Accounting for nearly half of all water in our oceans, the Pacific is a vast reservoir of heat and CO₂. However, the slow circulation of the North Pacific today curtails thermal exchanges with other regions and moderates the climatic impact of this ocean at high latitudes.

With the help of marine sediment cores taken from the bottom of the ocean, researchers have been able to reconstruct the circulation and climate of the North Pacific during the last glacial maximum (approximately 20,000 years ago). Today's circulation patterns are radically different from back then, when powerful currents warmed the region around the present-day Bering Sea. The discovery of this warming, which created favorable conditions for human settlement, helps solve the mystery of the first human colonization of this area.

According to genetic studies, the first inhabitants of the Americas lived free of any exchanges with other humans for several thousand years, at the height of the last ice age, before dispersing through the Americas. This hypothesis, called the "Beringian Standstill", raises an important question: where did this population live after its separation from its Asian counterparts and before its migration across the Americas (made possible by deglaciation)?

The new research suggests that these first Americans may have lived in a relatively warm refuge south of Beringia, on land now submerged by the Bering Sea.

"The warm currents revealed by our data may have created a much more pleasant climate than previously thought," says Will Gray, a researcher at the LSCE. "In the coastal regions of the North Pacific, this milder climate may have fostered more temperate ecosystems and allowed humans to survive a very hostile ice age. Moreover, our computer simulations show that changes in ocean circulation during the ice age would have increased the ocean's ability to trap CO₂ at depth, contributing to the decrease in atmospheric CO₂ observed during the ice age."

This work shows to what extent the earth's climate system is dynamic: changes in oceanic and atmospheric circulation can have major impacts on the habitability of a region. This approach is relevant for understanding how different regions will be affected by climate change in action.

This work was carried out by the School of Earth and Environmental Sciences at St Andrews, in Scotland.