In an interview over at Grist, Brad Udall reminds us that, as we think about the effect of climate change on the West, it’s not just the thorny question of whether precipitation rises or falls that matters. Despite the uncertainties surrounding that question, as temperatures rise (a projection about which there is considerably less uncertainty), available water falls:

[T]here were three years that were really bad at the beginning of the 2000s, 2001-2003. Almost all the years since then have been about average in terms of precipitation. But runoff has been significantly less. And we believe that these significantly high temperatures that we’ve been experiencing, especially over the last 10 years because of man’s emissions of greenhouse gases, have reduced the runoff in the river. What our research shows is a couple different things: We can develop relationships between temperature and runoff, and precipitation and runoff, and it appears that this system is very sensitive to increased temperatures.

To illustrate Brad’s point, some data from the Western Regional Climate Center. The first is precip for the Colorado headwaters climate division in eastern Colorado. You can see that precip has been low over the past decade, but we’ve seen dry spells like this before. Then look at temperature, specifically how much warmer it’s been than in previous droughts:

That’s what Brad’s talking about.