The last big ice age ended about 11,000 years ago, and not a moment too soon — it made a lot more of the world livable, at least for humans.
But exactly what caused the big thaw isn't clear, and new research suggests that a wobble in the Earth kicked off a complicated process that changed the whole planet.
Ice tells the history of the Earth's climate: Air bubbles in ice reveal what the atmosphere was like and what the temperature was. And scientists can read this ice, even if it'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. The logical conclusion? The greenhouse effect.
But the Antarctic was getting warmer even before CO2 levels went up. So which came first in the Antarctic, warming or CO2?
"The problem is, [the Antarctic is] just one spot on the map, and it's a dicey way to slice up global climate change by looking at one point," says Jeremy Shakun, a climate scientist at Harvard University. So he went way beyond the Antarctic — he collected samples of ice, rock and other geologic records from 80 places around the world and found that CO2 levels did, in fact, precede global warming.
Here's his scenario for what killed the ice age, which was 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. Gigantic ice sheets in the Arctic and Greenland melted.
"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," Shakun says.
The conveyer belt is how scientists describe the huge, underwater loop-the-loop that water does in the Atlantic: Cold Arctic water sinks and moves south while warm water in the southern Atlantic moves north.
The trouble is that the sudden burst of fresh meltwater didn't sink, so the conveyer belt stopped.
"It's like, you know, sticking a fork in the conveyer belt at the grocery store," Shakun says. "The thing just jams up; it can't keep sinking, and the whole thing jams up."
So warm water in the south Atlantic stayed put. That 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, a climate scientist at the British Antarctic Survey, isn't convinced a wobble was the trigger — the planet 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 created a runaway greenhouse effect worldwide.
"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," Wolff says.
But that's a process that has taken about 8,000 years. And Shakun's research found that the amount of CO2 it took to end the ice age is about the same amount as humans have added to the atmosphere in the past century.
Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.