It is now certain that drought will continue for another summer in many parts of California. On Monday, Gov. Jerry Brown issued an executive order making many water-conservation requirements permanent, and calling for stricter measures to be imposed through January 2017.

Most of us have never experienced a drought that lasted this long, and many Californians now wonder: how much worse can it get?

Benjamin Cook has some insight on that. Cook is a research physical scientist at NASA’s Goddard Institute for Space Studies in New York, and he has a Ph.D. in environmental science from the University of Virginia.

Last year, Cook was the lead author on a study published in the journal Science Advances that examined the risk of future droughts. Using computer models of future climate change and analyzing tree-ring growth patterns, the study found that the latter half of this century is likely to be drier than at any time in the past 1,000 years.

A key feature of the study is not just precipitation, but temperature and soil moisture. These are equally powerful measures of drought that are often overlooked. Cook found that even though precipitation is expected to increase slightly over parts of California in the future, droughts will still be worse because climate change will make the state hotter, thereby reducing soil moisture. This will create what the study calls “a level of aridity exceeding even the persistent megadroughts that characterized the Medieval era.”

We asked Cook to help us better understand his findings and the implications for California’s future.

Water Deeply: What did you learn about future droughts?

Ben Cook: A lot of our work has been focused on disentangling the potential that both the change in precipitation and temperature will play on drought. Most people think about drought in terms of precipitation. That’s fine to a certain extent. But at the end of day, as far as drought impacts go, we only care about those impacts to the extent that we have below normal soil moisture, or streamflow, or reservoir storage. That’s an important distinction, because those reservoirs can be filled up by precipitation, but they also can be emptied out by evaporation. As you warm things up, you’re going to evaporate more water off the surface and leave less for the things we care about.

What we find with all the evidence we’re seeing is that this temperature effect, which has been largely ignored historically, is playing a role in some of the droughts happening right now, and is likely to play an increasingly important role in the future.

Assuming we continue what you might call a business-as-usual scenario, where we don’t mitigate greenhouse gas emissions, what we see are many places, particularly in the Southwest and Central Plains, that are going to see really significant drying, largely unprecedented at any time in the historical record, even going back 1,000 years. That’s a consequence of not only precipitation changes, but on top of that increasing evaporation from warming temperatures.

Water Deeply: How did you figure this out?

Ben Cook: There are lots of observations both for the recent past – the last 100 years or so, from rain gauges and thermometers – but also further back in time from tree rings. In most of North America, trees grow well and put on a wide ring when it’s wet, and grow poorly and put on a narrow ring when it’s dry.

For the future, we use model simulations of the climate system. Analyzing them gives us different assumptions of greenhouse gases in the future, and what the likely climate impact is going to be. These models simulate all these sorts of processes in the climate system we care about. They simulate winds, precipitation, temperature at the surface. We can take outputs of the models, for example, of what soil moisture is going to look like in the future, and look at the past to compare.

Water Deeply: How accurate are these models?

Ben Cook: Certain things these models do very well, and certain things they don’t. We’re always analyzing as many models as we can get. Typically, we look at 15 to 20 models, which can potentially give us a range of different possibilities. Where those models tend to agree, we have higher confidence in our result.

Things like precipitation tend to be more uncertain. In the Western U.S., for example, there is very good convergence of different models that precipitation will decline with global warming over the 21st century. Interestingly enough, pretty much all the models everywhere really strongly agree on the temperature response, the warming. It’s this warming and increased evaporation that’s really driving the drought trends we see into the future.

Water Deeply: Why haven’t we paid more attention to this evaporation effect?

Ben Cook: Mostly because the climate hasn’t been changing. It’s only recently we’re starting to see these really significant trends in climate. Most of the big historical droughts, most of that drought variability, is from variability in precipitation. That’s been the predominant control.

But more recently, we’re starting to see global warming push our temperatures outside the range of natural variability. And in the future, it will definitely be outside that range. Because of that, we’re seeing temperature play an increasingly important role. Currently, it’s smaller than the precipitation role. But temperature is going to get increasingly important as we warm up.

Water Deeply: How accurate is it to compare future projections to the history shown in tree rings?

Ben Cook: For the Western U.S., I think it has very high accuracy, and we have very high confidence in what the trees are telling us. The reason is there are lots of trees out there, and this is an area where the trees are very, very sensitive to moisture. For example, trees in other areas like Canada or maybe New England may care a little bit more about temperature, because temperature in those areas is the bigger limiter on tree growth. But out West, by far the dominant limitation is water. So the trees really, really care a lot about that.

You also need to look at living trees so you can know the years those different rings were put down. But what you can do is you can start to take dead trees and kind of piece it together back into the past into a longer record, kind of like a puzzle. if you can find a tree that died but whose growth overlapped a living tree, if they’re from the same location, the way the living tree grew will be similar to the way the dead tree grew, in that similar year. You can use dead trees to extend the record much further back in time.

Water Deeply: How high is the risk of worse droughts?

Ben Cook: Our results suggest that after 2050, if we continue on our current course of greenhouse gas emission, the likelihood of Western North America experiencing what we call a megadrought – a drought that lasts 35 years or longer – is very likely above 80 percent.

Water Deeply: That’s a very high percentage.

Ben Cook: Yeah, it is. It’s the type of drought event we don’t have any experience with over the last 200 years. You can find similar types of droughts in the tree ring records during the 1100s and 1200s when most of Western North America was really arid. But a 35-year drought is something that nobody living has ever dealt with. So they could potentially present a real management challenge.

Water Deeply: This is based on a “business as usual” emissions scenario. Did you also look at a scenario in which we take a different path?

Ben Cook: We absolutely did. What we found is that the less we warm up, the less the risk is going to be. I think there’s good evidence there that suggests if we mitigate and prevent the same level of warming, then we can prevent at least some of these drought risks. Some warming is locked in at this point. But we can avoid some of the worst impacts of drought for Western North America.

Water Deeply: What are the solutions? How do we prepare for these kinds of megadroughts?

Ben Cook: The point I always try to make is that there are already lots of inefficiencies in the way that we use water, in the way we irrigate, the types of crops we grow, how much water we waste.

So I don’t think our results mean the West is doomed to be some kind of post-apocalyptic desert wasteland. But it does mean we need to change the way we use and value water. Because there simply isn’t enough water to do things as inefficiently as we have been. We just can’t treat water as abundant and free anymore.

There are decisions to make about what we grow and how we irrigate. Even recently Californians, on the municipal side, have really dramatically reduced their water use. That is encouraging, and it shows if we do take these issues seriously, we can really avoid the worst consequences.

Water Deeply: What about population growth?

Ben Cook: That’s part of it. At the same time things are starting to dry out, we’re seeing the West is also an area of the country that has some of highest population growth and the highest growth of economies in our country. What that means is, even if you don’t care about climate change, even when a normal drought occurs, the impacts are going to be felt that much more.

Water Deeply: It sounds like you’re fairly hopeful we can stay on top of this.

Ben Cook: Yes, I think so. The only bad move would be to ignore it. There are decisions to be made that are definitely above my pay grade. Large-scale decisions, certainly. But I am optimistic there are solutions if we take this problem seriously.