Before and after photo of Lake Powell comparing 1999 to 2021. Photo Credit: NASA Earth Observatory

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How severe has the Southwestern Drought been?

Widespread drought emerged in early 2020 in the U.S. Southwest, defined in this report as the four corners states (Arizona, Utah, Colorado, and New Mexico), Nevada, and California. Over the 20-month period from January 2020 through August 2021, the Southwest U.S. received the lowest total precipitation and had the third-highest daily average temperatures since 1895.

The drought has led to unprecedented water shortages in western reservoirs threatening drinking, agricultural, and tribal water supplies; electricity supply generated from hydroelectric plants; and fishing and recreational activities. The massive western wildfire seasons of 2020 and 2021 have been fueled by the lack of precipitation and surface moisture associated with the drought. Immediate economic losses are in the billions of dollars, while ultimate cumulative losses, which won’t be calculable until the event ends, will likely be in the tens of billions of dollars based on the costs of similar significant droughts in the past. 

U.S. Drought Monitor (USDM) map displaying drought categories across the Southwest for the week of September 14th, 2021. Source: drought.gov

What factors caused the Southwestern Drought?

The combined factors of record low precipitation and near-record high temperatures caused and have fueled the drought.

Precipitation in the Southwest comes from a number of sources. The two primary sources are: (1) storms that develop over the Pacific Ocean traveling along the jet stream and making landfall on the West coast, typically in the cool season, and (2) the North American Monsoon, which is a wind system that transports moisture from the Pacific Ocean, Gulf of California, and Gulf of Mexico and precipitates from thunderstorms generated over the Four Corners region. Successive precipitation season failures led to a build-up of dry conditions. First, the winter storms of 2020 brought below-average precipitation to California and Nevada generating low rainfall amounts and snowpack. A failure of the summer monsoon in 2020 led to the driest summer in history in the Southwest. A dryer-than-normal winter 2020-2021 throughout the Southwest then led to further precipitation deficits. 

Average temperatures in the Southwest have been approximately 3 ˚F above normal during the drought, and the period of January 2020 through August 2021 was the third hottest on record. High temperatures lead to an increase in evaporation of water from soils and vegetation, magnifying the effects of the precipitation deficits described above, and significantly intensifying the drought. Moreover, extreme air temperatures and drought are intertwined -- with less water in the soils to evaporate, that energy instead goes into heating the lower atmosphere, further exacerbating the drought and leading to heat waves.

Is the Southwestern Drought a symptom of long term climate change?

The report finds that the precipitation deficits result from a complex set of factors depending on the season  (cool season vs. monsoon). During the cool season, La Niña likely led to a change in the atmosphere that delivered less winter precipitation to the west coast. Much of this is likely due to natural variations. The causes of the failure of the summer monsoon are less well-understood. Various model results examined in the report suggest that successive low-precipitation seasons like this may occur by chance.

The high temperatures during this period, however, are significantly contributed to by climate change. These high temperatures have a number of direct impacts on the drought, including increased evaporation of surface and soil moisture and less snowfall (which reduces dry-season surface water resources from snowmelt). Indirect impacts such as the drought-temperature feedback are also important, and are influenced by climate change. High temperatures also significantly increase demand for water resources, placing an indirect strain on water systems.

Records of precipitation (left) and temperature (right) in the Southwest U.S. since 1895. Dots show individual 20-month periods corresponding to the analysis period in the report, while the solid gray line shows the 20-year average. The exceptional nature of the most recent 20-month period is clear in the long-term records, as is the multi-decadal upward trend in temperature.

Applying our best science to events like this reveals areas where more research is needed. Drought is a complex phenomenon that results from significant interactions between natural weather and climate variability, climate change, the ecosystem, and human management and use of water resources. The work in this report highlights the need for:

• Better understanding of the effects of climate change on atmospheric behavior leading to regional precipitation patterns
• Disentangling the relative importance of the inter-related variables that influenced this and other droughts
• Understanding the importance and influence of key ocean-atmosphere interactions that influence weather and climate variability over the United States (e.g., the influence of La Niña events, which were present in 2020-2021 and are expected in the winter of 2021-2022)
• Studying the changing nature of western snowpack and its implications
• Improvements to reduce forecast errors that impact water management decisions

This figure is an updated version of the left hand panel of Figure 4 of the report.  In Figure 4 of the report, annual averages were used and the annual average of 2020 was compared with the probabilities of annual average precipitation and vapor pressure deficit from the CMIP6 models.  Given that the drought has now lasted for two years, this figure shows 2 year averages over the observational record and assesses the probability of having a 2 year average period like 2020-2021 within the CMIP models.  Overall, similar conclusions can be drawn for this two year period as were drawn for 2020 with slight differences in the probabilities i.e., for precipitation, a 2 year period like 2020-2021 is an extreme event and the models do not indicate a systematic change in the likelihood of such an event as a result of anthropogenic forcing.  For vapor pressure deficit, a 2 year period like 2020-2021 does not happen in the CMIP models in the climate prior to the year 2000.  With climate change, that probability has increased but it is still an extreme event within the present day climate and by 2030-2050 the probability of having a 2 year period with VPD as low as 2020-2021 increases substantially.

NOAA DROUGHT TASK FORCE REPORT ON THE  2020-2021 SOUTHWESTERN U.S. DROUGHT

September 21st, 2021

This report was prepared by NOAA Drought Task Force, NOAA MAPP, and NIDIS personnel: 

Justin S. Mankin*, Isla Simpson*, Andrew Hoell*, Rong Fu, Joel Lisonbee, Amanda Sheffield, and Daniel Barrie with key contributions from H. Annamalai, Arun Kumar, Richard Seager, Martin Hoerling, Mingyue Chen, Cenlin He, Paul Dirmeyer Jason E. Smerdon, Flavio Lehner, Mathew Barlow, Christopher B. Skinner, and Ben Livneh.

*These authors contributed equally.

The NOAA Drought Task Force is organized by the Modeling, Analysis, Predictions and Projections (MAPP) Program of OAR/Climate Program Office.

This report was produced in partnership with the National Integrated Drought Information System (NIDIS).