As Winters Warm, The Threat of Nutrient Pollution Grows

CLIMATE | “Winter is changing,” says University of Vermont scientist Carol Adair, who recently revealed a significant new threat to U.S. water quality: as winters warm due to climate change, they are unleashing large amounts of nutrient pollution into lakes, rivers, and streams.

Her team’s landmark national study found that previously frozen winter nutrient pollution—unlocked by rising temperatures and rainfall—is putting water quality at risk in over 40 states across the Northeast, the Midwest and Central Plains, the Pacific Northwest, and the Sierra and Rocky Mountains.

Nutrient runoff—from phosphorus and nitrogen in fertilizers, manure, and more—has affected water quality during growing seasons for decades. But cold temperatures and a strong snowpack traditionally kept nutrients in place until spring thaw, when plants can help absorb excess nutrients.

But winter is now the fastest warming U.S. season. Winter rain and snowmelt are increasingly causing large, devastating floods that carry nutrients and soil through watersheds in winter when dormant vegetation cannot absorb them. As a result, winter runoff has transformed from rare or nonexistent to being far worse than other times of the year.

The study, published in Environmental Research Letters, found that “rainon-snow” events affect 53% of the contiguous United States., putting 50% of U.S. nitrogen and phosphorus pools at risk for export by ground or surface water. Where these factors converge, more than 40% of the contiguous United States is at risk of winter nutrient export and soil loss.

“This study is a wake-up call for government agencies and researchers,” says Adair, an ecologist and biogeochemist. “It reveals the existence of winter pollution over 40 states—but we don’t know how much, where it’s going, or the impacts on water quality and ecosystems.”

She says winter floods delivered a massive pulse of nutrients and sediment into the Mississippi River in 2019—far more than growingseason rainfall—contributing to the Gulf of Mexico’s eighth-largest dead zone on record, causing die-offs of fish and other aquatic animals.

To tackle the issue further, Adair has partnered with UVM engineer Raju Badireddy and others to measure changes to winter runoff with printable microsensors. With startup funds from UVM’s Gund Institute for Environment, they aim to transform our understanding of how watersheds work in a warming world—and strengthen our ability to predict and prepare for changing winters.