Ben Cook at NASA’s Goddard Institute for Space Studies has a new paper that offers a reminder of why the impact of climate change on our ecosystems and water supplies involves more than “will it rain less”?

In some sense this is an old and obvious point, which I link here just to repeat said old and obvious point. Drought is a combination of how much rain and snow falls from the sky and then what happens once it hits the ground. If it’s warmer, more evaporates, and it’s the net left behind after the puts and takes that defines our available water supply and drought or lack thereof. Here’s Cook:

For many regions, focusing on the precipitation response alone will be insufficient to fully capture changes in regional water resources such as soil moisture, runoff, or reservoir storage. Instead, increased evaporative demand will play a critical role in spreading drought beyond the sub-tropics and into the Northern Hemisphere mid-latitudes, regions of globally important agricultural production.

But Ben’s a smart scientist who is doing more than what I usually do here, which is simply restate the obvious. The new in the paper is the use of the latest round of models, known as CMIP5, along with a couple of different measures of dryness, to better flesh out the picture.

There’s been some argument over whether the Palmer Drought Severity Index (the most common measure for this sort of thing) is the best way to analyze future drought scenarios, so Cook and colleagues offer up alternatives, including the Standardized Precipitation Index and a look at “vapor pressure deficit” (a critical measure for thinking about the future of our forest – see Park Williams on this):

PDSI and SPEI projections using precipitation and Penman-Monteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America….

update: Dr. Cook kindly shared this link to an ungated copy of the paper (extremely large pdf)