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. Temperature reconstructions from the Northern Hemisphere (NH)generally indicate cooling over the Holocene, which is often attributed todecreasing summer insolation. However, climate model simulations predictthat rising atmospheric CO2 concentrations and the collapse of theLaurentide Ice Sheet caused mean annual warming during this epoch. Thiscontrast could reflect a seasonal bias in temperature proxies, andparticularly a lack of proxies that record cold (late fall–early spring)season temperatures, or inaccuracies in climate model predictions of NHtemperature. We reconstructed winter–spring temperatures during the CommonEra (i.e., the last 2000 years) using alkenones, lipids produced byIsochrysidales haptophyte algae that bloom during spring ice-out, preservedin sediments from Vestra Gíslholtsvatn (VGHV), southwest Iceland. Ourrecord indicates that winter–spring temperatures warmed during the last2000 years, in contrast to most NH averages. Sensitivity tests with a lakeenergy balance model suggest that warmer winter and spring air temperaturesresult in earlier ice-out dates and warmer spring lake water temperatures and therefore warming in our proxy record. Regional air temperatures arestrongly influenced by sea surface temperatures during the winter and springseason. Sea surface temperatures (SSTs) respond to both changes in ocean circulation and gradualchanges in insolation. We also found distinct seasonal differences incentennial-scale, cold-season temperature variations in VGHV compared toexisting records of summer and annual temperatures from Iceland.Multi-decadal to centennial-scale changes in winter–spring temperatures werestrongly modulated by internal climate variability and changes in regionalocean circulation, which can result in winter and spring warming in Icelandeven after a major negative radiative perturbation.

Winter–spring warming in the North Atlantic during the last 2000 years: evidence from southwest Iceland