As global temperatures rise, the atmosphere is becoming increasingly "thirsty," a term scientists use to describe growing evaporative demand—the amount of water the atmosphere could potentially draw from Earth’s surface given current weather conditions.
One key measure of this atmospheric thirst is standardized short-crop evapotranspiration (ETos), which estimates how much water would evaporate and transpire from a uniformly watered grass surface. ETos is widely used to evaluate evaporative demand over agricultural lands. Previous studies have shown that ETos has risen in response to factors such as increasing air temperature, solar radiation, declining humidity, and shifting wind patterns. However, little research has focused on long-lasting periods of exceptionally high evaporative demand.
To address this gap, scientists M. S. Kukal and M. Hobbins introduced a new concept: thirstwaves. Similar to heatwaves, thirstwaves are periods of extreme evaporative demand that can significantly disrupt the growing season. To qualify as a thirstwave, ETos must remain above the 90th percentile for at least three consecutive days. Their findings were published in the journal Earth’s Future.
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The researchers analyzed ETos data for the contiguous United States during growing seasons from 1981 to 2021. By assessing intensity, duration, and frequency at the county level, they identified patterns and regional trends, dividing the results into nine major geographic zones.
On average, thirstwaves occurred about 2.9 times per growing season (April through October), each lasting around four days. The longest thirstwave lasted 17 days, while some regions experienced as many as 20 events in a single season. The High Plains endured the most intense thirstwaves, while the South, Upper Midwest, Pacific Northwest, and West Coast had the longest-lasting events—averaging 4.5 days. The West Coast and South also recorded the highest number of thirstwaves, with roughly 3.5 per season.
Importantly, thirstwaves are becoming more frequent and expanding into regions that historically saw few or none, such as the Southwest, Northern Plains, and Northern Rockies. The chance that any given region will go an entire year without experiencing a thirstwave has significantly decreased.
Kukal and Hobbins emphasize that ongoing monitoring of thirstwaves will be essential for managing crops and water resources in a warming world. As the atmosphere continues to demand more moisture, understanding and preparing for these high-stress periods will be crucial for sustaining agriculture and adapting to the evolving climate.
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