Increased temperatures due to human-induced climate change transformed a typical drought into an unusually severe one that affected the American West from 2020 to 2022. A study conducted by scientists from UCLA and the National Oceanic and Atmospheric Administration (NOAA) revealed that evaporation was responsible for 61% of the drought’s intensity, while only 39% was due to decreased rainfall. The findings suggest that since the year 2000, evaporative demand has had a greater impact on drought conditions compared to lower precipitation levels, indicating that droughts may worsen as global temperatures rise.
“While research has indicated that higher temperatures lead to drought conditions, this is the first study we are aware of that shows moisture loss from increased demand surpasses moisture loss from decreased rainfall,” stated Rong Fu, a UCLA professor specializing in atmospheric and oceanic sciences and the lead author of the study published in Science Advances.
Ordinarily, droughts in the West have been primarily attributed to insufficient rainfall, with some contribution from evaporative demand. However, climate change induced by fossil fuel combustion has raised average temperatures, altering the landscape of drought causes. Although natural rainfall fluctuations can still result in drought, the heightened temperatures lead to more moisture being drawn from water sources, vegetation, and soil.
“For generations, drought has been typically linked to below-average precipitation,” remarked Veva Deheza, executive director of NOAA’s National Integrated Drought Information System and co-author of the study. “This research further confirms that we are in a new era where increasing temperatures are driving severe droughts, making precipitation a less significant factor.”
A warmer atmosphere can hold more water vapor until it reaches saturation, allowing for water to condense and create precipitation. For rain to occur, water molecules in the atmosphere need to combine. Heat causes these molecules to be more energetic and collide, preventing them from coming together. This results in a cycle where rising global temperatures cause more water to evaporate, yet a smaller proportion returns as rainfall. Hence, droughts can extend over longer periods, impact larger regions, and become more severe with each degree increase in temperature.
To analyze the influence of elevated temperatures on drought, the researchers differentiated between “natural” droughts resulting from changing weather patterns and those caused by human-induced climate change within a 70-year observational timeframe. Prior studies relied on climate models that considered rising greenhouse gas levels, concluding that increased temperatures contribute to drought. However, without real-time observational data, it was challenging to assess the influence of natural weather variations on evaporative demand.
By incorporating these natural weather variations, the researchers were taken aback to discover that climate change has been responsible for 80% of the rise in evaporative demand since 2000. During drought events, this percentage exceeded 90%, making climate change the leading factor in worsening drought severity and expanding drought areas since 2000.
From 2000 to 2022, the average area affected by drought in the American West increased by 17% compared to the period from 1948 to 1999 due to rising evaporative demand. Since 2000, drought conditions have become possible in 66% of historically drought-prone areas based solely on high evaporative demand, indicating that drought can occur even without a deficit in precipitation. This was only true for 26% of the area before 2000.
“During the 2020-2022 drought, the demand for moisture peaked significantly,” said Fu. “Although the drought initially stemmed from a natural drop in rainfall, its severity escalated from ‘moderate’ to ‘exceptional’ on the drought severity scale because of climate change.”
A ‘moderate’ drought is classified as within the 10-20% range for severity, while an ‘exceptional’ drought ranks within the top 2% in terms of intensity, according to the U.S. Drought Monitor.
Further simulations using climate models confirmed these findings. Projections suggest that due to greenhouse gases from burning fossil fuels, droughts similar to those of 2020-2022 could shift from being rare, occurring every thousand years, to happening every 60 years by the mid-21st century and every six years by the end of the century.
“Even if rainfall appears normal, drought conditions can still manifest due to the significant increase in moisture demand, which exceeds available water supplies,” Fu explained. “This isn’t an issue that can be resolved by building larger reservoirs or similar solutions; when the atmosphere warms, it will absorb more moisture universally. The only way to mitigate this is to curb temperature increases, which necessitates reducing greenhouse gas emissions.”
This research was supported by NOAA’s National Integrated Drought Information System and Climate Program Office, as well as the National Science Foundation.
Key takeaways
- Increased temperatures from human-induced climate change converted a regular drought into an extraordinary event impacting the American West from 2020 to 2022.
- A study by UCLA and NOAA scientists revealed that evaporation contributed to 61% of the drought’s severity, while precipitation reduction accounted for 39%.
- The findings indicate that since 2000, evaporative demands have become a more significant factor in drought conditions, which may worsen with rising temperatures.
