This article originally appeared in the Opinion section of The New York Times on March 16, 2014. 

_{Published: March 16, 2014
Author: Peter Gleick}

To the Editor:

Re “Global Warming? Not Always,” by Martin P. Hoerling (Sunday Review, March 9):

As California’s severe drought has worsened, there has been persistent debate about the links between drought and climate change (or indeed any extreme weather event). Three key, but very different, questions are often confused: Has climate change caused the current drought? Is it already influencing or affecting the drought (no matter its cause)? And how will climate change affect future droughts?

The most rigorous answer to the “causality” question for the California drought is neither yes nor no. We simply cannot say for certain.

But this is the wrong question to ask. The current drought has certainly been exacerbated by climate change for one simple reason: Temperatures in California are now higher today, as they are globally. This alone increases water demand by crops and ecosystems, accelerates snowpack loss, and worsens evaporation from reservoirs. There are other complicating effects, but the influence of higher temperatures on drought is already real and cannot be ignored.

We are now unambiguously altering the climate, threatening water supplies for human and natural systems. This is but one example of how even today we are paying the cost of unavoidable climate changes.

PETER GLEICK
JONATHAN OVERPECK
CONNIE WOODHOUSE
Oakland, Calif., March 10, 2014

Dr. Gleick is president of the Pacific Institute. Dr. Overpeck is a professor of geosciences and atmospheric sciences and Dr. Woodhouse is a professor of geography and geosciences at the University of Arizona, Tucson.