Characterizing global patterns of frozen ground with and without snow cover using microwave and MODIS satellite data products

How organisms respond to climate change during the winter depends on snow cover, because the subnivium (the insulated and thermally stable area between snowpack and frozen ground) provides a refuge for plants, animals, and microbes. Satellite data characterizing either freeze/thaw cycles or snow cover are both available, but these two types of data have not yet been combined to map the subnivium. Here, we characterized global patterns of frozen ground with and without snow cover to provide a baseline to assess the effects of future winter climate change on organisms that depend on the subnivium. We analyzed two remote sensing datasets: the MODIS Snow Cover product and the NASA MEaSUREs Global Record of Daily Landscape Freeze/Thaw Status dataset derived from SSM/I and SSMIS. From these we developed a new 500-m resolution dataset that captures global patterns of the duration of snow-covered ground (Dws) and the duration of snow-free frozen ground (Dwos) from 2000 to 2012. We also quantified how Dws and Dwos vary with latitude. Our results show that both mean and interannual variation in Dws and Dwos change with latitude and topography. Mean Dws increases with latitude. Counter-intuitively though, Dwos has longest duration at about 33°N, decreasing both northward and southward, even though the duration of frozen ground (either snow covered or not) was shorter than that at higher latitudes. This occurs because snow cover in mid-latitudes is low and ephemeral, leaving longer periods of frozen, snow-free ground. Interannual variation in Dws increased with latitude, but the slopes of this relationship differed among North America, Europe, Asia, and the Southern Hemisphere. Overall, our results show that, for organisms that rely on the subnivium to survive the winter, mid-latitude areas could be functionally colder than either higher or lower latitudes. Furthermore, because interannual variation in Dwos is greater at high latitudes, we would expect organisms there to be adapted to unpredictability in exposure to freezing. Ultimately, the effects of climate change on organisms during winter should be considered in the context of the subnivium, when warming could make more northerly areas functionally colder in winter, and changes in annual variation in the duration of snow-free but frozen conditions could lead to greater unpredictability in the onset and end of winter.