By taking no serious action to slash carbon pollution and put the world on a path to 2°C warming (or less), humanity is voluntarily choosing to sharply boost the chances of the worst kinds of droughts — including the kind of multi-decade megadroughts that in the past have overturned entire civilizations.
In their self-described “conservative” new study, “Assessing the risk of persistent drought using climate model simulations and paleoclimate data,” scientists have now quantified the risk of devastating, prolonged drought in the southwestern U.S. (and the world) due to global warming.
The researchers from Cornell, University of Arizona and U.S. Geological Survey conclude that “the risk of a decade-scale megadrought in the coming century [in the SW] is at least 80 percent, and may be higher than 90 percent in certain areas.”
Here is the risk (averaged over each state) of a decade-long drought occurring sometime within the coming century:
Yes, some of the most populated and arable land in the country faces a substantial risk of a drought lasting 10 years. The risk for California has been averaged over the entire state, but most of southern California faces a risk similar to that of the rest of the Southwest.
The authors note that because of some simplifying assumptions that they made, “the view of risk presented here is quite conservative.” For instance, as I discuss below, the study focuses on precipitation and not rising temperatures, which worsen any drought, especially the prolonged ones.
The study looks at two scenarios where the nation and the world take very aggressive action to keep carbon pollution levels low this century (450 and 500 parts per million of total greenhouse gases in the atmosphere) as well as the business-as-usual scenario where we keep taking no serious action (RCP8.5).
Here are the risks of a megadrought lasting 35 years or longer this century for each of those scenarios:
The risks of a devastating U.S. megadrought this century are thus quite substantial in the scenario where we keep doing little or nothing to slash carbon pollution. If that isn’t alarming enough, the authors point out:
We extend our analysis of megadrought risk in the western US to the rest of the world by examining raw [model] estimates of decadal drought and multi-decadal megadrought from the three RCP [emissions] scenarios. Risks throughout the subtropics appear as high or higher than our estimates for the US Southwest (e.g., in the Mediterranean, western and southern Africa, Australia, and much of South America).
This alarming finding is consistent with a great deal of recent research (see here). For instance, an important, if under-reported, 2012 study from the the National Center for Atmospheric Research “strengthened the case” that, unless we reverse emissions trends soon, we risk having a situation by the end of the century where “most of southern Europe and about half of the United States is gripped by extreme drought” a great deal of the time.
In a USA Today article, lead author Toby Ault of Cornell calls megadroughts the “great white sharks of climate: powerful, dangerous and hard to detect before it’s too late. They have happened in the past, and they are still out there, lurking in what is possible for the future, even without climate change.” Indeed, Ault labels megadroughts “a threat to civilization.”
Actually, the business-as-usual future is even more dangerous than this new study finds because of how conservative it is. The authors “based our analysis on precipitation projections.” They don’t look at the impact of “increases in temperatures,” which, of course, worsen any drought and lead to more evapotranspiration of surface moisture. They note that other studies have looked at “precipitation minus evapotranspiration,” which is the overall impact on soil moisture:
Such studies have found that drought conditions like the Dust Bowl will become normal in the Southwest and in other subtropical dry zones. If such transitions are indeed “imminent,” as stated in those studies, then the risk of decadal drought is 100 percent, and the risk of longer-lived events is probably also extremely high. By orienting our analysis around precipitation, the risks of prolonged drought we show here are in fact the lowest levels consistent with model simulations of future climates.
So this study is a best case scenario. So is their conclusion that “the probability of even longer events (> 50 year, or “permanent” megadrought) is non-negligible (5–10 percent).”
What are the implications of megadrought for the nation and the world? Well, what’s going on in California right now shows how devastating even a multi-year drought can be (see also “Study Ties Epic California DroughtTo Manmade Climate Change”).
For the kind of Dust-Bowlification caused by a megadrought, what does the word “adaptation” even mean? As I wrote in “The Next Dust Bowl,” my invited Comment piece for the journal Nature that reviewed the literature on prolonged drought:
Human adaptation to prolonged, extreme drought is difficult or impossible. Historically, the primary adaptation to dust-bowlification has been abandonment; the very word ‘desert’ comes from the Latin desertum for ‘an abandoned place’. During the relatively short-lived US Dust-Bowl era, hundreds of thousands of families fled the region. We need to plan how the world will deal with drought-spurred migrations and steadily growing areas of non-arable land in the heart of densely populated countries and global bread-baskets. Feeding some 9 billion people by mid-century in the face of a rapidly worsening climate may well be the greatest challenge the human race has ever faced.
To have any realistic chance at meaningful adaptation, we must start slashing emissions now.