Drought

California’s infamous 2011-2017 drought was made worse by climate change; scientists have identified telltale climate fingerprints on the drought’s hottest and driest years (2011-2014).[9] These ‘hot droughts’ are now twice as likely to occur in California due to climate change, just from higher temperatures drying out soil and evaporating water.[10] Record-high temperatures made the 2012-2014 drought 36 percent worse,[11] and were responsible for 8-27 percent of drier soil.[12]

These hot temperatures also significantly reduced California’s snowpack, which provides the state with much of its water. It is very likely (>80 percent chance) that climate change made the 2014 CA drought worse by reducing the snowpack and causing earlier snowmelt, leaving streams dry by summer.[13] Over 2011-2015, climate change reduced California’s snowpack by 25 percent.[14]

The effect on precipitation is less clear. There is, however, some evidence that the low precipitation in 2013 and 2014 could be linked to climate change,[15] partly by having made the ‘ridiculously resilient ridge’ - which blocked rain from reaching California - twice as likely.[16] Additionally, there is evidence that higher temperatures in the Northern Hemisphere due to climate change have displaced the tropical rain belt leading to less rainfall over Northern California.[2] Ultimately, whether or not climate change has already reduced precipitation, it has significantly exacerbated drought in California simply from hotter temperatures.

The effects seen in California can be seen in much of the Western US.

Streamflow in the Colorado River is decreasing due in large part to warmer temperatures that cause evaporation and early snowmelt.[17][18] The river’s annual average streamflow is decreasing by around 9 percent per degree Celsius (1.8 degrees Fahrenheit) of warming and warmer temperatures are responsible for half of the 16 percent decrease in flow in the Colorado River between 2000 and 2017.[19] Increasing risk of severe water shortages is expected.

In semi-arid environments - including the Southwest and large portions of the Great Plains - hotter temperatures can dry out land. In the Northern Great Plains, higher than average temperatures have contributed to lower streamflow since 1980 and exacerbated the “turn-of-the-century drought” in the region from 2000 to 2010.[20] Warmer temperatures made the 2017 Great Plains (the Dakotas and Montana) drought 50 percent more likely.[21]  Climate change also made the 2011 Texas drought more likely to occur; specifically, it greatly increased the frequency of hot temperatures by an order of magnitude and likely decreased precipitation totals.[22] Climate change has made extreme precipitation deficits (like those in the Central US in summer 2012) three to four times more likely.[23]

Just like California, climate change is leading to ‘hot droughts’ in the US Southwest. Since 2000, this region has been in perpetual drought conditions - a ‘megadrought’ - with human-caused warming responsible for about half the severity.[1] Within the megadrought, there have been exceptional drought years driven by climate warming.[4][24] In 2018, for example, hotter temperatures decreased the snowpack water content by 20 percent and reduced vegetation greenup - a proxy for rangeland conditions.[5]

The mountainous Western US is heavily dependent on a good winter snowpack. Climate change has led to more rain rather than snow, a lower overall snowpack, and early snowmelt: this has caused earlier runoff(streamflow), leaving late spring and early summer without water. At least half of this low late-winter snowpack and the earlier runoff is due to warmer temperatures.[25][26] These increases in hot temperatures - particularly daytime winter temperatures - are driven by climate change.[27] The largest effect on snowpack and number of snow days is in the Northwest,[28] where warm temperatures have significantly reduced snowpack.[29][30]