If you live somewhere with cold and snowy winters, you’re probably no stranger to the stark white blanket that takes over the landscape after a fresh snowfall, the terrain beautiful and sparkling, the air crisp and chilly. But have you ever thought about what it looks like underneath the snow? Probably not.
But to a small selection of researchers, that’s where all the action happens. Underneath the snow lies a whole other world, an ecosystem of tunnels where many rodents, insects, microbes, and hibernators live over the cold winter months. It’s called the subnivium, and as global temperatures rise, climate change threatens this little haven and the organisms that rely on it.
The subnivium forms under very specific conditions — conditions which can vary depending on the type of vegetation and climate of the area. In much of the northern Midwest, where University of Wisconsin wildlife ecologist Jonathon Pauli conducts fieldwork, the subnivium forms after a snowfall of around 15 centimeters (nearly 6 inches) and at temperatures at freezing or below.
According to Pauli, the snow has to reach a certain thickness before it’s able to provide any insulation for the animals that live down there — too thin, and it’s just going to melt, or the cold will get through. Usually, the subnivium temperature hovers around the freezing mark, Pauli says, much warmer than the sub-freezing temperatures above and the perfect conditions for the organisms that call the subnivium home to thrive.
“The way it forms is that you get enough snow that solar radiation will go through the snow and heat the ground. And so you'll get a heat differential — the soil will be slightly warmer than the snow, just above freezing,” Pauli says. “And when that happens, then you get melt. And warm air carrying that evaporated snow then moves up from the ground into the snow column, and it creates this pocket right at the interface of the soil and the snow.”
The subnivium can be as small as a little pocket of air beneath the snow in some areas, to as large as a network of tunnels navigable by small animals in others, Pauli said. The size and breadth of the subnivium depends on the conditions allowing it to form, and researchers still aren’t sure what its scale is in many places, since it’s so difficult to study.
Many of the organisms that live in the subnivium depend on it to survive the long winter. But as temperatures increase across the globe, the creation of the subnivium, its geographic distribution, and the length that it lasts could come under fire.
“The winter season is shortened because of warmer temperatures,” Pauli said. “Warmer temperatures mean that precipitation still falls, obviously, but rather than falling as snow, it's increasingly falling as rain. And so we're not getting the snowpacks that we used to see. That means the snowpack is getting lighter and that's not good for the subnivium. And then you get rain on snow events and that can lead to the compression of the subnivium.”
Recently, Pauli’s lab and fellow UW-Madison wildlife ecologist Benjamin Zuckerberg’s lab investigated the potential implications of climate change on the subnivium with a joint study using tiny greenhouses.
Kimberly Thompson, a graduate student in Zuckerberg’s lab, helped set up the study using greenhouses at nine different “warming sites” spread throughout the Midwest — six in Wisconsin, one in Minnesota, and two in the upper peninsula of Michigan — and in three different cover types — open areas, conifer forests, and deciduous forests. According to Thompson, the greenhouses were about eight feet square by 10 feet tall, which is smaller than a normal greenhouse, but still large enough to make setting up in natural forests difficult.
Each site had three greenhouses — one for control, with the inside temperature the same as the outside, one to simulate temperatures three degrees Celsius warmer, and one to simulate temperatures five degrees Celsius warmer. United Nations emissions experts estimate climate warming between 3 to 5 degrees Centigrade by 2050, so Thompson said they wanted to simulate these temperature changes. The greenhouses had retractable roofs to incorporate the natural rain and snow patterns.
Over the course of the experiment, the team found that at three degrees above, the subnivium didn’t experience too much change. But, in the five degrees above greenhouses, the subnivium area and duration went way down, demonstrating a clear reaction to the increased temperature.
To Thompson, the subnivium persisting at three degrees above current temperature was a positive surprise. But, Thompson said it’s important to act now against climate change to prevent dangerous implications down the line.
“I was happily surprised to see that contrary to my expectations, subnivia can still generally persist throughout the Great Lakes region with warming of 3 degrees Celsius,” Thompson said. “While we still hope that more sweeping, global interventions can prevent this level of warming, this resiliency could potentially buffer the subnivium from being completely eradicated and provide land managers with time to manage habitats in a way that promotes a longer lasting subnivium.”
To Pauli, protecting the subnivium for years into the future could mean a variety of wildlife management tactics. For one, managing the landscape to support snow persistence — for example, planting certain kinds of vegetation that might take longer for snow to melt off of. Or buffering landscapes to minimize the negative effects from snow loss — which could mean creating habitat features for these smaller animals that could take the place of snow. Or, as a last resort, setting up wildlife refuges in northern areas where the snowpack will continue to be high for years and years into the future, at least until we can start to mitigate climate change.
“There are a few different routes we can go to solve climate change, other than just thinking about carbon emission, other than just thinking about how we can attenuate climate change itself,” Pauli said. “We can also think about the downstream — what can we manage to promote these species?”