Great Lakes Ice Cover Peaks at 2nd Highest Level on Record

This winter's severe cold over the Great Lakes--Superior, Michigan, Huron, Erie, and Ontario--froze a near-record amount of ice this year. Ice coverage peaked at 92.2% on March 6, the 2nd highest ice cover on record, according to NOAA's Great Lakes Environmental Research Laboratory (GLERL). Dating back to 1977, only February 1979 (94.7% peak) had a greater ice coverage. Warmer temperatures this past weekend caused Great Lakes ice cover to dip to 91% as of Sunday, March 9, and temperatures in the 40s and 50s on Monday and Tuesday this week will further erode the ice cover. It is unlikely that the renewed cold blast expected later this week will be able to refreeze the lakes enough to break the 1979 Great Lakes ice cover record. On March 8th, the ice concentration on Lake Michigan reached a record high of 93.3%. The previous record was 93.1%, set in 1977, with records dating back to 1973. 


Figure 1. True-color MODIS satellite image of the Great Lakes on March 6, 2014. Two day later, on March 8, Great Lake ice cover peaked at 92.2%, the second highest coverage since record keeping began in 1977. Only Lake Ontario had large areas of ice-free water. Image credit: NASA.


Figure 2. Ice coverage on Lake Michigan peaked at 93.3% on March 8, 2014, setting a new record for the lake. The previous record was 93.1%, set in 1977, with records dating back to 1973. Image credit: NOAA's Great Lake Environmental Research Laboratory (GLERL).

Long-term trend: Great Lakes ice cover down 71% from 1973 - 2010
This year's near-record ice cover on the Great Lakes is a major departure from what happened in 2012, when the lakes' 5% coverage was the second lowest on record (the record low was set in 2002.) The long-term trend in recent decades is sharply downward; Great Lakes ice cover declined 71% between 1973 - 2010. A 2012 study published in the Journal of Climate by researchers at NOAA's Great Lakes Environmental Research Laboratory found that the biggest loser of ice during the 1973 - 2010 time period was Lake Ontario, which saw an 88% decline in ice cover. During the same time period, Superior lost 79% of its ice, Michigan lost 77%, Huron lost 62%, and Erie lost 50%. The loss of ice is due to warming of the lake waters. Winter air temperatures over the lower Great Lake increased by about 2.7°F (1.5°C) from 1973 - 2010, and by 4 - 5°F (2.3 - 2.7°C) over the northern Lakes, including Lake Superior. Lake Superior's summer surface water temperature warmed 4.5°F (2.5°C) over the period 1979 - 2006 (Austin and Colman 2007). During the same period, Lake Michigan warmed by about 3.3°F (1.7°C), Lake Huron by 4.3°F (2.4°C), and Lake Erie showed almost no warming. The amount of warming of the waters in Lakes Superior, Huron, and Michigan is higher than one might expect, because of a process called the ice-albedo feedback: when ice melts, it exposes darker water, which absorbs more sunlight, warming the water, forcing even more ice to melt. This sort of vicious cycle is also responsible for the recent extreme loss of Arctic sea ice. The increase in temperature of the lakes could be due to a combination of global warming and natural cycles, the researchers said. They noted a pronounced 4-year and 8-year oscillation in ice coverage, which could be caused by the El Niño/La Niña and Arctic Oscillation (AO), respectively.

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Figure 3. A tale of two winters: Lake Superior was choked with ice at the end of the winter of 2013 - 2014 (top), but was virtually ice-free at the end of the winter of 2011 - 2012 (bottom.) Image credit: NASA.

Great Lakes water levels recover from record lows
The near-record ice levels are good news for Great Lakes water levels, which have suffered from record or near-record lows in recent years. In February 2013, Lake Michigan and Lake Huron set an all-time low water record, 29 inches below the long-term mean. This record was due, in large part, to the steadily declining winter ice coverage on the lake, which allows much more water to evaporate in winter. The latest U.S. Army Corps of Engineers water level forecast for the Great Lakes shows that Lake Michigan and Lake Huron water levels have recovered to be about 13" below the long-term March mean, and are forecast to rise another 3" over the coming month.

Lower water levels have had a significant impact on the Great Lakes economy. Over 200 million tons of cargo are shipped every year through the Great Lakes. Since 1998, when water levels took a severe drop, commercial ships were forced to light-load their vessels. For every inch of clearance that these oceangoing vessels lost because of low water levels, $11,000 - $22,000 in profits were lost per day. Hydropower plants have also been affected by low water levels; several New York and Michigan plants were run at reduced capacity, forcing them to buy higher priced energy from other sources, and passing on the higher costs to consumers. Ice loss is also likely to accelerate shoreline erosion because of the increase in open water, and promote more algal blooms. It is uncertain if the Great Lake water levels will continue to fall as the climate warms, since the region is expected to see an increase in precipitation over the coming decades. In Michigan, annual precipitation increased by about 10% between 1895 - 2013, according to the National Climatic Data Center.

References
Austin, J. A., and S. Colman, 2007, "Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback," Geophys. Res. Lett., 34, L06604, doi:10.1029/2006GL029021.

Wang, J., X. Bai, H. Hu, A.H. Clites, M.C. Colton, and B.M. Lofgren, 2012, "Temporal and spatial variability of Great Lakes ice cover, 1973-2010," Journal of Climate 25(4):1318-1329 (DOI:10.1175/2011JCLI4066.1)

Jeff Masters