Today may be the first day of spring, but winter’s icy grip continues to linger for most of us in the Midwest. But as we move – we hope – into warmer weather, NOAA has provided an overview of the winter from which we have emerged. It released its latest monthly state of the climate data last week, which also included the data for this year’s meteorological winter (December-January).
Unsurprisingly, the report reveals that this winter was cold, but far from historically so. It was just the 34th coldest on record, and no state recorded its coldest winter ever. In contrast, California had its warmest winter ever, and Arizona had its fourth warmest. As the AP’s Seth Borenstein put it, this winter demonstrated a “bi-polar winter vortex.”
One climatological variable that did reach near-record levels was the extent of lake ice on the Great Lakes. Due to the spate of below-freezing temperatures in the Great Lakes region, ice cover reached a maximum of 91% this winter, far higher than the long-term average of 51.4%. Lake Erie, which typically has the highest level of ice cover of the five lakes, jumped from just 5% ice cover on New Year’s day to more than 80% ten days later as a result of the polar vortex on January 6-7.
One year does not make a trend, though. According to the National Climate Assessment (PDF), surface water temperatures have increased dramatically in the Great lakes since the mid-20th century. From 1973-2010, annual Great Lakes ice cover fell by 71%, a startling downward trend despite the noisy year-to-year variation. Additionally, the IPCC has noted the duration of lake ice throughout the entire Northern Hemisphere has decreased by approximately two weeks since the middle of the 20th century.
Cleveland’s winter was largely in line with the overall regional trend. January-February temperatures were 7.1ºF below the historical average, making it the sixth coldest such span in the past 75 years. Cleveland also recorded 65″ of snow this winter, 36% higher than average. That number would likely have been higher, however, had the brutal temperatures not iced over the lakes, effectively shutting down the lake-effect snow conveyor belt.
That got me thinking – as global warming continues to warm the lakes, could the Great lakes region actually see more lake-effect snow?
As Kunkel et al note (paywall), the presence of the Great Lakes provides the necessary heat and moisture to generate precipitation where none would otherwise exist. Lake-effect snow constitutes a major part of winter in the Great Lakes region, where it can account for up to 50% of winter precipitation (PDF).
Because lake-effect occurs (paywall), as a result of “the destabilization of relatively cold, dry air mass by heat and moisture heat fluxes from a comparatively warm lake surfaces,” ice cover can significantly influence the amount of lake-effect snow that falls in a typical winter. The existence of open water in the winter allows the development of a “significant surface-atmosphere temperature gradient,” which facilitates the development of lake-effect events. Accordingly, because we know that global warming has and will continue to reduce lake ice extent, it should also generate more lake-effect snow in the Great Lakes region.
So what does the evidence say? Do we have research to backup this seemingly counterintuitive outcome? In a word, yes.
In a 2003 study, Burnett et al examined long-term changes in lake-effect snowfall (paywall) and compared them with October-April snowfall totals and air temperatures from 1931-2001. According to the authors, lake-effect snowfall totals increased significantly at 11 of 15 sites studied. Their research also demonstrated that lake surface temperatures increased significantly at the majority of sites examined. Consequently, they concluded that
[I]ncreased lake-effect snowfall is the result of changes in Great lakes whole-lake thermal characteristics that involve warmer lake surface waters and a decrease in lake ice cover.
While the Burnett et al piece is now more than a decade old, several additional studies have largely supported its findings. Vavrus, Notaro, and Zarrin examined how ice cover affected a subset of 10 heavy lake-effect snow (HLES) events in order to quantify the impact of the ice. They found that “the suppression of open water [i.e. expansion of ice cover] on the individual lakes causes over an 80% decline in downstream” HLES. Ice cover on Lake Erie lowered snowfall by 73%, while Lake Michigan saw a reduction of nearly 100%.
The authors note that ice cover reduces heat fluxes over the lakes, lowers atmospheric moisture, stymies cloud formation, and depresses near-surface air temperatures. All of these changes can suppress lake-effect. For all five lakes, complete ice cover reduces downstream snowfall by 85%. As a result,
The results of the current study suggest that this change toward more open water should favor significantly greater lake-effect snowfall.
Wright, Posselt, and Steiner conducted a similar study, examining the relative amount and distribution of snowfall under four different models: a control (observed lake ice in mid-January 2009), complete ice cover, no ice cover, and warmer lake surface temperatures. The authors also show that moving from complete ice cover to no ice cover dramatically increases lake-effect totals. The total area seeing small (≤2mm) and large (≥10mm) lake-effect events increase by 28% and 93%, respectively. In contrast, while elevated lake surface temperatures do not increase the area affected by lake-effect, they do tend to increase the amount of heavy snowfall; areas that already experienced HLES saw 63% more snowfall.
It remains important to note that, while higher lake surface temperatures and reduced ice cover should lead to more lake-effect snow during the coming decades, a decrease in the number of cold-air outbreaks could work to counter this effect. But if lake-effect increases, as the preponderance of evidence suggests it will, it could carry major additional economic costs for Great Lakes states. According to a study of 16 states and two Canadian provinces from IHS Global Insight, snowstorms can costs states $66-700 million in direct and indirect losses per day if they render roads impassable. Great Lakes states had among the most significant losses, with Ohio forfeiting $300 million per day and New York leading the pack at $700 million.
Additional major lake-effect events will only serve to drive up this price tag even more, further constraining limited state and municipal budgets well into the future.