John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
A new analysis of weather data shows that in a few regions, it has become less common to see snow over the holidays.
He's a feature of so many holiday gatherings: the creaky codger, reminding you how in his day he had to slog through miles of chest-high snow to buy the Christmas ham. But is that memory correct—is modern December snowfall punier than the robust, puts-hair-on-your-chest powder of yore?
No and yes, according to a new analysis of weather during Christmas week across North America. In much of the country, the number of days with holiday snow on the ground has actually grown over the past several decades. But in a few regions—notably the Pacific Northwest, Montana, Nebraska, Alberta, Saskatchewan, and around the lower Great Lakes—it's become much less common to see snow frosting the grass over winter break.
Rutgers University and NOAA arrived at these findings by contrasting snow records during 1966-1989 and 1990-2013. Here's the map they made with the data, showing regions with a 25 percent more-frequent snow cover today in blue and 25 percent less in brown:
The map may appear a bit misleading in its suggestion that the years are getting colder and snowier—NOAA and Rutgers clarify this is not the case, and that "singling out a particular winter week for scrutiny isn't especially meaningful as an indicator of long-term climate change." Snow has been thinning through much of the Northern Hemisphere, especially during spring months. The researchers explain:
When it comes to meaningful indicators of how snow has changed over time, the scientists say, it's best to stick to monthly or seasonal averages. By those indicators, says David Robinson, who leads the Rutgers snow lab project, the pattern is clear: Northern Hemisphere snow cover is declining significantly at the end of the cold season (spring/early summer).
This pattern of snow disappearing earlier in the spring makes intuitive sense with respect to global warming. As temperatures rise, the impact on snow cover is likely to show up first in those seasons where the temperature is just barely cold enough for snow. Reductions in snow can also feed back on the atmosphere, amplifying warming. Where winter temperatures are well below freezing, however, temperatures will have to rise more significantly before snow cover is affected.
The vanishing powder is on view in these other NOAA maps, showing in brown the places have lost up to 60 snow-covered days a year (compared to a 1998-2010 baseline). Blue regions have packed on more snowy days, but you'll have a hard time finding them in the right-hand depiction of spring 2014. Notes the agency: "In April, Eurasia set a new record low [for snow-cover extent] since satellite observations began in 1967."