Ancient rainfall records stretching 550,000 years into the past may upend scientists’ understanding of what controls the Asian summer monsoon and other aspects of the Earth’s long-term climate, reports a University of Arizona-led international team of researchers in the May 25 issue of the journal Science.

The standard explanation of the Earth’s regular shifts from ice ages to warm periods was developed by Milutin Milankovitch in the 1920s. He suggested the oscillations of the planet’s orbit over tens of thousands of years control the climate by varying the amount of heat from the sun falling above the Arctic Circle in the summer.

“Here’s where we turn Milankovitch on its head,” said first author J. Warren Beck, a UA research scientist in physics and in geosciences. “We suggest that, through the monsoons, low-latitude climate may have as much effect on high-latitude climate as the reverse.”

During the northern summer, the subtropics and tropics north of the equator warm and the tropics and subtropics south of the equator cool.

Modern observations show the difference in heat propels atmospheric changes that drive the intensity of the monsoon. Beck said the monsoon can affect wind and ocean currents as far away as the North Atlantic and Arctic Oceans.

The Asian monsoon season is the biggest annual rainfall system on Earth and brings rainfall to about half the world’s population. The monsoon season occurs approximately April to September.

Beck and his colleagues found that over tens of thousands of years the changes in the intensity of the Asian summer monsoon corresponded to the waxing and waning of the polar ice caps.

The researchers suggest those long-term changes in the monsoon drove global changes in wind and ocean currents in ways that affected whether the polar ice caps grew or shrank.

Beck said this new explanation of the Earth’s past climate cycles will help climate modelers figure out more about the world’s current and future climate.

The new explanation of what drives the Earth’s climate system stems from a decade-long effort by Beck and his colleagues to develop a new record of rainfall in Asia reaching far back into the past.

Read more at University of Arizona