Simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation

According to Milankovitch theory, the lower summer insolation at high latit udes about 115,000 years ago allowed winter snow to persist throughout summ er, leading to ice-sheet build-up and glaciation(1), But attempts to simula te the last glaciation using global atmospheric models have failed to produ ce this outcome when forced by insolation changes only(2-5). These results point towards the importance of feedback effects-for example, through chang es in vegetation or the ocean circulation-for the amplification of solar fo rcing(6-9). Here we present a fully coupled ocean-atmosphere model of the l ast glaciation that produces a build-up of perennial snow cover at known lo cations of ice sheets during this period. We show that ocean feedbacks lead to a cooling of the high northern latitudes, along with an increase in atm ospheric moisture transport from the Equator to the poles. These changes ag ree with available geological data(10-15) and, together, they lead to an in creased delivery of snow to high northern latitudes, The mechanism we prese nt explains the onset of glaciation-which would be amplified by changes in vegetation-in response to weak orbital forcing.