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.
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. 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: http://dx.doi.org/10.1175/JCLI-D-13-00329.1
“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. http://onlinelibrary.wiley.com/doi/10.1002/2014GL059748/pdf
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.
 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.