April 9, 2015 4:07PM

California Drought: The Rest of the Story

Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”


When the national media covers a weather story these days, it almost certainly will find some angle in which to insert/assert an element of human-caused climate change, and that element will always be characterized such as to have made the situation worse. It would take a lot of thinking on our part to try to recollect a major weather-related story in which the global warming was suggested to have ameliorated the impact—this despite a scientific literature that is complex and nebulous as to the direction of most impacts, and even less certain regarding the current detectability of such impacts.

Take for example the coverage of the current drought in California. As the drought drags on with spring snowpack in the Sierra Nevadas at record low levels—a finding which has prompted California Gov. Brown to enact statewide water restrictions—the press is eager to finger our greenhouse gas-spewing modern economy as the prime culprit. The problem is most scientific research suggests that the lack of precipitation in California has its roots in natural variability (for example, this recent paper by Thomas Delworth and colleagues which finds a common cause to drought in the western U.S. and the hiatus in global warming).

Thus, for the time being at least, bedeviled by actual science, most in the press have resisted placing much of the blame on the lack of precipitation at the feet of anthropogenic global warming.

But, there are actually two components which contribute to drought: 1) lack of precipitation, and 2) high temperatures.

Stymied on the first, many in the media have turned to the second. 

After all, what’s a more obvious tie-in to global warming than higher-than-average temperatures? Or, in this case, even better yet, the record high temperatures that California has been experiencing as of late.

Check out the coverage of the drought from two prominent and popuar weather/climate oriented blogs—Andrew Freedman’s articles at Mashable, and Jason Samenow’s for Capital Weather Gang.

Freedman writes:

“We are now seeing the rise of a new, supercharged type of drought, in which global warming-related temperature extremes combine with dry conditions to transform what would otherwise be an ordinary drought event into a far more severe event.” 

And following Samenow’s headline “Hot hands: Fingerprints of climate change all over California drought” he tells us:

“California’s astonishingly low snowpack, a pathetic 5 percent of normal, and the severity of the drought afflicting the state isn’t some fluke. It’s a likely consequence of climate change, specifically the rising temperatures which are intensifying many of the processes causing the state to lose water at an alarming rate.”

If only things were so simple. In fact, they are not.

Several factors confound such a haste accusation.

First, drought begets high temperatures, and vice versa. When conditions are dry, instead of being occupied evaporating moisture from the soil, more incoming solar radiation (i.e., sunshine) goes towards raising the air temperature. And high temperatures enable the air to hold more moisture, which increases the process of evaporation. So, the lack of precipitation (itself linked to natural variability) helps lead to higher temperatures, which themselves, lead to worsening drought conditions.

No global warming necessary.

And second, many of the same elements of natural variability which have been linked to the precipitation deficit also act to elevate the temperature—something which is summarily being ignored by many in the media—but not by us!

Recall a couple of months ago we reported an a new study by Jim Johnstone and Nathan Mantua, researchers who, among other things, specialize in studying atmosphere/ocean  circulation patterns across the Pacific Ocean and how they impact the climatology/hydrology/ecology of the West Coast of the U.S. Johnstone and Mantua found that naturally occurring atmospheric pressure patterns across the northeast Pacific Ocean—patterns which they traced back more than 100 years—could explain virtually all of the increase in temperature that had occurred over large portions of the Pacific Northwest, and extending down into northern California. In southern California, the same patterns explained a bit more than half the long-term observed temperature increase. They wrote:

“These results suggest that natural internally generated changes in atmospheric circulation were the primary cause of coastal NE Pacific warming from 1900 to 2012 and demonstrate more generally that regional mechanisms of interannual and multidecadal temperature variability can also extend to century time scales.”

But their analysis ended in 2012. What about the years since then—the years which encompass the current California drought and record high temperatures?

We wrote to Dr. Johnstone and asked whether they had any updates to their analysis and he wrote back pointing us to this page on his website—a page dedicated to the study described above, and which included updates through March 2015. There we find that the naturally occurring atmospheric pressure pattern that Johnstone and Mantua linked to the long-term warming across California (and the Pacific Northwest) was very high during the past several years and, in fact, reached its highest anomalies on record during the past several months. In other words, the same mechanism which explains the majority of the observed temperature rise in California since the beginning of the 20th century also explains why it has been especially hot in California recently.

Again, no global warming necessary.

Dr. Johnstone did point out to us that the “The model underpredicts the magnitude of the current warm anomaly, but this is typical of strong peaks.” This type of behavior (underprediction of extreme values) is commonplace in the statistical methodology (linear regression analysis) employed by Johnstone and Mantua. And it is within this underprediction where the impact of global warming may be residing, although it is potentially sharing that space with drought-induced high temperatures, local landscape changes (e.g., urban warming), a non-linear response to atmospheric pressure forcing, and a host of other circulation factors.

In other words, any impact of human-caused global warming on the recent high temperatures in California (and their influence on drought conditions) is minor at best and most likely undetectable—and not deserving of the level of credit that good folks like Freedman and Samenow have bestowed upon it.

This is a conclusion supported by the scientific literature and basic climatology, but which is absent popular press coverage of the ongoing drought.

As the late Paul Harvey would say, “And now you know the rest of the story.”