Insolation and CO2 contribution to the interglacial climate before and after the Mid-Brunhes Event

Reconstructions of climate from marine sediment(1) and ice(2) cores show that the amplitude of glacial-interglacial climate cycles increased substantially after the Mid-Brunhes Event(3), about 430,000 years ago. Interglacial periods before the event seem to be characterized by larger continental ice sheets, lower sea level(4,5), cooler temperatures in Antarctica2 and lower atmospheric CO2 concentrations(6), relative to the more recent interglacials. Here we use an Earth system model of intermediate complexity to assess the contributions of insolation and greenhouse-gas concentrations to the climate associated with the peaks of all the interglacials over the past 800,000 years. Our simulations recreate the expected warmer interglacials after the Mid-Brunhes Event and suggest that later interglacials are warmer primarily because of increased global mean temperatures during Northern Hemisphere winters. This warmth arises from increased insolation during this season, relative to the interglacials that preceded the Mid-Brunhes Event, in conjunction with increased atmospheric greenhouse-gas concentrations. The effect of boreal winters and of the Southern Hemisphere, which is also warmer during austral winters, on the carbon cyle should be assessed when investigating the underlying causes of the higher CO2 concentrations during the later interglacials.