Correlation of paleoclimatic evidence with orbital changes shows that the build-up of polar ice accelerated when low obliquity coincided with perihelion in Northern Hemisphere winter. Under low obliquity the insolation was channeled to the tropics at the expense of both polar caps. As perihelion moved from winter solstice toward spring equinox, the solar beam in astronomic winter and spring became stronger than in summer and autumn. This orbital configuration under climate conditions like today would lead to warming of tropical oceans but cooling of the polar regions. The areally weighted global mean surface temperature, which is dominated by the low latitudes, would increase. Consequently, during the first millennia, the early glacial ice build-up was most likely accompanied by global warming. It was the associated increase of meridional insolation and temperature gradients, which were instrumental in the transition to a glacial. A significant part of the current global warming is due to the gradual temperature increase of the tropical oceans. As the changing orbital configuration today resembles that of the last interglacial/glacial transition, the warming is likely to have a natural component. © 2005 Elsevier Ltd. All rights reserved.
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