“Relative impacts of mitigation, temperature, and precipitation on 21st-century megadrought risk in the American Southwest”

Ault, T.R. et al. 2016. Science Advances, vol. 2, e1600873
Megadroughts are comparable in severity to the worst droughts of the 20th century but are of much longer duration. A megadrought in the American Southwest would impose unprecedented stress on the limited water resources of the area, making it critical to evaluate future risks not only under different climate change mitigation scenarios but also for different aspects of regional hydroclimate. We find that changes in the mean hydroclimate state, rather than its variability, determine megadrought risk in the American Southwest. Estimates of megadrought probabilities based on precipitation alone tend to underestimate risk. Furthermore, business-as-usual emissions of greenhouse gases will drive regional warming and drying, regardless of large precipitation uncertainties. We find that regional temperature increases alone push megadrought risk above 70, 90, or 99% by the end of the century,
even if precipitation increases moderately, does not change, or decreases, respectively. Although each possibility is supported by some climate model simulations, the latter is the most common outcome for the American Southwest in Coupled Model Intercomparison 5 generation models. An aggressive reduction in global greenhouse gas emissions cuts megadrought risks nearly in half.

In a week of headlines from California about flooding, mudslides, and a failing dam, it may seem counterintuitive to bring up the issue of drought. But I do want to bring up drought again . . . and not just any drought, but a catastrophic megadrought. As in: very large and very bad. I briefly visited this topic in a previous post about California’s drought—where, I want to note, notwithstanding this year’s precipitation filling up reservoirs, the depletion of the groundwater will take decades to reverse. But as the article I’m presenting now demonstrates, the problem certainly warrants much more discussion.

Toby Ault, a climatologist from Cornell, and his team used a modeling approach to predict the likelihood of future megadroughts in the Southwest. (I assumed the paper was worth reading because it came out in an excellent journal, Science Advances.) Their predictions are troubling. A megadrought in their definition is a very long period (35 years or more) of drought. Obviously, such droughts are almost of biblical dimension and would have a major impact on humans and the environment. I am not a climatologist, but as a biologist I am interested in climate because – together with the weather – it sets the conditions for evolution, and defines parts of the ecological niche of organisms.

But back to the paper: in their article the Ault et al. try to predict the likelihood of massive drought events in the Southwest under a couple of different scenarios. In their predictive models they consider mainly temperature and precipitation as factors. And they also model different scenarios for the amounts of greenhouse gases emitted. Here is the bottom line: without a very serious reduction of our greenhouse gas emissions megadroughts are virtually certain for the Southwest. In the authors words: “In a business-as-usual world (…), rising temperatures alone are sufficient to drive megadrought risks to unprecedented levels” (p. 5).

The recent drought has pushed California almost to the limits, but has also triggered or rekindled a major political dialogue about water. Under pressure from nature mindsets have changed, new policies were devised and implemented. But does this prepare the Southwest for droughts that last a whole generation?

Studies like the Ault et al. paper provide the factual basis for policy-makers’ decisions, but it is the whole population that needs to act. With unprecedented disasters looming, what social and political mechanisms do we have to cope? How can we create resilience to keep the Southwest a habitable area? What would the consequences be if there was a climate induced migration with large numbers of people leaving California? Those consequences would be nationally significant, because California is an economic giant. Are there engineering solutions to decreased water supply available either locally or globally? And finally, does the current political situation allow for a healthy, open-minded discussion of the problem?

One could, of course, argue that because the paper is based on modeling it is full of uncertainties and that there will be scientific debate about the findings. The authors talk about risks, not certainties. But should we use this as an excuse to ignore the prospect of megadrought? I don’t think this would be a sustainable approach. Just as the Navy is preparing for sea-level rise, the Southwest ought to prepare for the likely megadrought. Scientists have an increasingly important role to play in this preparation, both in assessing the risks, suggesting ways of mitigating them, and planning for necessary adaptations to their impacts if they come to pass.

Indeed, more scientists seem to coming out and finding their voices—interestingly, one of the responses to the current administration has been an open politicization of science. Fearful of losing their traditional role in society and feeling attacked by the Trump administration, suddenly scientists are willing to take to the streets and stand up for their, and the public’s, interests. After adding “citizen scientists” to the vocabulary, it seems we are now adding “scientist citizens.” This is probably not a bad thing.

B. I. Cook, T. R. Ault and J. E. Smerdon. 2015. ‘Unprecedented 21st century drought risk in the American Southwest and Central Plains.’ Science Advances, vol. 1, e1400082. An earlier paper to which Ault contributed.
A video of Ault discussing his research at Cornell.

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