Last updated December 1, 2020
Aug 1, 2020
- Ongoing

Western Drought 2020

Colorado
USA

Climate change is a major contributor to the intensity of the prolonged drought centered over the Four Corners region of the western US in 2020 as well as the underlying megadrought that has impacted the region since 2000.[1] Hotter temperatures melt snowpack earlier, evaporate water in streams and lakes, and evaporate water from soils, intensifying what would have been more moderate drought conditions. Climate change also increases the risk of seasonal precipitation extremes—including longer and hotter periods with little precipitation that dry out the soil as well as heavier rain and snow events leading to excessive runoff and flooding. Many events have contributed to the severity of the 2020 western drought including low rainfall during the 2019 monsoon season, a meager 2019-2020 snow season, and an exceptionally weak monsoon again in 2020.

Discover how Climate Signals are related

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Climate science at a glance

  • Climate change exacerbates drought conditions in the western US through a combination of hot temperatures and, in some areas, a lack of precipitation.
  • Climate change has increased the risk of temperature driven droughts, or ‘hot droughts’ in the Southwest, particularly since 2000.[1]
  • Streamflow in the Colorado River is decreasing, driven by warmer temperatures that cause evaporation and early snowmelt.[2][3]
  • Higher temperatures sharply increase the risk of megadroughts — dry periods lasting 10 years or more.[5]
  • Warmer and drier conditions brought by climate change account for nearly half the severity of the megadrought that has impacted the southwestern US since 2000.[4]

Background

What is a drought?

Drought is about water shortage - the result of complex physical interactions between the land surface and atmosphere. Droughts can develop quickly - flash droughts are short periods of warm surface temperature characterized by low and rapidly decreasing soil moisture - and they can take shape over much longer periods, as in the case of 'megadroughts', with extreme aridity lasting for decades.

Drought is sometimes defined by the weather or climate conditions that drive below average water supply, as in the case of 'hot droughts', driven by high temperatures that increase the evaporation of surface moisture into the air, or 'meteorological drought', when dry weather patterns lead to below average precipitation. Drought is also defined by the systems impacted by reduced water supply. 'Agricultural drought', for example, occurs when there is insufficient water or soil moisture to meet agricultural demands, and 'hydrological drought' occurs when there's a shortage in water resources, including groundwater and rivers.

Ultimately, we care about droughts when there is not enough water to meet needs.

Is the "nonsoon" or failed monsoon a signal of climate change?

In a typical summer, atmospheric high pressure over the southern Rocky Mountains draws moisture in from the Gulf of California and Gulf of Mexico, making July and August the rainiest time of year in Arizona, New Mexico, and the Four Corners region. This rainy period typically provides half of the year’s precipitation in some areas.

However, the 2020 monsoon brought little rainfall due to atmospheric high pressure that was centered farther south than usual, drawing in westerly winds. During the 2019 monsoon season, communities in northern Arizona, including Flagstaff and Phantom Ranch at the bottom of the Grand Canyon, experienced record low rainfall. Phoenix's season was the fifth driest recorded. St. George in southern Utah received just traces of rain and most of the Four Corners region didn't fare much better.

Whether and how climate change increases the risk or failed monsoon seasons is an active area of research, but scientists don’t expect a big change to the Southwest’s summer monsoons:

There is still a lot uncertainty with summer season precipitation because the climate models really struggle to handle all of the small regional nuances that drive regional monsoon precipitation. Largely, they continue to predict little change with maybe some increasing inten­sity with thunderstorm events and maybe fewer storms overall within the season, but, again, confidence is low."

Mike Crimmins, University of Arizona


Climate signals breakdown

Climate signals #1 and #2: Land surface temperature increase and drought risk increase

The average annual surface temperature of the Southwest increased 1.6°F (0.9ºC) between 1901 and 2016, increasing drought risk and taxing water resources. Higher temperatures also sharply increase the risk of megadroughts — dry periods lasting 10 years or more — such as the one that has impacted the southwestern US since 2000. Hotter temperatures draw out extra moisture from the soils, intensifying drought conditions.

Observations consistent with climate signals #1 & #2

  • The first week of August 2020, the US Drought Monitor assessment designated all of Colorado, and much of the West, in some stage of drought. It’s the fourth time in two decades — following 2002, 2006 and 2012 — that all of Colorado was designated as abnormally dry or in drought.
  • By mid-October, every part of Colorado was in at least moderate drought, with 16.72 percent in the most severe category of “exceptional,” mostly on the Western Slope.
  • On November 25, large percentages of the hardest hit states were in “exceptional drought”, the worst category, including Arizona (72 percent), New Mexico (51 percent), Utah (46 percent), and Colorado (27 percent).