Western states face a bleak future amid the worst drought in more than 1,000 years

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• Judy Woodruff:

  The so-called megadrought that's afflicting the American West is the worst in 1,200 years. That's according to a study published yesterday. It has dried up water supplies, threatened ranchers, and fueled wildfires.

  William Brangham explains more.

• William Brangham:

  That's right, Judy.

  Last summer, two of the largest reservoirs in North America, Lake Mead and Lake Powell, reached their lowest-ever recorded levels. Megadroughts have come and gone in the past, but this current one, now 22 years long, is being driven largely by human-caused climate change.

  According to the study, our warming atmosphere has made this drought much worse, 40 percent worse, and it's likely to continue.

  The new study was just published in the journal "Nature Climate Change."

  And its lead author, Park Williams, joins me now.

  Park, great to have you back on the "NewsHour."

  Could you just tell us a little bit more about the historic nature of this drought? I mean, 1,200 years, that this is the worst that we have seen, how do you go about proving something like that?

  Park Williams, University of California, Los Angeles: Well, in the early 1900s, tree ring scientists in Arizona discovered that tree rings actually correspond to drought in the Southwestern United States, and, in fact, after more work was done across most of Western North America.

  Trees are very sensitive to drought. And so the widths of their annual tree rings that grow within the tree trunks actually corresponds almost perfectly to the amount of water that the trees had available to them. From that research, the scientists realized that, back in the medieval period, between about 800 and 1600 A.D., there were repeated major drought events that they termed megadroughts.

  We used the tree ring records, along with climate observations, to try and tell, how bad has been — has the drought of the 2000s been relative to these megadroughts of the medieval period?

• William Brangham:

  So how is it — when you say that climate change is making this particular megadrought that we're in significantly worse, how do you go about proving that part of it?

• Park Williams:

  Well, we do an analysis called a counterfactual analysis, where we basically study the world without climate change.

  And the way that we do that is, we use climate models to tell us what the human-caused climate change has been over the last century. And then we recalculate what the climate records would have been without those human-caused climate trends.

  We can then calculate what the soil moisture balance would have been like in this counterfactual world where climate change never occurred.

• William Brangham:

  So, if you took out humanity's impact on the atmosphere, all the gasses that we have put up there in the last several hundred years, would we still be experiencing out West a significantly dry period?

• Park Williams:

  It would be a significantly dry period, even without human-caused climate change, because of just regular old bad luck.

  The tropical Pacific Ocean has been in a cold state for a lot of the last 22 years. That means kind of La Nina-like, and that usually means drought for the West. But those La Nina conditions have not been really extreme. And so we show that, of the drought that was observed, about 40 percent of that drought severity is explained by human-caused climate change.

• William Brangham:

  One other thing that I'm struck by in your report is that, even when the West gets significant snow or rain, that, because of the warming atmosphere and the warmed Earth, that that water, each drop of rain, each snowflake is not as effective as it used to be.

  Can you explain that?

• Park Williams:

  Sure.

  We're all familiar with this concept, actually, if we have houseplants. If you turn up the heater in your house, and you don't give your plants more water, then they're going to dry out faster. The same thing happens in the real world. Global warming is like turning on the heater in the atmosphere. The atmosphere's demand for moisture actually increases.

  And so plants and soil lose moisture more quickly in a warmer world than they would in the cooler world.

• William Brangham:

  So for people who say, well, December in January in California, they got a ton of rain, they got a ton of snow, that ton of rain and ton of snow just didn't do as much to replenish the aquifers and the entire landscape.

• Park Williams:

  The West has really been experiencing dry conditions for the vast majority of the last 22 years. Of the last 22, 18 have been drier than average. Just four have been wetter than average.

  And so we might have a wet month or even a wet year, but it's going to take a sequence of several really wet years to make up for the giant water deficit that's accumulated over the last couple of decades.

• William Brangham:

  So, I know that the majority of your work is about looking into the past.

  But, going forward, what does this research indicate it means for us? If we know that the temperatures are continuing to climb, greenhouse gases, emissions are continuing to climb, what does that mean for the West?

• Park Williams:

  Well, we have two really important lessons, one from tree ring records and one from climate modeling.

  The tree ring records tell us that, if the future is anything like the past, then water in the West is going to be like a yo-yo. There will be very dry periods, and there will also be very wet periods. Climate modeling tells us, though, that the average year is getting drier and drier.

  Now, no year actually matches the average, but the yo-yo going up and down is going to be slowly sliding down an escalator toward drying conditions. And that means that future droughts will be increasingly likely to be worse than ones we have experienced in the past. And when it does inevitably get wet, those wet periods very likely will not last as long as they would have historically.

• William Brangham:

  All right, Park Williams at UCLA.

  The most recent study is in the journal "Nature Climate Change."

  Always good to see you. Thank you very much.

• Park Williams:

  Thank you.