In ancient times, exceptional droughts could last for decades. As the world warms, we expect them to return with a vengeance. 

Overview Despite our advances in technology, agriculture, and governance, drought remains one of the most destructive natural hazards to affect society. But because in most places our memory of environmental history is quite limited, time and again, we underestimate just how severe or persistent extreme drought can be. Long intervals of unbroken drought are commonly part of mythological stories and religious history, with the Old Testament account of seven consecutive years of famine in pharaonic Egypt being the example best known by Western audiences. But during the last few decades, scientists have be able to recover long weather records from tree rings, lake sediments, and other natural archives that demonstrate in many parts of the world severe sustained drought is actually a quite common feature of our local climate. In this project, scientists combined weather records, tree rings, climate models, and statistics to estimate the future risk of megadrought -- droughts more severe or longer lasting than the worst recent droughts in a given place.

Intellectual merit We now have several lines of evidence suggesting that, relatively recently, the western United States was affected by droughts that persisted for several decades. The question is, how was that possible? What could have caused one half of the country to become locked into an extended dry spell lasting for a generation or more?

In physical climatology, there are two distinct schools of thought that have tried to explain this sort of behavior. The first idea is that megadroughts are the consequence of some kind of external factor acting on the Earth's climate. Volcanic eruptions, fluctuations in the Sun's output, and small changes in the Earth's orbit are three examples of external 'forcings', which are known to influence our planet's climate but are themselves not affected by climate in turn. Or it could just be a coincidence. General circulation models (GCMs) use the Navier-Stokes equations to simulate the flow of mass and energy through the atmosphere, ocean, cryosphere, and land surface. But even when the external forcings are kept the same, simulations from GCMs still include megadroughts.

We made a statistical model to test whether megadroughts could occur in the western United States simply due to chance. If our set of simulations did not include instances of megadroughts, we would have concluded that such exceptionally long-lasting droughts are only possible when the Earth's climate undergoes a lasting change. On the contrary, our statistical model ended up producing simulated megadroughts that looked very much like the real thing. In our simulations, megadroughts were just as long-lasting, covered as large an area, and were just as severe as the worst event inferred from tree rings. That result means megadroughts can occur in the western United States even if nothing else in the climate is different -- they really are just a matter of time.

In a business-as-usual world, rising temperatures alone are sufficient to drive megadrought risks in this region to unprecedented levels. Reducing carbon emissions cuts megadrought risks by nearly half, but does not bring those risks back down to pre-industrial levels. So regardless what actions we take to reduce future warming, water management decisions in the Southwest should plan for the possibility the region will be affected by a megadrought before the end of this century.

Broader impacts Paleoclimate research has shown us that long intervals of unbroken drought are not just a common plot device used by myth-makers, but rather are a normal feature of climate in many parts of the world. It very well may be that, by chance, the westward expansion of the United States coincided with a climatic interval that has been, by and large, megadrought-free. But if it is possible that the American West can slip into a decades-long drought even without any unusual climate forcing, then what happened before can certainly happen again. By drawing upon our improved understanding of the climate history of the America West, leaders responsible for managing water across the region might be able to ensure that future megadroughts will not bring the same consequences to today's society as they did to its predecessors.

Last Modified: 02/28/2022
Modified by: Scott St. George