Climate Change Could Revive Medieval Mega Droughts In The US

Around a dozen mega droughts hit the south west of the United States during the ninth to fifteenth centuries, but then mysteriously ceased aroundthe year 1600. What caused this agglomeration of mega droughts, that is, severe droughts that last for decades?

If scientists can understand why mega droughts occurred in the past, itcan help us better predict if, how and where they might occur in thefuture. A study published today in Science Advances provides thefirst comprehensive theory of why there were mega droughts in the south western United States. The authors discovered that ocean temperature conditions plus high radiative forcing, when the Earth absorbs more sunlight than it radiates into space, play an important role in the activation of mega waters. The study suggests an increasing risk of future mega droughts in the south western United States due to climate change.

Previously, scientists have studied the individual factors that contribute to mega droughts. In the new study, a team of scientists from the Lamont-Doherty Earth Observatory at Columbia University has analyzed how multiple factors of the global climate system work together and projected that climate warming can bring a new round of mega droughts.

By reconstructing the data on the aquatic climate and sea surface temperatures over the past 2,000 years, the team discovered three keyfactors that led to mega droughts in the south western United States:radiative forcing, severe and frequent La Niña events, cold temperatures of the surface of the tropical Pacific sea that cause changes in global climatic events and warm conditions in the Atlantic. The high radiative forcing seems to have dried the south west of the United States, probably due to an increase in solaractivity (which would send more radiation to us) and a decrease involcanic activity (which would admit more) at that time. The resulting increase in heat would lead to further evaporation. At the same time, temperatures of the surface of the Atlantic Sea warmer than usual, combined with La Niñas, very strong and frequent, decreased rainfall in the already dry area. Of these three factors, it was estimated that La Niña conditions were more than twice as important to cause mega agües.

While Lamont scientists say they were able to identify the causes of mega meanders in a more complete way than before, they say such eventswill remain difficult for scientists to predict. There are predictions about future trends in temperatures, aridity and sea surface temperatures, but the future activity of El Niño and La Niña remains difficult to simulate. However, theres earchers conclude that human-driven climate change is stacking thecover towards more megabytes in the future.

"Becausethe aridity of the baseline increases, in the future when I have alarge La Niña, or several of them in a row, it could lead to mega droughts in the American West," said lead author NathanSteiger, an Observatory of the Land of Lamont-Doherty hydroclimatologist.

During the time of medieval mega droughts, natural climatic variability caused greater radiative forcing. But today we are experiencing greater dryness in many parts of the world due to the forces created by man. Climate change is setting the stage for a greater possibility of megadroughts in the future through greater aridity, researchers say.

By:Preeti Narula
Content:https://www.sciencedaily.com/releases/2019/07/190724144152.htm