How a Warming Earth Once Cooled Off

April 14, 2008 -- New research on how the last great global greenhouse event chilled down shows that it took three separate cooling events to step down the Earth's temperature around 33 million years ago.

The new data comes from a rare conjunction of climate clues, or "proxies," in rocks from an Alabama quarry. As for just what caused the step-wise drop in temperature, that's less certain, although there are some promising candidates.

"We know that this is the biggest climate shift in the Cenozoic," said geoscientist Miriam Katz of Rensselaer Polytechnic Institute in Troy, New York. The Cenozoic is the most recent of the three great geological eras, beginning 65.5 million years ago and running to the present.

The global cooling event, known as the Eocene-Oligocene transition, is one of many great and lesser global climate changes which have occurred over Earth's history -- usually accompanied by mass extinctions. The current global warming is thought to be a natural warming cycle that is being exacerbated by humans releasing millions of years of stored, climate-warming carbon compounds into the atmosphere by way of fossil fuel burning.

The secrets of the Eocene-Oligocene transition could help scientists better understand the details of how Earth's climate can swing from one extreme to another. It's sort of like watching the present case of global warming running in reverse, Katz said.

In that sense, at the very least, it could be relevant to today's climate researchers. Fossil fuel burning, and the release of carbon dioxide and other greenhouse gases, is already projected to take global temperatures well beyond the millennial-scale ice ages and warm periods seen in recent geologic time.

What the Alabama climate proxies show is that the cool-down happened in three rapid steps, geologically speaking. Each step was several thousand years long and resulted in an overall two-degree cooling of the deep oceans -- the largest thermal reservoir on the planet -- and growth of the ice sheets in Antarctica, which were one-quarter more expansive than they are today.