Lower Sea Levels Influenced Tropical Climate During Ice Age

A vast pool of warm water stretches along the equator from Africa to the western Pacific Ocean. Now, scientists have discovered that this warm water, known as the Indo-Pacific warm pool, may reveal clues about the climate during the last ice age.

In this new study, researchers investigated preserved geological clues (called "proxies") of rainfall patterns that occurred during the last ice age. At the time, the Earth was dramatically cooler than it is today. Massive glaciers covered much of the planet, and the Arctic ice sheet extended much further than it does today. The researchers then compared these rainfall patterns with computer model simulations in order to find a physical explanation for the patterns inferred from the proxies.

"For our research, we compared the climate of the ice age with our recent warmer climate," said Pedro DiNezio, lead author of the study, in a news release. "We analyzed about 100 proxy records of rainfall and salinity stretching from the tropical western Pacific to the western Indian Ocean and eastern Africa. Rainfall and salinity signals recorded in geological sediments can tell us much about past changes in atmospheric circulation over land and ocean respectively."

So what did they find? The researchers discovered that much of the Indo-Pacific warm pool was far drier during this ice age. They also found that several regions, such as the western Pacific and western Indian Ocean were, surprisingly, wetter.

The researchers weren't done yet, though. They then matched up the rainfall and salinity patterns with simulations from climate models. They found that only one model of the 12 they used showed statistical agreement with the proxy-inferred patterns of the rainfall changes. The model that did agree, though, also corresponded with the rainfall and salinity indicators--two entirely independent sets of proxy data covering distinct areas of the tropics.

What does that mean exactly? The dry climate during the ice age was driven mainly by reduced convection over a region of the warm pool called the Sunda Shelf. Today, the shelf is submerged beneath the Gulf of Thailand. In the past, though, this shelf was above sea level when the oceans were almost 400 feet lower than they are today.

"Our research resolves a decades-old question of what the response of tropical climate was to glaciation," said DiNezio. "The study, moreover, presents a fine benchmark for assessing the ability of climate models to simulate the response of tropical convection to altered land masses and global temperatures."

The findings were published in the journal Nature Geoscience.