Rising levels of carbon dioxide have for the first time been definitively linked to the global warming that led to the last Ice Age.
A team at Oregon State University reconstructed globally averaged temperature changes during the end of the last Ice Age, in contrast to previous studies, which only compared CO2 levels with local temperatures.
They found that average temperature around the Earth correlated with – and generally lagged behind – rising levels of CO2.
"Carbon dioxide has been suspected as an important factor in ending the last Ice Age, but its exact role has always been unclear because rising temperatures reflected in Antarctic ice cores came before rising levels of CO2, says Jeremy Shakun, a former doctoral student at OSU and now a post-doctoral fellow at Harvard and Columbia.
"But if you reconstruct temperatures on a global scale – and not just examine Antarctic temperatures – it becomes apparent that the CO2 change slightly preceded much of the global warming, and this means the global greenhouse effect had an important role in driving up global temperatures and bringing the planet out of the last Ice Age."
The theory is that the Earth's natural wobble affected the amount of sunlight striking the northern hemisphere, melting ice sheets that covered Canada and Europe. Fresh water flowed into the Atlantic Ocean, where it formed a lid over the sinking end of the Atlantic Meridional Overturning Circulation.
This put an end to the movement of warm water up from the tropics which had delivered heat to the northern latitudes.
The discovery has implications, of course, for how human-generated carbon dioxide will affect the planet in future.
"CO2 was a big part of bringing the world out of the last Ice Age, and it took about 10,000 years to do it. Now CO2 levels are rising again, but this time an equivalent increase in CO2 has occurred in only about 200 years, and there are clear signs that the planet is already beginning to respond," says Shakun.
"While many of the details of future climate change remain to be figured out, our study bolsters the consensus view that rising CO2 will lead to more global warming."