California Drought Narrative Report FAQ

This FAQ covers questions asked about the report "Causes and Predictability of the 2011 to 2014 California Drought" which is an assessment report of the NOAA Drought Task Force Narrative Team.

1. What are the main findings in this report?

The severe drought in California over the last 3 years (2011-14) is primarily due to natural climate variability, key features of which appear to be predictable from knowledge of how California precipitation reacts to tropical ocean temperatures. There has been no long-term trend in California precipitation; however, California temperatures have been rising and record high temperatures during the drought were likely made more extreme due to human-induced climate change.

2. Why was this assessment done? Who can benefit from the information presented in this study? How can this information be used to help guide and inform their planning and preparedness?

Part of the mission of the National Oceanic and Atmospheric Administration mission is to provide early warnings and information that is relevant to preparedness. The current California drought is a noteworthy example of the fact that extreme climate events happen, and that these events can cause major societal impacts. When such an event happens, communities, businesses, and government entities inevitably have questions about why it happened, whether conditions will improve or worsen over time, and what they should do to plan and prepare. This report presents scientifically robust and timely information about the California drought’s causes. We hope this work will enable us to develop better early warning systems for future droughts in California.

The report also considers whether the California drought is linked to human-induced climate change, which may be of particular concern to long-term planning and policy-making interests.

3. Who was involved in this study?

This study was produced by the NOAA Drought Task Force, which is organized by the Modeling, Analysis, Predictions and Projections (MAPP) Program of the NOAA/OAR Climate Program Office. The Task Force is a multi-agency team of climate scientists with expertise in drought and predictions who are affiliated with universities, federal laborotories, and research institutions across the United States. The authors that produced this report are affiliated with a variety of institutions including the Lamont Doherty Earth Observatory of Columbia University, NASA’s Goddard Space Flight Center, the International Research Institute for Climate and Society of Columbia University, NOAA’s Earth System Research Laboratory, and NOAA’s Climate Prediction Center.

4. What caused the 2011-14 California Drought?

The 2011-14 California drought was primarily caused by natural climate variability. A prolonged, enhanced high-pressure ridge off the West Coast prevailed during the last three winters, which is typical of the state’s historical droughts. West Coast high pressure was rendered more likely during 2011-14 by effects of sea surface temperature patterns over the world oceans. The drought’s first year (2011/12) was likely the most predictable, when La Niña effects largely explained high pressure off the West Coast. Climate model simulations indicate that high pressure continued to be favored due to ocean effects in the second and third years (2012/13 and 2013/14).

5. Is there contradiction between the Report's finding that the 2011-14 California drought was mostly unrelated to human-induced climate change, and the overwhelming body of scientific evidence that human-induced climate change is real?

No, there is no contradiction. We agree with the overwhelming body of scientific evidence that warming of the climate system is unequivocal, and that human influences have been the dominant cause of observed warming since the mid-20th century. However, in the case of the 2011-14 California drought, we found that natural variations of precipitation patterns were much more potent factors than the effects of human-induced climate change impacts on precipitation. The precipitation deficits over California during the last 3 years were principally the result of natural variations in weather patterns, a finding that neither confirms nor refutes scientific evidence that climate change is real. Rather, this event (and others like it around the world) serves as an important reminder of how intense natural climate variability can be, and that the climate system has an inherent capacity to generate extreme events with or without additional human influences. The current California drought, however, has been accompanied by extreme high surface temperatures, which have exacerbated the stresses on water resources. The magnitude of the warmth during this drought is considerably greater than during previous California droughts, and cannot be reconciled with natural variability alone but is partly a consequence of climate change.

6. Could this event have been anticipated?

The results of retrospective climate simulations reveal that about one-third of the magnitude of California’s average precipitation deficits during 2011-14 were a consequence sea surface temperature (SSTs) conditions, principally in the tropical Pacific. The report provides evidence that California winter precipitation is sensitive to slowly evolving SSTs, in particular related to the well-known El Niño-La Niña cycle. The drought’s predictability was judged to be highest during the 2011-12 winter when La Niña conditions prevailed, though some predictability was also identified during the two subsequent ENSO-neutral winters.

7. How does human-induced climate change affect the likelihood of this event?

The severe 3-year deficit in California’s wintertime precipitation is not a harbinger of future precipitation change for the state. The projections from climate models that contributed to the IPCC 5th Assessment indicate human-induced climate change will increase California precipitation in mid-winter associated with an increase in westerly flow entering the central Pacific West Coast and a low pressure anomaly over the North Pacific. The models also indicate a decrease in spring precipitation. Also, future California hydroclimate will be impacted by increased evapotranspiration due to warmer temperatures, according to climate models, offsetting any increase in precipitation. The effect of a warmer climate on surface moisture implies droughts that occur will be initiated sooner and ended later than if human-induced climate change were not occurring.

8. How did diminished Arctic sea ice affect the drought?

We found no conclusive evidence that the loss of Arctic sea ice contributed to the California drought. We factored both the global sea surface temperature conditions and the global sea ice conditions during the drought period into the design of our model experiments. Our finding of an insensitivity of mid-latitude weather to diminished Arctic sea ice aligns well with other recent studies that have explored the implications of Arctic warming for weather at lower latitudes. Importantly, the report provides a physical interpretation for the North Pacific high pressure—and its year-to-year changes during 2011-14—that is supported by existing theories of tropical-to-mid-latitude interactions.

9. Is this Report the final analysis of the 2011-14 California Drought? If not, what next steps are being planned?

This report presents a comprehensive analysis of climate simulations that bring together seven different sophisticated models that, collectively, have simulated the period of the drought (and earlier decades) 160 times. It offers a robust evaluation of the factors involved in the drought, and expresses our confidence in those results in light of reproducibility across various models and across the full set of experiments performed with those models. We expect (and hope) that these model data will be used by other researchers to examine further aspects of the drought, including diagnosis of the precise mechanisms responsible for its extreme magnitude, reasons why the third year of the drought was the most severe even though the ocean forcing was more intense during the drought’s first year, etc. The assessment does not present a complete analysis of the drought’s predictability, and an important next step would involve analysis of the real-time climate prediction systems. A key question is: why didn’t our prediction systems perform better during each winter of the drought? A long-term research challenge is to understand the context of the drought within a decadal context of overall dry conditions over Southwest North America, and how such conditions may have acted to increase the odds for dry California conditions.

10. How did the dry and hot conditions combine to cause the drought? Are both hot and dry conditions required for a drought?

Within a definition of drought as being a period during which water demand isn’t met by water supply, then the current California drought resulted principally from failed precipitation not elevated temperature. In the last 3-year period, the equivalent of about one year of precipitation was lost. The coincidence of high temperature with the lack of precipitation in these three years of drought undoubtedly made impacts worse, though that effect remains to be quantified. Historically, winters with low precipitation in California have not been strongly related to temperatures -- low rainfall can occur under the influence of various atmospheric circulation patterns which can deliver either warm (dry) or cool (dry) air masses. Thus, both hot and dry conditions are not necessary for drought.

11. Does this report say anything how the drought will evolve in coming months?

The particular ocean temperature pattern that prevailed during the last three years, and that acted to increase drought risks for California during 2011-14, has significantly changed. In particular, an enhanced contrast in temperatures between the cold east Pacific and the warm west Pacific has recently evolved to a new pattern characterized by emergent El Niño conditions. Thus, there is reduced risk for another failure in California’s rainy season this winter. However, the risk hasn’t been eliminated. The report also shows that random atmospheric variability alone can prolong droughts, and that most of the current drought’s severity and duration is likely due to natural variability. Unfortunately, due to limits in current scientific understanding, our ability to predict such atmospheric behavior is limited.

In late Fall 2014, the equatorial east Pacific Ocean was several degrees warmer than it was during the 2011-12 winter, and the West-East contrast in ocean temperatures was weakened compared to 2011-14. The November 2014 Climate Prediction Center Winter Outlook indicates about a 45% chance of central to southern California precipitation being in the upper third of its historical distribution, though little guidance is available for northern California and the northern Sierras which is a key region for California’s water resources.

Note that El Niño conditions alone might not be a panacea for the existing drought, if they continue to develop. While it is reasonable to assume that California’s averaged precipitation amounts will likely be greater than during the last several winters, also note that two of the driest winters on record occurred during the 1976-77 and 1986-87 weak El Niño events. This report doesn’t address why during some El Niños there are statewide wet conditions, while during others there is drying. Further research is required to address that critical issue.


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