The US southwest could soon be ravaged by decade-long droughts so extreme they have not been experienced for centuries. Researchers examined the cause the 14 megadroughts the area experienced between 800-1600 AD. And the scientists discovered the climatic conditions present in this medieval period are fast returning now, suggesting megadroughts will soon return. The landmark study’s authors discovered ocean temperature conditions plus high radiative forcing – when Earth absorbs more sunlight than it radiates back into space – can together trigger megadroughts.
This indicates an increasing risk of future megadroughts in the American southwest due to man-made climate change.
Scientists have previously studied the individual factors contributing to megadroughts.
The latest study saw Columbia University’s Lamont-Doherty Earth Observatory scientists examine how multiple factors from the global climate system work together.
And they then projected that warming climate may bring a new round of megadroughts.
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The team discovered three key factors leading to megadroughts in the American southwest: radiative forcing, severe and frequent La Niña events and warm conditions in the Atlantic Ocean.
High radiative forcing appears to have dried out the American southwest, due to an increase in solar activity and a decrease in volcanic activity.
The resulting increase in heat would lead to greater evaporation.
Simultaneously, warmer than usual Atlantic sea-surface temperatures combined with very strong and frequent La Niñas decreased rain in the already dried-out area.
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Of these three factors, La Niña conditions are believed to be more than twice as important in causing the megadroughts.
While the Lamont scientists say they were able to pinpoint the causes of megadroughts in a more complete way than has been done before, they say such events will remain difficult for scientists to predict.
Although there are predictions about future trends in temperatures, aridity, and sea surface temperatures, but future El Niño and La Niña activity remains difficult to simulate.
But regardless, the researchers conclude human-driven climate change is stacking the deck towards more megadroughts in the future.
Lead author Dr Nathan Steiger, a Lamont-Doherty Earth Observatory hydroclimatologist, said: “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.
“”Several civilizations are thought to have been dramatically affected by these past droughts including the Maya and the Anasazi.
There is some evidence that the collapse of their civilisations could have been partially driven by megadroughts.”