Greenland Ice Cores Reveal a South-To-North Difference in Holocene Thermal Maximum Timings

Abstract

Holocene temperature evolution remains poorly understood. Proxies in the early and mid-Holocene suggest a Holocene Thermal Maximum (HTM) where temperatures exceed the pre-industrial, whereas climate models generally simulate monotonic warming. This discrepancy may reflect proxy seasonality biases or errors in climate model internal feedbacks or dynamics. Using seasonally unbiased ice core reconstructions at NEEM, NGRIP, and Greenland Ice Sheet Project 2, we identify a Greenland HTM of ∼2°C above pre-industrial, in agreement with other Northern Hemisphere proxy reconstructions. The firn-based reconstructions are verified through borehole thermometry, producing a multi-core, multi-proxy reconstruction of Greenland climate from the last glacial to pre-industrial. HTM timing across Greenland is heterogenous, occurring earlier at high elevations. Total air content measurements suggest a temperature contribution from elevation changes; regional oceanographic conditions, a weakened polar lapse rate, or variable near-surface inversions may also be important sensitivities. Our reconstructions support climate simulations with dynamic Holocene vegetation, highlighting the importance of vegetation feedbacks.