November 9, 2024 - By Holly Ober - Higher temperatures caused by anthropogenic climate change made an ordinary drought into an exceptional drought that parched the American West from 2020–2022. A study by UCLA and National Oceanic and Atmospheric Administration climate scientists has found that evaporation accounted for 61% of the drought’s severity, while reduced precipitation only accounted for 39%. The research found that evaporative demand has played a bigger role than reduced precipitation in droughts since 2000, which suggests droughts will become more severe as the climate warms. 

“Research has already shown that warmer temperatures contribute to drought, but this is, to our knowledge, the first study that actually shows that moisture loss due to demand is greater than the moisture loss due to lack of rainfall,” said Rong Fu, a UCLA professor of atmospheric and oceanic sciences and corresponding author of a study published in Science Advances.

Historically, drought in the West has been caused by lack of precipitation, and evaporative demand has played a small role. Climate change caused by the burning of fossil fuels has resulted in higher average temperatures that complicate this picture. While drought-induced by natural fluctuations in rainfall still exist, there’s more heat to suck moisture from bodies of water, plants and soil.

“For generations, drought has been associated with drier-than-normal weather,” said Veva Deheza, executive director of NOAA’s National Integrated Drought Information System and study co-author. “This study further confirms we’ve entered a new paradigm where rising temperatures are leading to intense droughts, with precipitation as a secondary factor.”

A warmer atmosphere holds more water vapor before the air mass becomes saturated, allowing water to condense and precipitation to form. In order to rain, water molecules in the atmosphere need to come together. Heat keeps water molecules moving and bouncing off each other, preventing them from condensing. This creates a cycle in which the warmer the planet gets, the more water will evaporate into the atmosphere — but the smaller fraction will return as rain. Therefore, droughts will last longer, cover wider areas and be even drier with every little bit that the planet warms.

To study the effects of higher temperatures on drought, the researchers have separated “natural” droughts due to changing weather patterns from those resulting from human-caused climate change in the observational data over a 70-year period. Previous studies have used climate models that incorporate increasing greenhouse gases to conclude that rising temperatures contribute to drought. But without observational data about real weather patterns, they could not pinpoint the role played by evaporative demand due to naturally varying weather patterns. 

When these natural weather patterns were included, the researchers were surprised to find that climate change has accounted for 80% of the increase in evaporative demand since 2000. During the drought periods, that figure increased to more than 90%, making climate change the single biggest driver increasing drought severity and expansion of drought area since 2000. 

Compared to the 1948–1999 period, the average drought area from 2000–2022 increased 17% over the American West due to an increase in evaporative demand. Since 2000, in 66% of the historical and emerging drought-prone regions, high evaporative demand alone can cause drought, meaning drought can occur even without precipitation deficit. Before 2000, that was only true for 26% of the area.

“During the drought of 2020–2022, moisture demand really spiked,” Fu said. “Though the drought began through a natural reduction in precipitation, I would say its severity was increased from the equivalent of ‘moderate’ to ‘exceptional on the drought severity scale due to climate change.”

Moderate means the 10–20% strongest drought, while “exceptional’ means the top 2% strongest drought on the severity scale, according to the U.S. Drought Monitor.

Further climate model simulations corroborated these findings. That leads to projections that greenhouse gases from burning fossil fuels will turn droughts like the 2020–2022 from exceedingly rare events occurring every thousand years to events that happen every 60 years by the mid-21st century and every six years by the late 21st century.

“Even if precipitation looks normal, we can still have drought because moisture demand has increased so much and there simply isn’t enough water to keep up with that increased demand,” Fu said. “This is not something you could build bigger reservoirs or something to prevent because when the atmosphere warms, it will just suck up more moisture everywhere. The only way to prevent this is to stop temperatures from increasing, which means we have to stop emitting greenhouse gases.” 

The study was supported by NOAA’s National Integrated Drought Information System and Climate Program Office, and the National Science Foundation.
Source: UCLA