WaterHow climate change is making California’s epic drought worse
California is undergoing a record-setting drought that began in 2012, the worst in at least 1,200 years. California and other southwestern states have suffered through multi-year droughts in the past, but how does climate change figure into what’s happening now? Can scientists separate the effect of rising greenhouse gas levels on the current drought from other factors? Detecting and attributing observed or projected impacts to man-caused climate change is not an easy task. But there is some supporting evidence from improving numerical climate models and the record of several diverse meteorological and hydrological events already happening, including heat waves, flooding, or droughts. Scientists can run climate model experiments that include only natural variability and then include manmade factors, such as greenhouse gases. These tools serve to highlight and distinguish the dominant mechanisms responsible for particular air circulation characteristics. These climate model simulations show that the extreme and persistent circulation patterns that have caused droughts on the West Coast this century are due to anthropogenic external forces, not natural causes. Studies suggest that climate change might give rise to a new climate regime, one in which the years of low precipitation will be accompanied by warm conditions, creating the aforementioned “warm drought.”
California is undergoing a record-setting drought that began in 2012, the worst in at least 1,200 years. It can be seen in many ways: most of the freshwater reservoirs are drying up, crops are wilting in the fields and groundwater is rapidly depleting.
Whatever definition is used for drought, based on meteorological, hydrological, agricultural, or socioeconomic references, all the indicators are showing an extreme situation due to particularly warm and dry conditions. Last year had the lowest calendar-year precipitation on record, leading to acute water shortages, groundwater overdraft to replace the missing rain, critically low stream flow and high wildfire risk.
California and other southwestern states have suffered through multi-year droughts in the past, but how does climate change figure into what’s happening now? Can scientists separate the effect of rising greenhouse gas levels on the current drought from other factors? To answer these questions, we need to consider the meteorological forces that drive California’s weather.
Eye on the Pacific
California’s winter precipitation normally comes from east-traveling North Pacific storms, which are guided by the strong river of wind known as the jet stream. Year-to-year variations in precipitation are affected by a number of factors, including the El Niño cycle of warming Pacific Ocean temperatures as well as long-term oscillations in climate that cause natural variability over decades or hundreds of years. Also at play are normal short-term changes in the atmosphere and the effect of long-term climate change.
Droughts are caused when there is a substantial reduction in the number of precipitation-carrying Pacific storms to reach the West Coast. They are blocked by a region of persistently high atmospheric pressure, called an upper-level ridge. This ridge of high pressure directs the jet stream toward a much more northerly track. The result is that most precipitation systems miss California, with some of them going as far north as British Columbia and Alaska.
In 2013 and 2014, the sea surface temperature in the North Pacific was unusually high, which created the conditions for a persistent high-pressure ridge and thus dryness over California. In addition, reduced Arctic sea ice is associated with warmer surface temperature in the Pacific and reduced rain and snowfall in the western part of the US from the northward shift in the storm track.
