Atmospheric CO2 levels have played the lead role in dictating global climate patterns for at least the past 2.7 million years, and continue to do so today, researchers have argued this week. A team led by Brown University’s Timothy Herbert has made this conclusion after showing that sea surface temperatures in the tropics have varied over this period in parallel with climate change at the poles.
The geologists used rock samples as a measure of the sea surface temperature records in the tropics. They found that, from about 2.7 million years ago, tropical ocean surface temperatures dropped by 1-3ºC during each Ice Age as ice sheets were spreading and oceans cooling across the Northern Hemisphere. When Ice Age temperature cycles switched from roughly 41,000-year to 100,000-year intervals, so did the tropics. “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.
Climate scientists had previously determined that as temperatures fell over this period, so did the amount of CO2 in the atmosphere, as the gas was absorbed by oceans near the poles. Writing in leading journal Science yesterday, Herbert and colleagues interpret the similarity between polar and tropical temperatures as reflecting the driving role of the atmosphere, and the gases in it. “It seems likely that changes in carbon dioxide were the most important reason why tropical temperatures changed,” Herbert said. Read the rest of this entry »