The cold weather continues across much of the Unites States, Europe, and central Asia as the Arctic Oscillation  remains in a strong “negative” state, forcing cold Arctic air down  to the mid-latitudes. A couple of weeks ago I explained  why more frequent heavy snowfall events could be a consequence of global warming for mid-latitude areas near large bodies of water, like Washington, D.C., and Syracuse, New York (see figure).
|The average amount of annual snowfall has been increasing in Syracuse, New York, for most of the past century. (SOURCE: Increasing Great Lake–Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming? Journal of Climate vol. 16, pp. 3535-3342, Figure 1)|
On January 31, I noticed a forecast for lake-effect snowfall around the Great Lakes on Weather.com : “Lake-effect snows are also possible near the central and western Great Lakes today and tonight.”
This forecast caught my eye because lake-effect snow events usually subside around this time of year for Lake Erie because it usually has a lot of ice on it by now. Yet the map above forecasts lake-effect snow for all of the lakes, including Erie (white circles in figure). It might seem counterintuitive that Lake Erie would have little ice during an unusually cold winter, but all the hot summers and warm winters we’ve had in recent decades have warmed the lake over time. It would take more than one cold winter to cool down such a large body of water.
Data compiled by the Great Lakes Surface Temperatures  reporting system at Michigan State University show that surface-water temperatures on January 27 in the western end of Lake Erie, the shallowest part that freezes first during the winter, were still above freezing (see figure).
|Surface water temperatures in western Lake Erie were above freezing (32°F) on January 27, 2009. (SOURCE: the Great Lakes Surface Temperatures  reporting system at Michigan State University)|
The EPA’s Large Lakes and Rivers Forecasting Research Branch (LLRF ) keeps an eye on changes in Lake Erie’s ice cover  and presents the following graph showing that the maximum winter ice cover has been getting smaller in recent years (see figure).
|The annual maximum ice cover, expressed as a percent of the Lake Erie’s surface area, has been getting smaller on average in recent years. (SOURCE: Detroit River-Western Lake Erie Basin Indicator Project, INDICATOR: Lake Erie Ice Cover . USEPA Large Lakes and Rivers Forecasting Research Branch)|
The LLRF  research group is concerned about the social, economic, and ecological effects of this trend:
“Climatic warming may cause a dramatic reduction in ice cover with as yet unknown consequences to Lake Erie and the other Great Lakes.”
Looking a bit further, I found two peer-reviewed science papers that looked directly at Great Lakes ice cover. The first paper  by scientists from NOAA and the University of Michigan was published in an academic journal called Climatic Change . The research documents trends of decreasing ice cover in the Great Lakes since the early 1980s. The paper concludes that these trends: “may be harbingers of a period of even lower annual mean ice cover in the 21st Century.”
The second paper , published by scientists at several U.S. and Canadian universities in the Journal of Climate , goes a step further to document trends in the amount and source of snowfall in the eastern Great Lakes region. They found a convincing difference (see figure) between snowfall trends in locations known to receive lake-effect snow, like Syracuse, New York, compared to locations in the same climate zone that were far enough removed from the lakes that they do not receive lake-effect snow.
|The amount of annual snowfall increased in lake-effect sites but did not increase in nearby non-lake-effect sites, showing that the warming of the Great Lakes is likely responsible for increased regional snowfall. (SOURCE: Increasing Great Lake–Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming? Journal of Climate vol. 16, pp. 3535-3342, Figure 3).|
The authors used a neat technique to confirm that the increased snowfall was actually coming from the Great Lakes rather than from large-scale winter storms that import atmospheric moisture from the Gulf of Mexico or the Atlantic Ocean. They detected an increase in a chemical signature of Great Lakes water in the smaller Finger Lakes  of central New York, which lie in the snow shadow of lakes Erie and Ontario. Great Lakes water has a different oxygen isotope ratio than that found in ocean water, so if the amount of lake effect snow has been increasing, then the amount of Great Lakes-like oxygen isotope ratios should have increased over time in the Finger Lakes.
Lakes are handy for reconstructing local environmental histories because the sediments at the bottom build up in annual layers (deeper sediments are older than shallow sediments). By taking vertical samples of the sediment, the researchers were able to see if the ratio of different oxygen isotopes had changed over time. Indeed, the oxygen isotope ratio had changed over time in the direction expected if the Great Lakes were producing more snowfall (see figure).
|Oxygen isotope ratio in Finger Lakes Sediments. The decreasing isotope ratio indicates that an increasing fraction of total precipitation is coming from the Great Lakes. (SOURCE: Increasing Great Lake–Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming? Journal of Climate vol. 16, pp. 3535-3342, Figure 5).|
This clever study shows convincingly that lake-effect snow in the eastern Great Lakes region has increased over the past century. I will give the study authors the last word on the implications of their results:
“Records of air temperature, water temperature, and lake ice suggest that the observed lake-effect snow increase during the twentieth century may be the result of warmer Great Lakes surface waters and decreased ice cover, both of which are consistent with the historic upward trend in Northern Hemispheric temperature due to global warming. Given projected increases in future global temperature, areas downwind of the Great Lakes may experience increased lake-effect snowfall for the foreseeable future.” (SOURCE: Increasing Great Lake-Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming? Journal of Climate vol. 16, pp. 3535-3342)
Jay Gulledge is Senior Scientist and Program Manager for Science & Impacts