For some species, surviving winter would be difficult to impossible without a layer of snow. Certain plants, animals, and microbes rely on the protective and insulating habitat that lies in the space between the bottom of the snow layer and the top of the frozen ground. But this zone of refuge—called the “subnivium”—is at risk in many areas around the planet.
Likai Zhu of Linyi University and colleagues used data from NASA and the Japan Aerospace Exploration Agency (JAXA) to conclude that global warming has already affected this important habitat, particularly in the mid-latitudes of the Northern Hemisphere. The research, published in Nature Climate Change, shows that organisms now have to endure more winter days with frozen ground but less protective snow cover. Global climate models also show that such changes will continue into the future.
The patterns of change are visible in these maps. The map above shows projected changes in the duration of winter snow cover for the 2071-2100 period compared to 1982-2014. Dark blue areas indicate where the number of days of snow cover are expected to decline the most.
Averaged globally, winter snow cover in the modeled future period is expected to last 110 days, down from 126 days for the historical period. In some locations, species will have to endure nearly an extra month without the protective cover of the subnivium. The largest declines are expected between 40 and 50 degrees latitude across North America and Asia.
The researchers note that the duration of snow cover is only half of the story. The survival of some species will largely depend on the conditions they will have to endure in the absence of snow cover—that is, how long will the bare ground stay frozen? It turns out that some areas are seeing longer bouts of bare, frozen ground—exposing subnivium-dependent organisms to colder air temperatures and also leaving them more vulnerable to predators.
Changes in the duration of bare and frozen ground is shown in the second map, again projected for the 2071-2100 period compared to 1982-2014. Blue areas show where the number of days of bare, frozen ground are expected to increase, notably in the western United States, the eastern Tibetan Plateau, Mongolia, and Europe. The greatest increases (dark blue) are focused in the Northern Hemisphere’s mid-latitudes, where the average number of days with bare and frozen ground would increase from 39 to 45 days.
The changes are likely to have an array of effects on cold-adapted species. The researchers note that animals such as rodents are likely to be more vulnerable to predators and could struggle to find food in the frozen ground. Meanwhile, plants might struggle with freeze-thaw cycles. And colder soil temperatures could affect the diversity of microbes. Some species might adapt to the new environment, either evolving to tolerate the new conditions or by migrating toward the poles.
“The subnivium is important for the overwintering success of snow cover-dependent organisms, and changes will result in broad-scale shifts in species distributions and further biodiversity patterns,” Zhu said. “We should consider the vulnerable but often-overlooked subnivium habitat when understanding and conserving biodiversity.”
NASA Earth Observatory images by Lauren Dauphin, using data from Zhu, L. et al. (2019). Story by Kathryn Hansen.