Global warming and drought are turning the Golden State brown

There’s a rapidly growing body of scientific research finding that California is in the midst of its worst drought in over a millennium, global warming has made the drought worse, and decades-long mega-droughts could become the norm in the state later this century. A new study published in the Proceedings of the National Academy of Science (PNAS) by scientists at Stanford University adds to this bleak picture for the Golden State.

There has been some confusion about the human contribution to California’s drought, now entering its fourth consecutive year, because some reports have said that humans have not influenced the amount of precipitation falling in the state thus far. This is a subject of debate – some studies have found evidence of a human ‘fingerprint’ in the high pressure ridge that’s diverted storms away from California over the past three years. But overall, while precipitation has been low, there have been a few years in the historical record where it was lower.

However, evidence indicates that California is in the midst of its worst drought in over 1,200 years. The new PNAS paper helps reconcile these two facts. As an accompanying commentary by Michael Mann and Peter Gleick notes,

Part of the challenge is that the term “drought” can be defined in different ways: for example, meteorological, hydrological, agricultural, and socioeconomic drought. Drought, most simply defined, is the mismatch between the amounts of water nature provides and the amounts of water that humans and the environment demand.

California’s worst droughts have historically happened in years that are both dry and hot. While humans may or may not be influencing the amount of rain falling in the state, we are indisputably making it hotter. If we could flip coins representing precipitation and temperature each year, the first could come up wet or dry, but humans are weighting the second such that it will increasingly come up hot. This will make conditions like those that caused California’s current record-breaking drought return more often as the planet keeps warming.

California temperature (°F) and precipitation (inches) anomalies from January 1895 to November 2014, plotted as 3-year anomalies relative to 1901–2000 mean. Data from the National Climatic Data Center nClimDiv dataset.  Source: PNAS; Mann & Gleick (2015).

The PNAS paper summarizes the significance of these findings.

California ranks first in the United States in population, economic activity, and agricultural value. The state is currently experiencing a record-setting drought, which has led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk ... we find that human emissions have increased the probability that low-precipitation years are also warm, suggesting that anthropogenic warming is increasing the probability of the co-occurring warm–dry conditions that have created the current California drought.

Mann and Gleick similarly note that the Golden State is quickly becoming an arid brown state, but that we still have an opportunity to stop this dangerous transition.

As drought has taken hold, the Golden State is slowly becoming a more arid, brown state, where constraints on water availability threaten a large and growing population (up nearly 80% since the severe drought of 1976–77), unique ecological resources, a major source of agricultural produce, and one of the largest economies in the world ... That might sound like bad news, and certainly the trends are moving rapidly in the wrong direction. The good news, however, is that this is only one possible future. If society works to limit global warming to under 2 °C, which is still possible (1), then we can likely avoid committing to a brown California. California still has a chance to remain the Golden State.

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Posted by dana1981 on Monday, 30 March, 2015

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