When It Rains, It Pours: The Climate Link Between Extreme Precipitation and Drought

By Allison Crimmins

flooding image with traffic light in foregroundFrom the photos my Colorado friends posted this summer, I wasn’t surprised to learn that 2013 has been the wettest on record for Boulder. However, Boulder also experienced drought, the most destructive wildfire in Colorado’s recorded history, and a week of record heat. How is it that Colorado can experience both extreme wetness and extreme dryness in one year?

These seemingly conflicting events fit a pattern that scientists expect to occur under global climate change—a pattern that has been developing for the past few decades over many parts of the United States. Precipitation (rain, snow, etc.) is increasing, and more of it is coming in the form of downpours, blizzards, and other intense bursts, with longer dry spells in between. If you add up the percentage of land in the U.S. where a greater-than-normal amount of total precipitation fell in the form of intense single-day events, eight of the top 10 years for extreme precipitation have occurred since 1990.

How does it work? Imagine our atmosphere as a sponge passing over the land’s surface, soaking up moisture through evaporation and occasionally wringing out the collected water through precipitation. Warmer air can hold more water vapor, so the sponge absorbs more in a warmer world, leaving the land drier than it used to be. Eventually, the sponge becomes waterlogged—and when it’s finally wrung out, often miles away from where it picked up the water, it releases a huge amount all at once. This pattern occurs now in many parts of the U.S. and around the globe. Warmer air causes more evaporation, which leaves dry areas drier, but also results in heavier rainfalls.

If current trends continue, we can expect more droughts as well as more devastating deluges, like the one that resulted in loss of life, massive property damage, and thousands of evacuations throughout Boulder and Larimer Counties. As my friends join the rest of Boulder in rebuilding, city planners and emergency managers will keep these growing risks in mind so the city is better prepared for future extreme weather.

About the Author: Allison Crimmins is an environmental scientist with EPA’s Climate Change Division, where she focuses on the impacts and risks associated with climate change. Prior to joining the EPA she studied oceanography, climate science, and public policy. She lives, works, and cooks a mean strawberry rhubarb pie in Washington, DC.

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