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.
"What was special about our location is we can get three proxies in one place," Katz told Discovery News. She and her colleagues published their discovery in the April 13 issue of Nature Geoscience.
The first line of evidence is oxygen-18, which is trapped in the fossil shells of microscopic organisms called forams. During cold times there is more oxygen-18 in the oceans because the lighter oxygen-16 evaporates more readily and ends up locked up in polar ice. So an up-tick in the oxygen-18 in the shells over time is a strong signal that the climate is cooling globally.
The proxy is the relative amount of magnesium and calcium in the shells that formed in waters once more than 650 feet (200 meters) deep. It turns out that the amounts of those two elements, used by animals to make their shells, are closely tied to temperature. So the magnesium-calcium ratios in the shells reveal ancient seafloor temperatures.
Finally, there are the rocks themselves. Such things as silty, deep-water muds and near-shore sands reveal how deep the water was and suggest a 300-foot (100-meter) drop in sea level. That matches what's expected to happen on a cooling planet where lots of water is being locked up in ice sheets.
"As far as I know, this is the first study to measure all three proxies from one study site," said earth scientist Katharina Billups of the University of Delaware. "So all in all, yes, I think it is unusual that they were able to do this."
As for what might have caused the step-wise cooling, that's a tougher question. One likely factor was the opening of the Drake Passage, which severed the connection between South America and Antarctic.
That allowed ocean currents to encircle Antarctica and cool it down dramatically.
Another suspect is what's known as Milankovitch cycles -- combinations of cycles in Earth's orbit and tilt that can periodically reinforce each other to push climate in one direction or another.
"It just kind of nudged it a little more," said Katz of the these cycles.
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