Wisconsin climate change research confirms effects of warming winter nights

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The first scientific report of its kind in a decade highlights that climate change is altering weather patterns, wildlife habitats, industry conditions and a myriad of cultural traditions across Wisconsin.

Among the most pronounced changes observed by scientists is a rapid warming of low temperatures during winter nights. While warmer lows may save some fingers and toes on long winter nights, they also threaten Wisconsin wildlife and challenge industries that depend on the cold.

Warming winter nights are one of many changes scientists are facing in the 2021 review published by the Wisconsin Climate Change Impacts Initiativea partnership of scientists from University of Wisconsin-Madison Nelson Institute for Environmental Studies and the Wisconsin Department of Natural Resources. Governor Tony Evers ordered the report in 2019 to follow up on the first report of its kind published a decade earlier. The 100-page assessment documents the many ways climate change is manifesting in Wisconsin.

In terms of average temperatures, nighttime lows and winters as a whole warmed faster than daytime highs and summers, the report said. Between 1950 and 2020, nighttime lows in Wisconsin have warmed between 4 and 7 degrees Fahrenheit, compared to a warming of 1 degree Fahrenheit for summer daytime highs. This winter warming was most pronounced in northwestern Wisconsin, the coldest region of the state.

Wisconsin’s warming trend is not evenly split between nighttime lows (top) and daytime highs (bottom), and across seasons, winter night lows warm the most quickly. (Credit: Courtesy of Wisconsin Climate Change Impacts Initiative)

This phenomenon, in which a warming climate disproportionately warms the coldest periods of the state, corresponds to scientific expectations. It depends Dan Vimonta climatologist at UW-Madison who led the report climate science work. The Wisconsin data adds to evidence that have been accumulating in the world for decades.

The phenomenon is partly due to increased atmospheric carbon dioxide, which acts as an increasingly powerful blanket by keeping more solar energy trapped near the Earth’s surface at night. Another reason the nights heat up faster is simply because the part of the atmosphere affected by surface conditions – called boundary layer — is thinner at night than during the day and therefore requires less energy to warm up.

Vimont said the phenomenon of more intense winter and nighttime warming emerged as a robust indicator of climate change that the Wisconsin data confirms and reinforces.

“The more evidence we see like this, the more inclined we are to be able to say things like, ‘Yeah, it’s definitely climate change,'” he said.

The report details many of the ongoing effects of climate change on communities in Wisconsin, from the Great Lakes to the Mississippi River, including increased levels of extreme rainfall and flooding, as well as the deterioration of water quality. A number of aspects of Wisconsin’s economy, ecosystems and culture are already feeling the effects of warming winters and winter nights.

These effects have an impact on the large forest industry, which depends in part on cold winter temperatures to freeze the ground enough to allow heavy machinery to access remote off-road locations. Warmer winters also reduce snowpack, which insulates trees, and provide more opportunities for invasive and pest species to wreak havoc in state forests.

A dense stand of pine trees with snow speckled trunks stands in a wintry landscape.

Snow plays an important role in Wisconsin’s forest ecosystems. (Credit: PBS Wisconsin)

Reduced snow cover is also a threat to snowshoe hare, which the report identified as “critical species for forest ecosystems” in Wisconsin. This is because snowshoe hares’ coats turn white in response to shorter days in winter, a response intended to help camouflage the hares against a snowy landscape. Snowshoe hares are “increasingly incompatible with Wisconsin winters that have less and less snow cover,” according to the report.

Additionally, the report notes that decreasing snow cover and increasingly frequent winter “thaws” can threaten crops such as alfalfa when followed by a sudden and extreme drop in temperature, such as a experienced during a polar vortex in early 2019. Alfalfa is an important forage crop that supports the state’s dairy industry.

Meanwhile, warmer average winter temperatures could reduce soil quality on farmland, in part by allowing microorganisms more time to break down nutrient-rich organic matter in the soil during these months.

Then there are the impacts on winter tourism in Wisconsin, much of which depends on a constant snowpack and constantly frozen inland lakes. For example, a study 2020 Researchers from York University in Ontario have documented how warming winters in regions that historically experience long periods of ice cover have made freezing conditions less predictable, finding that accidental winter drownings have increased in these locations .

A series of eight maps show predicted daytime and nighttime warming in Wisconsin by 2050.

Wisconsin will continue to warm in all seasons for the next century. Projections show that winter nights will warm the most and summer days the least. These maps show projected warming by mid-century compared to conditions at the end of the 20th century. (Credit: Courtesy of Wisconsin Climate Change Impacts Initiative)

Vimont pointed to even more potential impacts on the horizon, as the report predicts that winter nights will continue to warm fastest in Wisconsin through 2050. This warming will likely see more northward migration of insect species it can cause disease, he says.

“The Zika-carrying mosquito [virus] by mid-century it will likely be able to overwinter occasionally here in Wisconsin,” Vimont said. “It’s not fun to think about.

While present and future impacts may not be pleasant to consider, Vimont said he hopes the report will provide a catalyst for more individual and group action around Wisconsin to mitigate and adapt to climate change. Such actions would include a further shift from reliance on fossil fuels to greater use of state-generated renewables, such as wind and solar, and moving away from car-centric routines and development.

“What strikes me about the process of this report is that a big difference from 10 years ago is all the stories of people who are currently experiencing climate change and all the stories of those people who are already taking action,” he said. “It is no longer something hypothetical. We understand it, we are affected by it and we are taking action.”

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