In a report released Wednesday that we wrote for the National Oceanic and Atmospheric Administration, we found that this drought was triggered by an exceptional multi-season period of low rain and snowfall. The drought was intensified by extreme temperatures fueled by climate change that dried out soils, rivers and vegetation. It is projected to cause tens of billions of dollars in impacts.
The intense 2020 and 2021 wildfire seasons, Western heat waves, and profound challenges to energy, water and recreational sectors are interconnected with this drought, which is anticipated to last into 2022, and potentially beyond.
How bad is this drought?
Drought occurs when the supply of water cannot meet demand and is typically caused by a period of low precipitation. Droughts often intensify themselves — as soils dry because of a lack of precipitation, the air warms, which can increase snowmelt and further desiccate soils, rivers, vegetation and reservoirs. A warmer atmosphere evaporates more water from the ground.
In particular, the report documents how successive dry winter seasons in 2019-2020 and 2020-2021, together with a failed 2020 summer southwestern monsoon, led precipitation totals since January 2020 to be the lowest on record since at least 1895 over the entirety of the Southwest. At the same time, temperatures across the six states considered in the report (Arizona, California, Colorado, Nevada, New Mexico and Utah) were at their third highest on record.
Together, the exceptionally low precipitation and warm temperatures reduced snowpack and increased evaporation of soil moisture, leading to a persistent and widespread drought over most of the American West.
Wildfires have been a major consequence of this drought. The 2020 wildfire season burned more than 10 million acres, with exceptionally large areas burned in California, Colorado and Arizona. This year, wildfire risk has been centered on California, but the fires there have been intense enough to pollute the air on the other side of the country, and beyond. With the nation’s wildfire resources fully committed to fighting fires by mid-July, 2021 has rivaled 2020 as an extremely costly and lengthy wildfire season.
Moreover, many surface water reservoirs, which store water and are designed to buffer periods of drought, dropped to historic lows: about 57 percent of average spring capacity across all six states, leading into summer, when water demands peak. Such historically low reservoir levels throughout the Southwest have led to electricity blackouts amid record-setting heat waves as electricity demand is at its highest because of air conditioning.
As the economy recovers from the coronavirus pandemic, the low reservoir levels also disrupt fishing and recreation (as boat launches close in national parks, for example), slowing recovery for many tourism-based economies. In fact, the immediate economic losses associated with the drought for 2020 alone are approximately between $515 million and $1.3 billion, not including losses from associated wildfires, which raises the costs to be between $11.4 billion and $23 billion. The total cost of an event like this will take years to estimate, as drought impacts are felt deeply and broadly throughout the national economy.
What caused this drought?
The report examined both precipitation and temperature to answer this question.
On precipitation, the report found that the 2020-21 Southwestern drought was initially triggered by an unfortunate (and, most likely, natural) sequence of low-precipitation seasons beginning in 2020, but then was made much worse by both natural and, in particular, human-caused warming.
Both observations and climate model simulations suggest that randomness in atmospheric circulation was the main cause of the initial precipitation declines. Conditions in the remote oceans have a significant effect on weather and climate in the United States. In the case of this drought, cooler-than-average sea surface temperatures in the tropical Pacific and warmer-than-average sea surface temperatures in the tropical Atlantic also may have depressed precipitation in some seasons and regions.
But the report found that the incredibly warm air temperatures the Southwest experienced in 2020 and so far in 2021 far exceeded those that can be explained by random year-to-year variations in climate. While the extremely low precipitation and resulting dry soils contributed to these high temperatures, it is virtually certain that warming driven by greenhouse gas emissions played a major role in making this drought so severe and widespread. Beyond evaporating water in soils, rivers and plants more quickly, such high temperatures from regional warming accelerated melting of mountain snowpack so crucial to water supplies in Western U.S. basins.
To make its claims, the report presents an analysis of observations and 33 coupled state-of-the-art computer models, assessing the role of greenhouse gas-driven warming on the temperature and precipitation that occurred during 2020 and 2021. The results show that the exceptional temperatures helped drive significant vapor pressure deficits, a key variable that measures the atmospheric thirst for moisture from the land. The vapor pressure deficits observed in 2020 were essentially impossible in the climate before 2000. Such extraordinary atmospheric moisture demand has been made more likely by the warming from greenhouse gas emissions.
When will this drought end?
Seasonal outlooks and the onset of a La Niña event in the tropical Pacific mean that it is unlikely that winter precipitation in the Southwest will be plentiful enough to end the drought. As such, it is expected that dry and warm conditions will persist into 2022, and perhaps longer.
Even if precipitation returns this winter, it will take several wet (and preferably cooler) years to make up for the water shortage in reservoirs and rivers to fully end the drought across the region. Arizona provides a case in point — the state enjoyed a particularly wet summer monsoon this year, which improved the drought conditions measurably. However, 87 percent of the state is still in drought, with 40 percent of the state in severe or worse drought conditions.
While several seasons of above-average rainfall and high-elevation snowpack can help the Southwest eventually emerge from the drought, there is still the future to consider. This extraordinary drought is just the latest expression of a longer-term aridification the region is undergoing because of both natural long-term variations in precipitation as well as greenhouse-gas-induced warming. For example, flows in the Colorado River have been declining steadily over the past 20 years and had approached the threshold for a basin-wide water shortage declaration even before this drought first emerged.
With continued warming, the atmospheric demand for soil moisture will increase unabated, making even randomly occurring low or near-normal precipitation years a more sensitive drought trigger — meaning droughts like this one will happen more often. In fact, the report shows, based on projections from the latest group of climate model simulations provided in support of the Intergovernmental Panel on Climate Change, that the record-high vapor-pressure deficit seen in this exceptional drought would occur more than once a decade by 2030-2050. What is clear is that the long-term increase of vapor-pressure deficit and associated aridity over the Southwest United States will not end without curbing greenhouse gas emissions.
While 2020-21 was an exceptional period of low precipitation, the drought that has emerged is a harbinger of a future that the more arid Southwest must take steps to manage.
Fu is a professor of atmospheric sciences at the University of California at Los Angeles; Simpson is a climate scientist at the National Center for Atmospheric Research; Mankin is a professor of climate variability and change at Dartmouth College; Hoell is a climate scientist at NOAA; and Barrie is climate program manager at NOAA.