AMHERST, Mass. – A new climate modeling study by climate scientists Michael Rawlins and Raymond Bradley at the University of Massachusetts Amherst, with others, projects a decline in annual freezing days by mid-century across North America. They also estimate how much of the continent will no longer see a “frozen season,” that is, where there will be no winter days when the daily average temperature falls below freezing.
Lead author Michael Rawlins, manager of the Climate Science Research Center at UMass Amherst, and colleagues at Rutgers and the universities of Colorado and Montana say, “The decline in freezing days ranges from less than 10 days across north-central Canada to nearly 90 days in the warmest areas of the continent that currently undergo seasonally freezing conditions.” They estimate that by mid-century around 1 million square kilometers of North America will no longer see sub-freezing winter temperatures, approximately 6 percent of the region that now experiences freezing weather.
Results suggest that large parts of the United States, particularly western areas, will experience “considerable declines in freezing days,” with the greatest decreases expected near the southern boundary of areas that now have seasonal freezing conditions. Details appear in the current issue of the Journal of Climate.
The study suggests that the spring thaw will arrive earlier than the autumn freeze will delay in parts of the western and central U.S. However, over much of Canada, the opposite is expected to be true, with a more pronounced delay in autumn freeze than an advance in spring thaw dates.
The researchers used two atmospheric reanalysis data sets and a suite of climate model data from the North American Regional Climate Change Assessment Program (NARCCAP) to quantify the expected changes. In the NARCCAP, a set of high resolution regional climate models (RCMs) were driven by a set of general circulation models (GCMs) over an area encompassing most of North America, Rawlins explains.
The team used the RCM air temperatures and an approach which accounts for biases in climate model data to arrive at their conclusions. Rawlins notes, “Our projections for freezing day declines drawn from the RCM and reanalysis data sets, together with the bias-correction method, represent a considerable improvement over estimates obtained from GCM data alone.”
As the authors point out, seasonal freezing conditions and freeze/thaw timing are an integral element of a region’s ecosystem processes, recreational activities and economy. “For example, freezing air temperatures play a key role in the overwinter survival of many insects that must rely on external sources to provide their heat,” they note.
Snowfall has declined sharply across the western U.S., they add, and Rawlins notes that fewer sub-freezing days in areas that rely on snow cover for skiing and other winter sport-related tourism can be expected to cause economic hardship. These trends are expected to continue, he and colleagues write.“Relatively large advances in spring thaw projected by the models are driven, in part, by a continuation of the decline in spring snow cover,” says Rawlins.
The research team notes that the pattern of projected change across North America is similar when a uniform warming is used in place of the climate model outputs, indicating agreement between two different methods.“This result raises confidence in our estimates of the magnitude of freezing day declines over different parts of the continent,” says Rawlins.
He and colleagues recommend continued development of models at higher spatial scales to improve projections of regional variation and local impacts of season transition and freezing condition changes.
This work was supported by the Department of Interior Northeast Climate Science Center at UMass Amherst and the U.S. Geological Survey. NARCCAP is funded by the National Science Foundation, the U.S. Department of Energy, the National Oceanic and Atmospheric Administration and the U.S. Environmental Protection Agency Office of Research and Development.