The Growing Influence of Climate Change on the California Drought

Over the past three years (and indeed, for 10 of the past 14 years) California has experienced a particularly deep drought. How bad is the drought? Is it the worst in the instrumental record? The worst in over a century? The worst in 1200 years? The worst “ever”? And why has it been so bad?

There is no single definition of “drought.” Drought, most simply defined, is the mismatch between (1) the amounts of water nature provides and (2) the amounts of water that humans and the environment demand. As the National Drought Mitigation Center puts it:

“In the most general sense, drought originates from a deficiency of precipitation over an extended period of time -- usually a season or more -- resulting in a water shortage for some activity, group, or environmental sector. Its impacts result from the interplay between the natural event (less precipitation than expected) and the demand people place on water supply, and human activities can exacerbate the impacts of drought. Because drought cannot be viewed solely as a physical phenomenon, it is usually defined both conceptually and operationally.”

Droughts aren't a new problem for California. Like any other region of the world, the state is subject to extreme hydrologic events, including both floods and droughts. Long-term climatic data developed from tree-ring reconstructions, other “paleoclimatic” assessments, and the more recent instrumental and satellite records provide a record of extensive and persistent natural droughts going back more than a thousand years.

By any measure, the current California drought is severe, to the degree that Governor Brown made an emergency drought declaration almost a year ago, state and federal water agencies have been forced to greatly cut back deliveries of water to cities and farms from dangerously depleted rivers and reservoirs, and local utilities are asking customers for a mix of voluntary and sometimes mandatory water-use reductions. And the current drought is more severe than in the past in part because of the growth in the state’s population. Today California has 16 million more people than during the severe 1976-77 drought, and nearly 10 million more than during the long 1987-92 drought (Figure 1).

California population from 1900 to 2013. Data from CA Dept. of Finance. California population from 1900 to 2013. Data from CA Dept. of Finance.


But a new factor must also be acknowledged:

The current California drought is bad because for the first time ever, scientists from many different fields see parallel lines of evidence for the influence of human-induced climate changes, including the fingerprints of higher temperatures and changes in the atmospheric circulation patterns. In short, climate change has made the current drought worse. [A summary of some of the recent peer-reviewed literature is provided at the end of this column for readers wanting to dig deeper.]

There is rapidly growing evidence from a combination of basic climate science, models, and real-world observations that human-caused climate change has influenced and worsened the current drought.[1] Indeed, California is not alone in experiencing the growing impacts of climate change: evidence that climate change is influencing extreme hydrologic events all over the world is now pouring in, from heat waves to coastal damages during extreme tides and storms, flooding from more intense precipitation events, drastic loss of Arctic ice, and droughts.

The rainy season has started again (as of the beginning of the official “water year,” October 1), and there is the hope and chance that California will see an average or even a wet year. But if there is any lesson to be learned from the past few years, it is that California is moving rapidly into a new water regime, where hydrologic extremes, including both droughts and floods, are likely to be both more frequent and increasingly severe, and where the influence of human-induced climate change is ever more apparent.

Even without the new factor of a changing climate, it is time to acknowledge that California is in permanent long-term shortage: even in a “normal” rainfall year more water is now demanded and used than nature provides, leading to growing political conflict, unsustainable groundwater overdraft, and ecological destruction of the state’s rivers, streams, and wetlands. Human-caused climate change just worsens this mix.

Business-as-usual water policies and politics cannot continue. California’s water community must face up to a new reality – a new “normal” – and work to bring our water use back into balance.


The Science Background (A few recent relevant papers)

“The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures.”

Griffin, D. and K.J. Anchukaitis. 2014. How unusual is the 2012-2014 California drought? Geophysical Research Letters. DOI: 10.1002/2014GL062433

“Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.”

Seager, R. and M. Hoerling, 2014: Atmosphere and Ocean Origins of North American Droughts. J. Climate. Vol. 27, 4581–4606. doi:

“Although the recent drought may have significant contributions from natural variability, it is notable that hydrological changes in the region over the last 50 years cannot be fully explained by natural variability, and instead show the signature of anthropogenic climate change.”

Cayan et al., 2010. Future dryness in the southwest US and the hydrology of the early 21st century drought, PNAS, Vol. 107, December 14, 2010, pp 21271-21276

“Climate change is linked to CA’s drought by two mechanisms: rising temperatures and changing atmospheric patterns conducive to failing rains. The first link is firmly established, and there is considerable and growing body of evidence supporting the second.”

Swain, D. L., M. Tsiang, M. Haugen, D. Singh, A. Charland, B. Rajaratnam, and N.S. Diffenbaugh. 2014. “The extraordinary California drought of 2013-2014: Character, context, and the role of climate change.” BAMS, Vol. 95, No. 9, September 2014 (Special Supplement), pp. S3-S7.

There is growing observational data, physical analysis of possible mechanisms, and model agreement that human-caused climate change is strengthening atmospheric circulation patterns in a way “which implies that the periodic and inevitable droughts California will experience will exhibit more severity…” “there is a traceable anthropogenic warming footprint in the enormous intensity of the anomalous ridge during winter 2013–2014 and the associated drought.”

S.-Y. Wang, L. Hipps, R. R. Gillies, and J-H. Yoon. 2014. Probable causes of the abnormal ridge accompanying the 2013-2014 California drought: ENSO precursor and anthropogenic warming footprint. Geophy. Research Letters, Vol. 41, Issue 9, pp. 3220-3226, May 16, 2014.

AghaKouchak, A., L. Cheng, O. Mazdiyasni, and A. Farahmand. 2014. Global Warming and Changes in Risk of Concurrent Climate Extremes: Insights from the 2014 California Drought. Geophysical Research Letters (in press). DOI: 10.1002/2014GL062308

“Increased heating from global warming may not cause droughts but it is expected that when droughts occur they are likely to set in quicker and be more intense.”

Trenberth, K. E., A. Dai, G. van der Schrier, P. D. Jones, J. Barichivich, K. R. Briffa, and J. Sheffield, 2014. Global warming and changes in drought. Nature Climate Change, 4, 17-22, doi:10.1038/NCLIMATE2067.

All models, regardless of their ability to simulate the base-period drought statistics, project significant future increases in drought frequency, severity, and extent over the course of the 21st century under the SRES A1B emissions scenario.

Wehner et al., 2011. Projections of future drought in the continental United States and Mexico. Journal of Hydrometeorology, Vol. 12, December 2011, pp 1359-1377.

"Over the past millennium, late 20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains and in their north-south synchrony across the cordillera... the snowpack declines and their synchrony result from unparalleled springtime warming that is due to positive reinforcement of the anthropogenic warming by decadal variability. The increasing role of warming on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western United States."

Pederson et al., 2011. The unusual nature of recent snowpack declines in the North American Cordillera. Science, Vol. 333, 15 July 2011, pp 332-335.


[1] None of these studies, and no scientists that I know of, have argued that the drought is “caused” by climate change – that is the wrong question. As I have discussed in an earlier column, the evidence points to the “influence” of climate change worsening these extreme events.

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By Hal Helsley (not verified) on 08 Dec 2014 #permalink

The SW region of Western Australia is also getting used to a new normal. The solution so far here has been to build desalination plants. There are also permanent water restrictions.

By Duncan Farrow (not verified) on 08 Dec 2014 #permalink

Dear Mr. Gleick: Here is some background to assess whether this three year drought is the worst in California, over the last 1,200 years, ever, or whatever.

A cursory Google search brings up this article, among others.

From The New York Times, July 19, 1994.

"BEGINNING about 1,100 years ago, what is now California baked in two droughts, the first lasting 220 years and the second 140 years. Each was much more intense than the mere six-year dry spells that afflict modern California from time to time, new studies of past climates show. The findings suggest, in fact, that relatively wet periods like the 20th century have been the exception rather than the rule in California for at least the last 3,500 years, and that mega-droughts are likely to recur.

The evidence for the big droughts comes from an analysis of the trunks of trees that grew in the dry beds of lakes, swamps and rivers in and adjacent to the Sierra Nevada, but died when the droughts ended and the water levels rose. Immersion in water has preserved the trunks over the centuries."

This information seems to put the lie to the notion that the current drought (still going on, in spite of the rain) is the worst in 1,200 years. It also renders suspect any assertion that a drought in California needs anthropogenic help.

It took all of 5 minutes to find this online. Why don't people check their facts anymore, when others will?

Here is a link to the article, by the way. It is well worth reading.…

By Torgeir Hansson (not verified) on 09 Dec 2014 #permalink

Apart from motivating a lot of water conservation work, the droght has left me with Strange New Respect for the agronomists and AgEntrepreneurs who pioneered the planting of pomegranate and pistachio groves in antecipation of this trend !

This pinot disaster has been a vintage year for pomegranate juice

By Russell Seitz (not verified) on 10 Dec 2014 #permalink

Adelaide too. We've got desal now here too. Even though the current water restrictions aren't as bad as during the worst of the drought, you'll notice A Lot of people who still have lawns let them go brown in most summers. (Last year lawns and parks were pretty green up until, and even beyond, Xmas.)

Though we've also been the state with the highest rate of domestic rainwater tanks. I notice that our local Bunnings (a hardware chain) now has a few gigantic tanks which they use for watering their plant stocks. Californians need to get on board with a similar strategy. Even if they only use rainwater for toilet flushing and car washing, maybe laundry, apart from gardening, it certainly makes an aggregate difference to water consumption for a street/ suburb/ region.

(One of the reasons for South Australians to have maintained their passion for rainwater tanks was that, until recent decades, the taste of Adelaide's tapwater was diabolical as well as being the "hardest" water supply in the country. Soap companies had no need to advertise here, we already used twice as much as anyone else. Turns out that was A Good Thing so that it's easy to revive enthusiasm for rainwater capture here now.)