As California is entering its sixth year of drought, things aren’t looking good for the Golden State. But as the planet continues to warm, its chances of being hit by an even bigger drought this century, one lasting 35 years or more and known as a "megadrought", increases by 50 percent. The impact of this on California would be drastic, as the current one is already wreaking havoc.
“The increase in risk is not due to any particular change in the dynamic circulation of the atmosphere,” said Toby Ault, co-author of the study that initially reported on the increasing risk, and lead of another study looking at how to mitigate it. “It’s because the projected increase in atmospheric demand for moisture from the land surface will shift the soil moisture balance. If this happens, megadroughts will be far more likely for next millennium.”
Ault explains how the increase in regional air temperatures, which could be more than 4°C (7.2°F), would mean that the balance between rain and evaporation will be shifted to favor the latter. Such a dramatic and extreme rise in temperatures, Ault says, would see the land surface dry out to such an extent that there would be an astonishing 70 to 99 percent chance that a megadrought would hit. “Megadroughts are rare events, occurring only once or twice each millennium,” explained Ault. “In earlier work, we showed that climate change boosts the chances of a megadrought.”
In a newer study, published in Science Advances, Ault has found that even though it might not sound like it, the fact that the severity of the drought is dictated by variations in the air temperature is a good thing. This is because this, in turn, is impacted by the concentrations of carbon dioxide in the atmosphere, and that is something we can do something about. If we implemented aggressive policies to keep the region's temperatures in the Southwestern US below a rise of 2°C (3.6°F), this risk of experiencing a megadrought is cut in half.
Whether or not it will be possible to keep climate change to within 2°C is another question entirely, but with the Paris climate agreement set to come into force by next month, it seems that the wheels may be set in motion to at least attempt to do so.