Scientists find that climate change is contributing to droughts in the American West, even without rainfall deficits

The Enterprise Bridge passes over a portion of nearly dry Lake Oroville on September 30, 2014 in Oroville, California. Credit: Andrew Innerarity/California Department of Water Resources
Rising temperatures caused by anthropogenic climate change turned a normal drought into an abnormal one, drying out the American West from 2020 to 2022. A study by climate scientists at UCLA and the National Oceanic and Atmospheric Administration found that evaporation accounts for 61% of drought severity, while reduced precipitation accounts for only 39%. The study found that evaporation demand has played a larger role in droughts since 2000 than decreased precipitation, suggesting that droughts will become more severe as the climate warms. There is.
“While studies have already shown that rising temperatures contribute to droughts, to our knowledge this is the only study to actually show that water loss due to demand is greater than water loss due to lack of rainfall. “This is the first time,” said Professor Long Fu. Professor of Atmospheric and Oceanic Sciences at UCLA and corresponding author of the study published in Science Advances.
Historically, droughts in the West have been caused by a lack of precipitation, with evaporative demand playing a minor role. Climate change caused by the burning of fossil fuels is complicating this situation by increasing average temperatures. Droughts caused by natural rainfall fluctuations still exist, but there is increased heat to draw water from bodies of water, plants, and soil.
“For generations, drought has been associated with drier-than-normal weather,” said Veba Deheza, executive director of the NOAA National Integrated Drought Information System and co-author of the study. . “This study further confirms that we have entered a new paradigm in which rising temperatures cause severe droughts with precipitation as a secondary factor.”
A warmer atmosphere holds more water vapor before the air parcel becomes saturated and the water condenses and forms precipitates. For rain to fall, water molecules in the atmosphere must come together. Heat keeps water molecules moving and repelling each other, preventing condensation. This creates a cycle in which the warmer the Earth becomes, the more water evaporates into the atmosphere, but a smaller portion returns as rain. Droughts therefore last longer, cover larger areas, and become drier every time the earth warms even a little.
To study the effects of high temperatures on droughts, researchers separated “natural” droughts caused by changing weather patterns from droughts caused by anthropogenic climate change in 70 years of observational data. Previous studies have used climate models that incorporate increases in greenhouse gases to conclude that rising temperatures are contributing to the drought. However, without observational data on actual weather patterns, it was not possible to pinpoint the role played by evaporative demand due to naturally changing weather patterns.
When including these natural weather patterns, researchers were surprised to find that 80% of the increase in evaporative demand since 2000 was due to climate change. During periods of drought, that number increases to more than 90%, with climate change being the only contributing factor. The biggest factor is the increasing severity of droughts and expansion of drought areas since 2000.
Compared to the 1948-1999 period, increased evaporative demand resulted in a 17% increase in average drought area in the American West from 2000 to 2022. Since 2000, in 66% of historic and newly drought-prone regions, high evaporative demand alone can cause droughts, which can occur even in the absence of rainfall deficits. It means that. Before 2000, that was the case in only 26% of the region.
“During the 2020-2022 drought, water demand really spiked,” Hu said. “Droughts began with a decrease in natural precipitation, but climate change has increased their severity from ‘moderate’ to ‘exceptional’ on the drought severity scale. ”
According to the U.S. Drought Monitor, “moderate” means the most severe drought in the 10 to 20 percent range of severity, and “exceptional” means the most severe drought in the top 2 percent of severity.
Further climate model simulations supported these findings. Greenhouse gases from burning fossil fuels are therefore causing droughts like the one in 2020-2022 to go from being extremely rare events that occur every 1,000 years to occurring once every 60 years by the mid-21st century, and by the second half of the 21st century. This has led to predictions that this will change to a phenomenon that occurs every six years.
“Even if rainfall appears normal, droughts can still occur because the demand for water has increased so much that there isn’t enough water to keep up with that increased demand.” said Mr Fu. “This isn’t something you can prevent by creating larger reservoirs or anything, because as the atmosphere warms, it just sucks more moisture everywhere. The only way to prevent this is to We need to stop the rise in greenhouse gas emissions. ”
Further information: Yizhou Zhuang et al., Anthropogenic warming has ushered in an era of temperature-dominated drought in the western United States, Science Advances (2024). DOI: 10.1126/sciadv.adn9389
Provided by University of California, Los Angeles
Citation: Scientists find climate change contributes to drought in western U.S., even without rainfall deficit (November 6, 2024) https://phys.org/news/2024-11-climate- contributing-drought- Retrieved November 6, 2024 AmericanWest.html
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