2.75 million years ago


Climate Change – News and Comments and Updating The Chilling Stars

Why the big freeze 2.75 million years ago?

The CO2 folk are flummoxed. In the current issue of Science (14 May) William F. Ruddiman of the University of Virginia wrings his hands over the mismatch between unchanging carbon dioxide levels and the drastically cooling climate over the past 20 million years. “Major glaciations began in the Northern Hemisphere around 2.75 million years ago, after a long prior interval of climatic cooling,” Ruddiman says, “… but our understanding of the driving forces behind the cooling remains incomplete.”

For Henrik Svensmark and me, an explanation for that big freeze of 2.75 million years ago is the “jewel in the crown” of climate history, because of its importance for the subsequent origin of the first human beings.Here’s a picture from The Chilling Stars of one of the earliest known stone tools, which were made less than 200,000 years after the big freeze began.

But if you want CO2 to be the big driver of climate change, as Ruddiman evidently does, the commonly used data are disobedient. Here are his graphs.

(A) Oxygen-18 index of deep-ocean temperature and ice volume (after Miller & Fairbanks1987 and Zachos et al. 2001). (B) Estimates of past CO2 concentrations from alkenones (after Pagani et al. 2005). Source: W.F. Ruddiman, Science, 14 May 2010, p.839.

Ruddiman thinks that the data must be wrong. He suggests pushing the CO2 up a little, 20 to 10 million years ago (using boron/calcium ratios) and finding a decline between 5 and 2 million years ago in new alkenone data. He concludes, “Geochemists still have work to do in refining the CO2 proxies.”

But that would be wasted effort if CO2 were not the driver. In The Chilling Stars Henrik Svensmark and I tell a completely different story about what was happening 2.75 million years ago. In Postscript 2008 we relate how the Sun and Earth, wandering through the Galaxy, blundered into a region of space packed with extra cosmic rays – just what was required to chill the world by making more low clouds, in accordance with the Svensmark hypothesis.

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