STEVE INSKEEP, HOST:
Nothing is more disposable than a news headline, which can be outdated in an instant. But with this next story, we'll take a longer view. It's a story of the last big Ice Age which ended about 11,000 years ago. Retreating glaciers made a lot more of the world livable for humans. But what caused the thaw is not exactly clear, even today. New research suggests a wobble in the Earth kicked off a complicated process that changed the whole planet.
NPR's Christopher Joyce reports.
CHRISTOPHER JOYCE, BYLINE: Ice tells the history of the Earth's climate. Air bubbles in ice reveal what the atmosphere was like, what the temperature was. Scientists can read ice that's been buried for thousands of years. But when it comes to the last Ice Age, ice has a mixed message.
The conventional wisdom is that carbon dioxide increased in the atmosphere starting about 19,000 years ago. Then the ice melted. Logical conclusion: greenhouse effect. But the Antarctic was getting warmer even before CO2 levels went up. So which came first in the Antarctic: warming or CO2?
JEREMY SHAKUN: The problem is its just one spot on the map. And it's a dicey way to slice up global climate change by looking at one point.
JOYCE: That's climate scientist Jeremy Shakun of Harvard University. He went way beyond the Antarctic. He collected samples of ice, rock, and other geologic records from 80 places around the world. He found that rising CO2 levels did, in fact, precede global warming.
Here's his scenario for what killed the Ice Age, published in the journal Nature this week. About 20,000 years ago, the Earth - the whole planet - wobbled on its axis. That happens periodically. But this time, a lot more summer sunlight hit the Northern Hemisphere as a result. Gigantic ice sheets in the Arctic and Greenland melted.
SHAKUN: That water is going to go into the North Atlantic. And that happens to be the critical spot for this global conveyer belt of ocean circulation.
JOYCE: The conveyer belt is how scientists describe the huge, underwater loop-de-loop that water does in the Atlantic. Cold Arctic water sinks and moves south, warm water in the southern Atlantic moves north. Trouble is that sudden burst of fresh melt-water didn't sink. So the conveyer belt stopped.
SHAKUN: It's like, you know, sticking a fork in the conveyer belt at the grocery store - the thing just jams up. It can't keep sinking and the whole thing jams up.
JOYCE: So, warm water in the south Atlantic stayed put. It made the Antarctic warmer. Eventually, ocean currents and wind patterns changed and carbon dioxide rose up out of the Southern oceans and into the atmosphere.
Eric Wolff is a climate scientist at the British Antarctic Survey. He not convinced a wobble was actually the trigger. The Earth had wobbled before and not melted the ice. But he says, whatever did start the process during the Ice Age, the subsequent increase in CO2 did create a runaway greenhouse effect worldwide.
DR. ERIC WOLFF: The CO2 increase turned what initially was a southern hemisphere warming into a global warming. That's a very nice sequence of events to explain what happened between about 19,000 and 11,000 years ago.
JOYCE: That took about 8,000 years. Shakun's research found that the amount of CO2 it took to end the ice age is about the same as humans have added to the atmosphere in the past century and a half.
Christopher Joyce, NPR News.
INSKEEP: The long view on MORNING EDITION from NPR News.
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