Typically, leftover agricultural chemicals from manure and fertilizer freeze during the winter. In much of the United States, frigid temperatures and a blanket of snow lock the pollution in place, where it can’t do much environmental harm. Then, the ice thaws in the spring, when plants can help soak up the nutrients and prevent them from running off into nearby waterways.
But as human-caused climate change warms the Earth, winter rains could release this nutrient pollution at a time when dormant plants can't absorb it, leading it to flow into streams, lakes and rivers, according to a new paper published in the journal Environmental Research Letters last week. This process, which researchers say could play out in more than 40 states, has the potential to cause big problems, including algae blooms and fish die-offs.
Scientists have long been aware of nutrient pollution during the warmer growing seasons. But now, they’re beginning to understand its effects during other times of the year. With climate change increasing winter temperatures, nutrient pollution during the winter has gone from “rare or nonexistent” to “far worse” than during other seasons, according to a statement.
“The assumption that discharge and nutrient transport remains low during the winter months no longer holds,” the researchers write in the paper.
To reach this conclusion, the team zeroed in on so-called “rain-on-snow” events, or instances where rain falls on top of snow that’s already on the ground and causes it to melt.
They looked at flooding in the Mississippi River Basin in 2019 and found that winter rain-on-snow events sent huge amounts of sediment and nutrients into the Gulf of Mexico—much more pollution than a similar amount of rain in the warmer months would have caused.
“Winter events tend to carry a little bit more pollution than the same size event in the growing season,” says Carol Adair, an environmental scientist at the University of Vermont and one of the study’s authors, to KCUR’s Eva Tesfaye. “That's largely because there are no plants around taking things up.”
The infusion of nutrients into the gulf contributed to what’s known as a “dead zone,” which occurs when bacteria proliferate and remove too much oxygen from the water, causing fish and other marine wildlife to die off or leave the area. The 2019 Gulf of Mexico dead zone was the eighth largest in the gulf on record and spanned 6,952 square miles, according to the National Oceanic and Atmospheric Administration (NOAA).
Though other factors were also at play, “consistent increases in winter floods, runoff and winter discharge” were likely big contributors to the massive dead zone, the researchers write in the paper.
Using geospatial data, they also studied U.S. regions with large pools of nitrogen and phosphorus, two chemicals present in manure, animal feed and fertilizer.
They found that half of nitrogen and phosphorus pools across the country are at risk of losing the chemicals to ground and surface water as a result of rain-on-snow events. In total, this means that more than 40 percent of the country has the potential to experience harmful nutrient runoff when winter rain washes snow away.
Large portions of the Rocky Mountains, the Pacific Northwest, the Northeast, the northern Midwest, the central plains and the Sierra Mountains could face winter nutrient pollution as the climate continues to warm, researchers say. And, they write in the paper, their projections likely represent a “conservative estimate of risk,” because they only looked at one type of phenomenon—rain-on-snow—and other events can also cause winter runoff.
The researchers hope their findings serve as a wake-up call for other scientists and government officials, who they argue should take a closer look at winter pollution as the climate continues to evolve.
“Climate change impacts on winter are often overlooked,” says Aimée Classen, a biologist at the University of Michigan and one of the study’s authors, in the statement. “If we care about our water quality, we can no longer ignore how climate change impacts winter precipitation.”