A dozen megadroughts struck the American Southwest between the 9th and 15th centuries. Scientists warned that it could happen again.

In a new study, scientists at Columbia University's Lamont-Doherty Earth Observatory investigated the factors that caused megadroughts: severe droughts that last for decades. Understanding why megadroughts happened in the past can help scientists predict if it will happen in the future.

Why Megadroughts Happen

To investigate, the team reconstructed aquatic climate data and sea surface temperature from the last 2,000 years. They identified three factors that caused the occurrence of megadroughts in the American Southwest: radiative forcing, severe and frequent La Niña, and warm conditions in the Atlantic.

The scientists explained that high radiative forcing was caused by an increase in solar activity, which would have expelled more radiation toward Earth and a decrease in volcanic activity that can help cool the planet. This led to an increase in heat that caused greater evaporation.

Meanwhile, the sea-surface temperature of the Atlantic and the La Niña events decreased precipitation in areas that are already dry. All three factors combined caused megadroughts that lasted for decades.

The study noted that La Niña was the more important factor that can cause a megadrought.

"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 Nathan Steiger, hydroclimatologist and the lead author of the study.

Climate Change Might Bring Back Megadroughts

In the past, a high radiative force was a result of natural climate variability. However, climate change is not.

Modern climate change is setting up the planet for another megadrought in the American Southwest in the future and no one will be able to predict when it happens.

"Based on modeling studies of future climate, we would expect to see increased drying in the American West as a result of increased CO2, which will make megadroughts more likely," added Steiger to Newsweek. "But the trigger for these megadroughts will most likely be severe and/or frequent La Ninas which are not very predictable at all. So we won't know what decades in the future will experience a megadrought in the American West, but we're unfortunately confident that they'll happen again."

The study was published in Science Advances on Wednesday, July 24.