Cold season warming in the North Atlantic during the last 2000 years: Evidence from Southwest Iceland

Abstract. Temperature reconstructions from the Northern Hemisphere (NH) generally indicate cooling over the Holocene which is often attributed to decreasing summer insolation. However, climate model simulations predict that rising atmospheric CO2 concentrations and the collapse of the Laurentian ice sheet caused mean annual warming during this epoch. This contrast could reflect a bias in temperature proxies, and particularly a lack of proxies that record cold (late fall–early spring) season temperatures, or inaccuracies in climate model predictions of NH temperature. We reconstructed winter–spring temperatures during the Common Era (i.e. the last 2000 years) using alkenones, lipids produced by Isochrysidales haptophyte algae that bloom during spring ice-off, preserved in sediments from Vestra Gislholtsvatn (VGHV), southwest Iceland. Our record indicates cold-season temperatures warmed during the last 2000 years, in contrast to NH averages. Sensitivity tests with a lake energy balance model show that this warming is likely driven by increasing winter–spring insolation. We also found distinct seasonal differences in centennial-scale, cold-season temperature variations in VGHV compared to existing records of summer and annual temperatures from Iceland. Sustained or abrupt cooling in VGHV temperatures are associated with the cumulative effects of solar minima and volcanic eruptions, and potentially ocean and sea-ice feedbacks associated with cooling in the broader Arctic. However, multi-decadal to centennial-scale changes in cold season temperatures were strongly modulated by internal climate variability, i.e. the North Atlantic Oscillation, which can result in winter warming in Iceland even after a major negative radiative perturbation.