We usually think of climate change in terms of summer heat waves, warming oceans and extreme weather events. But what is happening in winter, when the ground should be frozen or covered in snow? Statistics show that winter is actually the season that is warming fastest in the U.S., and this is having some serious and unforeseen consequences. 

In a first-of-its-kind national study, scientists from the Universities of Vermont, Colorado, Kansas, and Michigan have identified a significant new threat to U.S. water quality that is directly related to the warming associated with climate change. In cold regions, accumulated nutrients from farming activities, such as phosphorus and nitrogen from fertilizers, manure, animal feeds and other sources, have historically remained frozen in snowpack throughout winter. When the spring thaw has arrived, the nutrients have been carried away into rivers and lakes where plants have been able to use these nutrients for their growth.

But with winters becoming warmer, cold snaps becoming shorter and the number of days with temperatures below 32°F decreasing each year, the seasonal snowpack in much of the U.S. has become less stable. Increased rain-on-snow, snowmelt, and rainfall events now carry the nutrients into streams and rivers during winter instead, when the vegetation is dormant and cannot absorb the excess. As a result, nutrient pollution from winter runoff has quickly progressed from rare or non-existent to far worse than at other times of the year.

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