Nutrient pollution, a major water quality threat, is now happening in U.S. winters due to climate change, a new national study finds. Photo: Tom Fisk
Key Points:
- Climate change is putting water quality at risk in at least 30 states.
- Increased rain-on-snow, snowmelt, and rainfall events now carry nutrients and soil into streams and rivers during winter when dormant vegetation cannot absorb them.
- Winter runoff impacts on nutrient pollution has quickly progressed from rare or nonexistent to far worse than during other times of the year.
A first-of-its-kind national study finds that previously frozen winter nutrient pollution—unlocked by rising winter temperatures and rainfall—is putting water quality at risk in 40% of the contiguous U.S., including over 40 states.
“We are clearly seeing much larger amounts of cloudy water and sediment traveling through U.S. watersheds in winter,” said Carol Adair, study author from the University of Vermont. “The idea of winter nutrient pollution is new, because it’s a relatively recent impact of climate change with the potential to cause significant problems for people and the environment—from algae blooms that make swimming dangerous to ‘dead zones’ that kill fish stocks.”
Of particular concern are so-called “rain-on-snow” events, which can cause large, economically and environmentally devastating floods. The team used geospatial datasets to explore the impacts of rain-on-snow events in U.S. regions with large pools of nitrogen and phosphorous.
The scientists found that rain-on-snow affects 53% of the contiguous U.S. and puts 50% of U.S. nitrogen and phosphorus pools at risk of export to groundwater and surface water. Where these factors converge, more than 40% of the contiguous U.S. is at risk of nutrient export and soil loss from rain-on-snow events.
Analyzing the Mississippi River floods of 2019, researchers found rain-on-snow events delivered a large pulse of nutrients and sediment into the river and Gulf of Mexico—at much greater levels than a similar growing season rainfall event would —contributing to the Gulf of Mexico’s eighth largest dead zone on record. Dead zones occur when bacteria that thrive on excess nutrients remove too much oxygen from the water, causing massive die-offs of fish or other aquatic animals.
“We hope this study is a wake-up call for government agencies and researchers, because it reveals that 40% of the U.S. is producing winter pollution—but no one is tracking exactly how much, where it’s going, or the impacts on water quality and ecosystems," said Carol Adair, a researcher at UVM’s Gund Institute for Environment. “That’s a big problem that urgently needs addressing.”
In addition to winters being the fastest warming season in the U.S., the longest cold snaps are becoming shorter, and the number of days with temperatures below 32°F is expected to continue to decline across the country, researchers say. Rain is also becoming more frequent than snow, a trend predicted to continue across the U.S.
Information provided by University of Vermont.