October 2013

What Does the Drought Mean for the Future?


Jonathan Overpeck


Drought means many things to many people, but to most, it means not getting enough rain to meet your needs. Not enough rain to maintain healthy range or forest conditions, supply enough water, produce a good harvest, or sustain other needs. Drought can also be a rainfall deficient from some “normal” conditions.


In the Southwest, we have plenty of drought experience. The region has been in drought much of the last 14 years, including several years of unprecedented drought—first early in the 21st century, and then eclipsed by the burning dryness of the last two years. Burning dryness because we’ve literally seen unprecedented wildfire, but also because Southwest droughts of the last two decades have been hotter than any time since we started keeping track.


Droughts are tough on our water supply, forests, range and much more. They seldom leave human and natural systems better off than before the drought, but each drought does provide clues about what might lie ahead, and what we can do about it. As a climate scientist who tries to read and understand everything about drought, especially in the Southwest, I see key lessons emerging from the hardships of our current drought.


First, droughts can happen for a variety of reasons and have a variety of impacts. In the Southwest, droughts just happen—they happen when ocean temperature patterns line up right, and they seem to do this all too often. The science of drought tells us a great deal about causes, but not enough to say when the next drought will hit, or exactly where, or how bad. But, one thing you can take to the bank—there will be more drought. No doubt about it.


Second, a lot of folks want to ignore the issue of human-caused climate change. On one hand, that’s OK, because droughts can happen naturally, and it’s a solid no-regrets strategy to always be planning for the next drought. Our recent drought, as bad as it has been, is nothing compared to some of droughts the Southwest has seen over the last 2000 years. There is little doubt that the tree-ring records are correct in telling us that droughts lasting several decades can happen. Imagine that. One of the most severe droughts affecting the headwaters of the Río Grande—and more—lasted 51 years, with only one year of above-normal rainfall. Hard to imagine that.


Third, although a megadrought lasting decades can happen naturally, they are still pretty rare. We lack a good understanding of why they persist so long, and thus we can’t even say whether the drought we’ve been in will end next year or 20 years from now. Best bet is that it will persist at least another year, as we continue to wait for a strong El Niño or some other driver of big winter rain and snow to hopefully push our region back out of drought.


Fourth, the story is more complex because of the influence humans have on the climate system. There is now almost no doubt among climate scientists that global warming is real, and that it is driven mostly by humans releasing greenhouse gases to the atmosphere. Global warming is the reason the Southwest has been warming so dramatically, and another reason why our recent droughts have been so severe. The last 14 years have been dry, but they have also been hotter than any comparable period for many centuries.


This last lesson is perhaps the most important. As the Earth’s atmosphere warms up, it can hold more water, and it will therefore demand more water from wherever it can get it; soil and plants are a prime source. We can measure this, and we see it happening as the Earth gets warmer. And everyone reading this probably knows firsthand what happens on those really hot, dry days to soil and plants that don’t get enough water: they wilt, and in some cases, they die. This lesson is key because we know why it’s warming, and why drought impacts are getting worse—it’s like finally understanding why a prized animal is sick, and knowing that there is a cure if we want to pay for it.


What’s ahead then? More drought—sure bet. Hotter drought—ditto. The climate baseline is changing so that the old “normal” will look wetter and wetter as we move further into a progressively drier future. The best science also says that continued warming will bring less snow and less flow in our big rivers. The last two years have started to give us a clear vision of what climate change will mean to the Southwest. Hotter, drier and more drought-prone. And as I’ve already noted, if you know the cause, you know how to stop it if you want. The first step is to take climate change seriously and decide what we can and can’t adapt to. Then we have to talk about how to limit the climate change we can’t adapt to. With the drought, we all have a better idea of what’s at stake.


We will still get good years—wet years—but they will be more and more the exception. A better bet is to expect more drought and plan for it. And, if you don’t like the idea of an increasingly hot and dry Southwest, then join in conversations about what to do about climate change. It is fixable, and the fixes will likely mean more jobs in the sunshine—and wind-rich Southwest.



Jonathan Overpeck, professor of Geosciences and Atmospheric Sciences at the University of Arizona, Tucson is the opening speaker at the 2013 Quivira Conference.





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