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Over the past 50 million years, the Earth's climate has been cooling (Fig. 1). Although Antarctica has been glaciated for at least the past 35 million years, large ice sheets did not appear in the Northern Hemisphere until about 2.7 million years ago. Earth scientists largely agree that overall climate cooling is associated with decreasing levels of carbon dioxide in the atmosphere. and that ice sheets can only grow if sufficient moisture is available and winter snow survives the summer heat.
Geochemical evidence suggests that, 2.7 million years ago, the seasonal temperature contrast of the subarctic Pacific Ocean sea surface became larger as summers warmed and winters cooled. Warmer summer sea-surface temperatures result in a warmer atmosphere that can hold more moisture. Like a snow gun blasting away at ski slopes, westerly winds blow the moisture onto the cold North American continent where it falls as snow and accumulates as ice.
Haug et al. have combined geochemical expertise with numerical modelling to present an integrated approach to the origin of the ice ages. Evidence comes from the floor of the subarctic Pacific Ocean, on which the remains of certain species of marine plankton (diatoms, coccolithophores and foraminifera) have accumulated over time. The primary evidence for summertime warming 2.7 million years ago stems from the biochemistry of coccolithophores, which varies according to temperature. Augmenting this well-established index are the 18O/16O ratios in the siliceous tests of diatoms, a comparatively more complex measure of palaeotemperatures.
What, then, caused the sudden increase in late summer temperatures?[...]At 2.7 million years ago, the abundance of diatom remains plummets, suggesting a decrease in the nutrient availability, like that brought about by the sea surface being cut off from the deeper ocean, at least on a seasonal basis. At the same time, the reduction in vertical mixing allows the sea surface to warm.




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