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Scientific Evidence



Great Lakes Region Climate

Two major factors that shape the region’s climate – its location in the middle of the North American land mass and the presence of the Great Lakes. Being in the middle of the continent, far from the oceans, means swings in air temperature between warm summers and cold winters.

In the winter, bitterly cold Arctic air masses can move southward into the region, and the polar jet stream is often located near or over the region. This causes frequent storm systems that bring cloudy, windy conditions and rain or snow. See Section 5.2.

In the summer, a high-pressure system that tends to stay in the subtropical Atlantic Ocean forces warm, humid air into the Great Lakes region, particularly into the southern portions.

The Great Lakes themselves affect the climate. Large bodies of water gain and lose heat more slowly than the surrounding land. The surface water temperatures in the lakes tend to be warmer than the land during the late fall and early winter. The reverse is true in the late spring and summer. This moderates air temperatures near the shores of the lakes. We see this effect especially on the downwind sides, where it helps to create microclimates such as the wine-growing regions of southwestern Michigan and Ontario.

Another well-known aspect of the Great Lakes influence downwind is “lake-effect ” snow. Cold air masses sweep across the warmer lakes, picking up heat and moisture. This generates extreme snow storms on the lee sides of the lakes. (The lee side is the shore across from the direction of the prevailing winds.) The winds generally come from the north-west, so the lee side is the south-eastern shore. (Picture a snow-fence. The snow drops on the lee side of the fence, the side away from the wind)

Changing Climate

Natural climate variability can be quite large, with year-to-year differences of several degrees in annual temperature or swings from very wet years to drought. However the scientific evidence strongly suggests that changes in the atmosphere caused by human activity are the primary cause of the climate shifts now being observed worldwide. Analyses of data from the National Climate Data Center (1895-2001) and the Midwest Climate Center (1900-2000) reveal shifts in temperature, total precipitation, and extreme events in recent decades.

  • Temperatures over the past 30 years have ranged from near average to somewhat warmer than average. In the past four years, however, annual average temperatures have ranged 1 to 2 Co warmer than the long-term average and up to 4 Co above average in winter.
  • The past two decades have seen the hottest months in recorded history, although extended heat waves have been infrequent since the 1950s. A few episodes of extreme cold occurred in the 1990s, but most years saw fewer cold waves.
  • The last spring freeze has been occurring progressively earlier. Current dates are approximately one week earlier than at the beginning of the 1900s. Growing seasons have also begun to lengthen.
  • Both summer and winter precipitation has generally been above average for the past 30 years, making this the wettest period of the twentieth century. However, water levels in the Great Lakes were higher during the mid- to late-nineteenth century, indicating even wetter conditions then.
  • The frequency of 24-hour and 7-day intense rainfall events that result in flooding, has been fairly high over the past 50 years, relative to the long-term average.

Water Temperatures

  • Water temperature records of the Great Lakes and other inland lakes show trends in temperature change. Five of seven monitoring sites in the eastern Great Lakes area have a lengthened period of summer stratification which has increased by one to six days per decade.
  • Increasingly over the last 80 years, warmer spring and autumn water temperatures have been recorded. Summer water temperatures have also increased, though less dramatically.

Duration and Extent of Lake Ice

The extent of ice and how long it stays on the lakes are very sensitive indicators of climate variability. Shifts in the ice cover on lakes and streams can provide early signs of ecosystem responses to climate change.

Researchers have found consistent historical changes in ice cover in the inland lakes and in the bays of the Great Lakes themselves.

  • The ice cover is not lasting as long. Freeze-up has been occurring later in fall and the loss of ice cover in spring has been occurring earlier for the past century. This change is accelerating. The rate of change has been greater in the past 20 years than over the preceding 80 years. Recently, the fall freeze has been moving later by 1.5 days per decade and spring breakup earlier by two days per decade. Records over the past 100 to 150 years consistently show increasingly shorter periods of ice cover
  • In the Great Lakes themselves, the extent of ice cover has been highly variable from 1963 to the present with no long-term trend. In recent years the Great Lakes have had little ice cover.
  • Periods of greatly reduced or no ice cover have become more frequent, while periods of extensive ice cover have decreased in frequency.

Shifts in ice cover have a number of impacts. Reduced ice cover allows greater evaporation from open water in winter. This contributes to lower water levels, loss of winter recreation on lakes, and perhaps an increase in lake-effect snows (depending on air temperature and wind direction).

Observations from 1846 to 1995 show that both the length of the ice cover season and the area of the ice cover have decreased in the Great Lakes Region. During this time the temperature also increased 1.2 C degrees per century. Ice Break up is now an average of 6.5 days earlier and freeze up 5.8 days later. In the last 150 years the lakes and rivers in Ontario have gained almost 2 weeks more of open water. See Albedo affect, Lake Snow effect.

Ports and commercial shipping schedules have changed. The Hudson’s Bay ice cover has decreased one-third since 1971. Shipping grain through Churchill as a port leading to the prairies and to the USA is cheaper than the ports on the St.Lawrence Seaway. Since 2002 one-third of all grains shipped have come through Churchill in spite of the fact that the port at Thunder Bay has an ice-free season that is twice as long. The change from Great Lakes ports to Churchill saved $10 million U.S. in 2002.

The good news is that Canada has developed better ice-mapping systems and better ice detection for better navigation. We are changing our behavior due to reduced ice cover.

Both commercial shipping and recreation seasons, such as ice fishing, are changing to meet the changing climate.

The graph below highlights the Ice Cover on Lake Simcoe between 1853-1993 and reflects the changing climate.

Source: Martin Futter. Climate, Nature, and People: Indicators of Canada’s Changing Climate, Canadian Council of Ministers of the Environment, 2003 www.ccme.ca

1. Describe the trends seen in this graph of ice cover on Lake Simcoe.
2. Changes in fish populations are already happening according to the creel census of 2004. What key characteristic will the new dominant fish populations have?

An earlier-than-expected spring break up recently caused ice-fishermen to be stranded on broken ice floes. Rescue by helicopter from Lake Simcoe was extremely costly. Find an article in the newspapers that has more details.