Is California Getting Drier

Dr. Benjamin Cook is a scientist at the Lamont-Doherty Earth Observatory and the NASA Goddard Institute for Space Studies, where he studies drought and the interaction between hydrology and climate. We asked him to tell us more about drought and aridity in California.

You've studied drought all over the world. How do you measure drought?
When most people think about drought, they think about precipitation: rainfall and snow. That's obviously very important, because that's how most droughts start. When we talk about water resources needed by people and ecosystems, however, we're really talking about soil moisture, streamflow, and groundwater. I tend to focus more on these aspects of drought because they're not just dependent on rainfall-they also depend on things like evaporation, which is sensitive to temperature. Generally speaking, as temperatures rise, evaporation also rises, drying out the soil; this is why we expect soils to become drier with climate change in many regions.

In California, we keep hearing about increasing aridity. What does that mean, and does it reflect changes in climate?

Aridity refers to how dry on average a given region is. For example, Phoenix, Arizona is more arid than New York City-that's the climatological difference between the two places. Drought refers to a deviation in the water balance from the baseline climate: is it dryer or wetter than normal? Drought in Phoenix and drought in New York look very different because the baseline aridity is so different.

This brings up an interesting problem when we look at climate change projections. With climate change, we're starting to see an aridification trend in many regions, including in the southwestern United States. As warming continues, this is shifting regions towards drier average conditions. That becomes a challenge: how do we interpret droughts in the context of a continuously drying baseline? And when do we decide it's no longer a drought but a shift in aridity to a more permanent, drier state? There's a lot of discussion in the community about this.

California is kind of funny, because one of its defining features is whiplash, where you'll go very quickly from a really bad drought year to a year with flooding or record-breaking snowfall. For California, it's harder to say if it's going through an aridification trend. We can say more confidently that climate change is likely to amplify the extremes. When you get a drought, it's going to be hotter and drier than it would have been. And probably when you get these atmospheric rivers, a warmer world means they'll have more water, which could lead to more flooding.

In the West, there are droughts all the time. Everyone's worried when it's going on, but as soon as it ends, people feel they have nothing to worry about. I always say that when a drought ends, it's time to start preparing for next one.

There are numerous policy implications for growing aridity in California. What issues are at the top of your list?

There are some clear pain points here. The vast majority of the water used in the western US-about 70%-is used for agriculture. Any solution will have to reckon with that. There may be a need to change cropping practices. On the other side, the Colorado River Basin has a very complicated network of water rights. Some of these rights go back generations with different levels of seniority, and that adds a layer of complexity to finding a solution that works for everybody.

California will probably have to adapt to changes in snowpack. The state manages reservoirs for flood control during the wintertime, with the expectation that in the spring, all the snow in the Sierras will melt and refill the reservoirs, and we'll have that available for everybody during the dry season. With climate change, that snowpack will get smaller, because more of it will come down as rainfall, and you'll have earlier snowmelt and even sublimation (where snow evaporates directly into the atmosphere). In a warmer future, you can't count on snowpack to be there in spring to capture and use for the dry season. We'll have to adapt management around that.

Anything else you're excited about?

We're now looking at the opposite of droughts: pluvials (extended periods of above-average wetness). The early 20th century was one of the wettest periods in western North America, and the water allotments in the Colorado River Compact are based on measurements made during this time. The 1980s and '90s was also one of the wettest 20-year periods in the last thousand years, and there was massive population and economic growth. These periods of abundant water very likely shaped much of the expansive development in western North America.

We're looking more deeply at these really wet periods to understand them better. How often have they happened in the past, and what kind of dynamics underlie them? What might they look like in the future? Pluvials are really critical for drought management. For example, groundwater in California is recharged during wet years, and this groundwater is used during dry years to mitigate drought impacts. There's an underlying, implicit assumption that whenever droughts end, these pluvial events will come and fill up the water resources for use during the next drought. We're trying to determine how credible that assumption is.