Researchers have found that glaciation of the northern hemisphere was initiated recently in geological terms. They have also found that it may well
have been triggered
by seasonal warming, indicated by a change in the diatom paleo-population.
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
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.
Please visit the link provided for the complete story.
Another facet in the complex and sometimes counter intuitive question of climate behaviour.
I am a bit perturbed that the author of the article concludes that this is an 'exemplary' study, even tho all the 'individual climate indicators
may not have withstood the uncertainties and assumptions that limit each of them', however the study seems reasonable enough.
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[edit on 25-2-2005 by Banshee]