Carbon dioxide has been the main factor controlling global temperatures for millions of years, according to a new study.
A research team led by Brown University says the climate in the tropics has for 2.7 million years changed in step with the spread and retreat of northern ice sheets.
"We think we have the simplest explanation for the link between the Ice Ages and the tropics over that time and the apparent role of carbon dioxide in the intensification of Ice Ages and corresponding changes in the tropics, but we don't know why," said Timothy Herbert of Brown University. "The answer lies in the ocean, we're pretty sure."
The team analyzed seabed cores from the Arabian Sea, the South China Sea, the eastern Pacific and the equatorial Atlantic Ocean.
They showed that climate patterns in the tropics have mirrored Ice Age cycles for the last 2.7 million years, and that carbon dioxide has played the leading role in determining global climate patterns.
Looking at the chemical remains of tiny marine organisms, the scientists were able to work out the ocean surface temperature for the last 3.5 million years - and found that it dropped by one to three degrees C during each Ice Age.
"The tropics are reproducing this pattern both in the cooling that accompanies the glaciation in the northern hemisphere and the timing of those changes," Herbert said. "The biggest surprise to us was how similar the patterns looked all across the tropics since about 2.7 million years ago. We didn't expect such similarity."
The scientists matched the data with records of carbon dioxide levels for the last 800,000 years - spanning the last seven Ice Ages - from ice cores taken in Antarctica.
They found that carbon dioxide in the atmosphere fell by about 30 percent during each cycle, and that most was absorbed by the North Atlantic and the Southern Ocean. This pattern began 2.7 million years ago, and the amount of atmospheric carbon dioxide absorbed by the oceans has intensified with each successive Ice Age.
"It seems likely that changes in carbon dioxide were the most important reason why tropical temperatures changed, along with the water vapor feedback," Herbert said.