Yes, Manmade Global Warming Is Worsening California’s Epic Drought

California’s epic drought got even worse last week. The U.S. Drought Monitor reports that the area of California under moderate drought (or worse) rose from 94.6 percent of the state to a stunning 99.8 percent. The area under extreme or exceptional drought rose from 65.9 to 71.8 percent, encompassing the entire agriculture-rich Central Valley

Back in January, we interviewed 8 of the leading climate and drought experts in the country, who explained in great detail how climate change is worsening California’s epic drought in multiple ways. As I discussed in my 2011 literature review in the journal Nature — even in regions where climate change does not alter the amount of precipitation, it will have these effects:

What precipitation there is will probably come in extreme deluges, resulting in runoff rather than drought alleviation. Warming causes greater evaporation and, once the ground is dry, the Sun’s energy goes into baking the soil, leading to a further increase in air temperature…. Finally, many regions are expected to see earlier snowmelt, so less water will be stored on mountain tops for the summer dry season.

The president’s science advisor John Holdren made precisely the same points in a recent paper.

These points are disputed by very few. So why is there any confusion? A relatively small subset of experts are focused very narrowly on the issue of whether global warming caused a reduction in precipitation — but they generally fail to make clear how narrow their perspective is. NOAA’s Martin Hoerling did this in a recent New York Times piece, asserting “At present, the scientific evidence does not support an argument that the drought there is appreciably linked to human-induced climate change.”

Now, in fact, there is science supporting the argument that the reduction in precipitation is directly linked to human-induced climate change, specifically Arctic ice loss. I broke that story last June (see here) and updated it again in March.

But even setting aside the precipitation issue, our leading scientists have repeatedly made clear that global warming is worsening the drought. In a letter to the NY Times, three top drought experts — Peter Gleick, Jonathan Overpeck, and Connie Woodhouse — explain that the issue of what “caused” the drought “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.

As if to underscore this point, last week NOAA released its climate analysis of the U.S. winter, reporting:

California had its warmest winter on record…. The California winter temperature was 48.0°F, 4.4°F above the 20th century average, far exceeding the previous record, set in 1980/81, by 0.8°F.

And as Tamino explains, in California, “If we look at an actual measure of drought — the Palmer Drought Severity Index, or PDSI — then there is a decreasing trend (which means, toward more and/or more extreme drought) which is statistically significant.”

Finally, our favorite climate videographer Peter Sinclair has interviewed a variety of scientists on this subject in yet another must-see video: