Global Warming? Not Always

California is now in the midst of the third year of one of its worst droughts on record. As our planet gradually warms from our rampant burning of fossil fuels, it’s only natural to wonder what role climate change has played in California’s troubles.

The answer is this: At present, the scientific evidence does not support an argument that the drought there is appreciably linked to human-induced climate change.

The drought has many attributes of historical droughts over that region — in particular, a lack of storms and rainfall that would normally arrive from the Pacific Ocean with considerable frequency. It resembles the droughts that afflicted the state in 1976 and 1977. Those years were at least as dry as the last two years have been for the state as a whole.

In short, the drought gripping California has been observed before. And it has occurred principally because of a lack of rain, not principally because of warmer temperatures. Indeed, it should be quite familiar to anyone who lived in California in the mid-1970s, as I did. We can also say with high confidence that no appreciable trend toward either wetter or drier conditions has been observed for statewide average precipitation since 1895. This drought is not part of a long-term drift toward reduced precipitation over the state.

What’s different this time, however, is that the demand for water has greatly increased in the state, and it may very well be that the current stress created by the failed rains is more severe than for similar rainfall deficits 40 years ago. It is at least intuitive that growth patterns, population increases and the rising value of the state’s agricultural sector have increased California’s vulnerability to drought and reduced its resiliency — that is, the state’s ability to adapt and cope with less precipitation.

Other indicators and aspects of rainfall behavior could also be conducive to drought. These include the gap between rainy days, the intensity of rains when they do occur and the effects of warming temperatures in California and for the planet as a whole. Even if the average seasonal rainfall isn’t changing, changes in these other factors could alter the risk of drought. What is the evidence there?

One way of accounting for the combined effects of rainfall and temperature on drought is to examine soil moisture. Long-term soil moisture observations are not readily available, but have been estimated using sophisticated models. The 2012 report on extreme events by the Intergovernmental Panel on Climate Change examined the evidence for regional changes in soil moisture since 1950, and made the following assessment for western North America:

“No overall or slight decrease in dryness since 1950; large variability; large drought of the 1930s dominates.”

The team of 42 scientists who made that assessment assigned it a rating of “medium confidence” that they were correct. The report also assessed the scientific evidence for how drought over western North America will change in the 21st century, finding an “inconsistent signal in consecutive dry days and soil moisture changes.”

A 2013 report by the I.P.C.C. reaffirmed that finding and concluded: “Recent long-term droughts in western North America cannot definitively be shown to lie outside the very large envelope of natural precipitation variability in this region, particularly given new evidence of the history of high-magnitude natural drought and pluvial episodes suggested by paleoclimatic reconstructions.”

Thus, the scientific evidence does not support an argument that human-induced climate change has played any appreciable role in the current California drought.

But that is not to say that a warmer climate cannot and will not act to decrease soil moisture and amplify the severity of future naturally occurring droughts. It simply reminds us that the current drought, like its ancestors, continues to be strongly driven by shifts in the location of storm tracks that may or may not deliver rains to the narrow strip of the West Coast.

Why should we care about what caused this drought, or for that matter, other extreme events like this winter’s severe cold over the Midwest or last year’s floods in Colorado? An accurate interpretation of the cause or causes can provide a preview of the future. It can help us understand whether the current experience is normal, or may instead foretell a new normal. The diagnosis is key to the prognosis.

Both answers have critical implications for how we assess risk and develop strategies to mitigate future problems and dangers. This is particularly true in the West, where we have built expensive and complex water resource systems to provide drinking water and support agriculture and industry. And it is also important for the energy infrastructure in the Midwest and the East Coast to protect against frigid winters and hot summers.

The bottom line is, the correct diagnosis matters.

This article is adapted from Andrew Revkin’s blog, Dot Earth.

Martin P. Hoerling is a research meteorologist, specializing in climate dynamics, at the Earth System Research Laboratory of the National Oceanic and Atmospheric Administration.

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