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.

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