While medieval Europe was recovering quite well from a long period of excessive cold and rain, things were a little different in the land that would one day become the Southwestern United States.

Between the 9th and 15th centuries, about a dozen megadroughts—that is, droughts that lasted for decades—struck the American Southwest. Scientists are struggling to understand why these periods of extremely dry weather happened, and why they mysteriously stopped around the year 1600.

A study from Columbia University, recently published in the journal Science Advances, has provided the first comprehensive theory for why there were megadroughts in the Southwest. The study’s authors found that ocean temperature conditions plus a phenomenon called high radiative forcing, a situation where the Earth absorbs more sunlight than it radiates back into space, were important to the phenomenon.

The Columbia scientists looked at the way multiple factors from the global climate system—not just ocean temperatures and radiative forcing—worked together and have projected that a warming climate may bring a new series of megadroughts to the American Southwest. By reconstructing aquatic climate data and sea surface temperatures from the last 2,000 years; they found that radiative forcing, severe and frequent La Niña events, cool tropical Pacific sea temperatures and warm conditions in the Atlantic were the triggering events.

High radiative forcing, probably due to an increase in solar activity and a decrease in volcanic activity, dried out the American Southwest. The increase in heat from the sun most likely led to greater water evaporation. At the same time, warmer sea surface temperatures in the Atlantic, combined with those very strong and frequent La Niña events, decreased precipitation even more, further drying the Southwestern deserts. The scientists believe that the La Niña events are more than twice as important in causing megadroughts.

Despite this new theory, scientists say these events still remain difficult to predict. It’s hard to model future El Niño and La Niña events, for example. Nevertheless, the researchers do say that anthropogenic climate change is making megadroughts more likely in the future.

“Because you increase the baseline aridity, in the future when you have a big La Niña or several of them in a row, it could lead to megadroughts in the American West,” said lead author Nathan Steiger, a Columbia hydroclimatologist.

During the medieval years, megadroughts were caused by natural climate variations, but today we are experiencing increased dryness around the world due to human-caused climate change. This, the researchers, could lead to an increased possibility of excessive dryness in the future due to greater aridity.

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