The precipitation forecast for the next 15 days is neither encouraging or completely dismal. While the heaviest precipitation will likely stay to our north and our east, there are signs that we’ll see another storm or two through the first half of April.
Information from a recent scientific paper that attempts to explain the California drought and the state’s increased risk of drought due to warmer temperatures.
The title of the paper is “Anthropogenic warming has increased drought risk in California” and is freely available to read here: http://m.pnas.org/content/early/2015/02/23/1422385112.full.pdf
Below copied relevant passages from the text of the study and tried to display them in an order that honestly and quickly summaries the paper:
California ranks first in the United States in population, economic activity, and agricultural value.
California is currently in the midst of a multi-year drought. The event encompasses the lowest calendar-year and 12-month precipitation on record, and almost every month between December 2011 and September 2014 exhibited multiple indicators of drought.
Although precipitation deficits are a prerequisite for the moisture deficits that constitute “drought” (by any definition), elevated temperatures can greatly amplify evaporation, thereby increasing overall drought intensity and impact.
Temperature is especially important in California, where water storage and distribution systems are critically dependent on winter/spring snowpack, and excess demand is typically met by groundwater withdrawal.
The impacts of runoff and soil moisture deficits associated with warm temperatures can be acute, including enhanced wildfire risk, land subsidence from excessive groundwater withdrawals, decreased hydropower production, and damage to habitat of vulnerable riparian species.
Because California’s dry season occurs during the warm, summer months, soil moisture loss through evapotranspiration is typically high—meaning that soil moisture deficits that exist at the beginning of the dry season are exacerbated by the warm conditions that develop during the dry season, as occurred during the summers of 2013 and 2014.
While a recent report by Seager et al. (30) found no significant long-term trend in winter precipitation in California during the 20th and early 21st centuries, which is consistent with our findings…
…we found that the probability that below normal precipitation co-occurs with warmer than average temperatures has increased recently, with warm+dry years occurring more than twice as often in the past two decades as in the preceding century.
Although there is clearly value in understanding possible changes in precipitation, our results highlight the fact that efforts to understand drought without examining the role of temperature miss a critical contributor to drought risk.
Analyzing historical climate observations from California, we find that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm.
Indeed, our results show that even in the absence of trends in mean precipitation, or trends in the occurrence of extremely low-precipitation events, the risk of severe drought in California has already increased due to extremely warm conditions induced by anthropogenic global warming.
Climate model experiments … reveal that human activities have increased the probability that dry precipitation years are also warm.
Further, a large ensemble of climate model realizations reveals that additional global warming over the next few decades is very likely to create ∼100% probability that any annual-scale dry period is also extremely warm.
Numerous paleoclimate records also suggest that the region has experienced multi-decadal periods in which most years were in a drought state, albeit less acute than the current California event. Although multi-decadal ocean variability was a primary cause of the mega-droughts of the last millennium, the emergence of a condition in which there is ∼100% probability of an extremely warm year substantially increases the risk of prolonged drought conditions in the region.
Explanation of the last bullet point: There have been big droughts in the past 500-1000 years, and these droughts were caused by natural, non-human factors. But, the recent warming of the earth, most likely caused at least in part by humans, shifts the probabilities toward drought in California, simply because of the warmer temperatures.
So what does all of this mean?
While years with low precipitation in California do not seem to be on the increase, and are not forecasted to increase during the coming decades, it is clear that temperatures have increased and are forecasted to continue to increase. This increase in temperatures increases the risk of drought because it strains water supplies due to more evaporation and faster snow melt (or more precipitation falling as rain instead of snow).
To be clear, this paper is NOT saying that there will be less precipitation in California in our future climate, but simply that, due to warmer temperatures, the risk of drought has and will increase.