The sensitivity of the Greenland ice sheet to climate forcing is of key importance in assessing its contribution to past and future sea level rise. Surface mass loss occurs during summer, and accounting for temperature seasonality is critical in simulating ice sheet evolution and in interpreting glacial landforms and chronologies. Ice core records constrain the timing and magnitude of climate change but are largely limited to annual mean estimates from the ice sheet interior. Here we merge ice core reconstructions with transient climate model simulations to generate Greenland‐wide and seasonally resolved surface air temperature fields during the last deglaciation. Greenland summer temperatures peak in the early Holocene, consistent with records of ice core melt layers. We perform deglacial Greenland ice sheet model simulations to demonstrate that accounting for realistic temperature seasonality decreases simulated glacial ice volume, expedites the deglacial margin retreat, mutes the impact of abrupt climate warming, and gives rise to a clear Holocene ice volume minimum.

a b s t r a c tThe last deglaciation is the most recent interval of large-scale climate change that drove the Greenland ice sheet from continental shelf to within its present extent. Here, we use a database of 645 published 10 Be ages from Greenland to document the spatial and temporal patterns of retreat of the Greenland ice sheet during the last deglaciation. Following initial retreat of its marine margins, most land-based deglaciation occurred in Greenland following the end of the Younger Dryas cold period (12.9e11.7 ka). However, deglaciation in east Greenland peaked significantly earlier (13.0e11.5 ka) than that in south Greenland (11.0e10 ka) or west Greenland (10.5e7.0 ka). The terrestrial deglaciation of east and south Greenland coincide with adjacent ocean warming. 14 C ages and a recent ice-sheet model reconstruction do not capture this progression of terrestrial deglacial ages from east to west Greenland, showing deglaciation occurring later than observed in 10 Be ages. This model-data misfit likely reflects the absence of realistic ice-ocean interactions. We suggest that oceanic changes may have played an important role in driving the spatial-temporal ice-retreat pattern evident in the 10 Be data.

Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10 Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10 Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.

The Younger Dryas (12.9 ± 0.1 to 11.7 ± 0.1 ka) was a return to cold conditions in the Northern Hemisphere during the last deglaciation. This climatic event was hypothesized to have been caused by a change in glacial Lake Agassiz (north-central North America) overflow from its routing to the Gulf of Mexico to an easterly route to the North Atlantic due to Laurentide ice-sheet retreat from the Lake Superior basin, which caused a reduction in Atlantic meridional overturning circulation. Alternative models argue that Lake Agassiz triggered the Younger Dryas via northwestward routing to the Arctic Ocean. We present new 10 Be surface exposure ages that directly date ice retreat from eastern Lake Agassiz outlets and show that the area was ice free at the onset of the Younger Dryas. The southernmost eastern channels opened at 14.0 ± 0.4 ka and 13.6 ± 0.2 ka, but an ice-free route through the Lake Superior basin only opened after 13.5 ± 0.5 ka. The main eastern channel to the eastern Great Lakes and North Atlantic opened at 13.0 ± 0.1 ka to 12.7 ± 0.3 ka. This channel opening was concurrent with decreased runoff to the Gulf of Mexico and increased runoff through the lower Great Lakes to the Gulf of St. Lawrence and North Atlantic. Gulf of St. Lawrence runoff records and isostatic-rebound modeling suggest eastern outlet abandonment at ca. 12.2 ka, with possible northwestward routing of runoff. Our results confirm that Lake Agassiz overflow could have been routed eastward to the North Atlantic at the Younger Dryas onset and caused the canonical abrupt climate change event.

PALSEA2 2015 Workshop; Tokyo, Japan, 22–24 July 2015