By Ellen Leventry
periodiCALS, Vol. 6, Issue 2, 2016
Toby Ault, assistant professor of earth and atmospheric sciences, caused a big splash in 2015 when he and co-authors from Columbia University and NASA published a paper showing that, because of climate change, a megadrought will likely occur later this century throughout the Southwest and Great Plains and could last three decades. Subsequent work has found that taking action now to lower greenhouse gas emissions could lower that risk.
Q: Are megadroughts a new phenomenon?
A: We know megadroughts have happened in the past from both the geologic record and the “tree ring record” of the American Southwest. During sequences of exceptionally dry years, tree rings tend to be relatively narrower than in wet years. In fact, we know that several megadroughts occurred in North America during medieval times (1100–1300 CE). They have also been linked to the demise of several pre-industrial civilizations, including the Khmer Empire of Cambodia and the Mayans. Normally, megadroughts are extremely rare events—maybe occurring only once or twice per millennium, but megadrought risk will increase because of climate change.
Q: Is it possible to decrease the risk of megadrought?
A: Megadrought risk depends so strongly on temperature that—surprisingly—precipitation could increase and megadrought risk should still go up. This is because there is a constant “tug of war” between precipitation supply and evaporation demand. The increases in average regional temperatures due to climate change could be so dramatic—more than 7.2ºF—that evaporation wins out, which in turn dries out the land surface and makes megadroughts much more likely. The good news is that because it is so temperature dependent, an aggressive strategy for cutting greenhouse gas emissions—including reducing the use of fossil fuels—could keep regional temperature changes below 3.6ºF. This lower level of total average warming cuts megadrought risk almost in half compared to predictions based on continued high emissions.
Q: What do the predictions mean for water conservation efforts?
A: Because some models for the Southwest predict a higher likelihood of megadrought, despite precipitation increases, our study serves as a cautionary note for areas like the Northeast that expect to see more rainfall in the future. Furthermore, even if we aggressively reduce carbon emissions, megadrought risk doesn’t just go away. Efficient uses of water resources are critical to help all areas of the country thrive in a changing climate.