Causes Of California Drought Linked To Human-caused Climate Change

Image Caption: The drought crippling California is by some measures the worst in the state's history. Credit: NOAA

National Science Foundation

The atmospheric conditions associated with the unprecedented drought currently afflicting California are very likely linked to human-caused climate change, Stanford scientists say.

In a new study, a team led by Stanford climate scientist Noah Diffenbaugh used a novel combination of computer simulations and statistical techniques to show that a persistent region of high atmospheric pressure hovering over the Pacific Ocean that diverted storms away from California was much more likely to form in the presence of modern greenhouse gas concentrations.

The research, published on Sept. 29 as a supplement to this months issue of the Bulletin of the American Meteorological Society, is one of the most comprehensive studies to investigate the link between climate change and Californias ongoing drought.

Our research finds that extreme atmospheric high pressure in this regionwhich is strongly linked to unusually low precipitation in Californiais much more likely to occur today than prior to the human emission of greenhouse gases that began during the Industrial Revolution in the 1800s, said Diffenbaugh, associate professor of Environmental Earth System Science at Stanford and a senior fellow at the Stanford Woods Institute for the Environment.

The exceptional drought currently crippling California is by some metrics the worst in state history. Combined with unusually warm temperatures and stagnant air conditions, the lack of precipitation has triggered a dangerous increase in wildfires and incidents of air pollution across the state. A recent report estimated that the water shortage would result in direct and indirect agricultural losses of at least $2.2 billion, and lead to the loss of more than 17,000 seasonal and part-time jobs in 2014 alone. Such impacts prompted California Governor Jerry Brown to declare a drought emergency, and the federal government to designate all 58 California counties as natural disaster areas.

Scientists agree that the immediate cause of the drought is a particularly stubborn blocking ridge over the northeastern Pacific popularly known as the Ridiculously Resilient Ridge, or Triple Rthat prevented winter storms from reaching California during the 2013 and 2014 rainy seasons.

Blocking ridges are regions of high atmospheric pressure that disrupt typical wind patterns in the atmosphere. Winds respond to the spatial distribution of atmospheric pressure, said Daniel Swain, a graduate student in Diffenbaughs lab and lead author of the study. We have seen this amazingly persistent region of high pressure over the northeastern Pacific for many months now, which has substantially altered atmospheric flow and kept California largely dry.

Blocking ridges occur periodically at temperature latitudes, but the Triple R was exceptional for both its size and longevity. While it dissipated briefly during the summer months of 2013, it returned even stronger by fall 2013 and persisted through much of the winter, which is normally Californias wet season. At its peak in January 2014, the Triple R extended from the subtropical Pacific between California and Hawaii to the coast of the Arctic Ocean north of Alaska, said Swain, who coined the term ridiculously resilient ridge last fall to highlight the unusually persistent nature of the offshore blocking ridge.

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Causes Of California Drought Linked To Human-caused Climate Change