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Temperature
The northern Midwest, including the upper Great Lakes region, warmed by almost 4°F (2°C) in the 20th century (NAST 2000).
Based on historical records, extreme heat events are occurring more frequently in the Great Lakes region (NAST 2000, Wuebbles et al. 2003).
Data for Lakes Michigan, Huron and Superior show that summer water temperatures are rising. Lake Superior's summer surface water temperatures increased by 4.5°F (2.5°C) between 1979 and 2006, a rate approximately double the rate of air temperature rise during the same period. An earlier start of the stratified season significantly increases the period over which the lake warms during the summer months, leading to a stronger mean summer temperature trend than would be expected from changes in summer air temperature alone (Austin and Colman 2007).
Over 15 years leading up to 2002, two-thirds of the winters in the Midwest had temperatures above the long-term historical winter average. The last spring frost is coming earlier and the first autumn frost is coming later (Kling et al. 2003, Wuebbles and Hayhoe 2003).
Between 1968 and 2002, mean annual air temperature increased at an average rate of 0.037 °C (0.067°F) per year at Lakes Huron, Erie and Ontario, resulting in an overall 1.3 °C (2.3°F) increase for the 34-year period. August surface water temperature has risen 0.084 °C annually at Lake Huron and 0.048 °C (0.086°F) annually at Lake Ontario, resulting in overall increases of 2.9 °C (5.22°F) and 1.6 °C (2.88°F), respectively, for the same time period. Temperatures at Lake Erie rose a small, but not significant, amount over the 34-year span (Dobiesz and Lester 2009).
Water Cycle
Ice around the Great Lakes and tributary streams is declining and melting earlier (Robertson et al. 1992, Anderson et al. 1996, Magnuson et al. 2000, Austin and Colman 2007).
The timing of Lake Superior's summer overturn advanced two weeks between 1979 and 2006. On average the date of the summer overturn has been half a day earlier each year (Austin and Colman 2007).
Based on historical records, winter precipitation is becoming more variable than summer precipitation (Wuebbles et al. 2003).
Lake level fluctuations in Lakes Michigan and Huron prior to 1980 were predominantly driven by changes in precipitation; however, evaporation has begun to significantly contribute to lake level changes for the first time on record. Increasing summer water surface temperatures, which correlate with decreasing winter ice cover, have caused evaporation rates to more than double since 1980 (Hanrahan et al. 2010).
Based on historical data, Lakes Huron, Erie, Michigan, and Ontario experienced a statistically significant trend toward increased precipitation between the years 1930 and 2000. During the same time period, stream flows increased in three connecting channels of the Great Lakes: the St. Clair, Niagara and St. Lawrence Rivers (McBean and Motiee 2008).
In Bayfield, Wisconsin, between 1857 and 2007, the onset of ice cover occurred an average of 1.6 days later per decade, and break-up of ice cover has occurred an average of 1.7 days earlier per decade due to rising air and water temperatures. Taken together, these changes have resulted in an approximately 3-day per decade (45 days over 150 years) reduction in the ice cover period. The most significant changes influencing this average occurred since 1975, with the ice season beginning an average of 11.7 days later and ending 3.0 days earlier every decade (Howk 2009).
In Minnesota, measurements of stream flow based on 53 to 101 years of historical records in the years leading up to 2002 show increase in peak flows due to summer rainfall events. The number of high flow days also increased. Summer and winter base flow increased significantly, likely due to wetter summers and more frequent snow melt events due to warmer winters. Mean annual flow rates changed at a rate of 2% per year over the last 50 years of record, increasing to 8% per year over the last 15 years (Novotny and Stefan 2007).
Wildlife
Under either forest expansion or contraction, relative mixtures of species in forest communities will change. This may cause some animal species to be at risk. Greater rates of change are associated with greater disequilibrium between the habitat needs of the species and the habitat realities (i.e., temperature and precipitation) (Watson et al. 1997, Parmesan and Yohe 2003).
Changes in climate are having significant effects on breeding and winter distribution of birds in North America (Watson et al. 1997).
Vegetation
The growing season has been expanding as spring arrives sooner. The average length of winter freeze has been decreasing, with frost days declining in the U.S. The first freeze occurring later and last freeze occurring earlier has expanded the growing season (Wuebbles et al. 2003).
Increasing levels of carbon dioxide affect the physiology of vegetation (Watson et al. 1997).
Cool adapted tree species, such as sugar maple and birch, are projected to have smaller habitat in the northeastern U.S., shifting largely to Canada. Oaks, hickories, and pines may see an expansion of potential habitats, although expansion may be limited by soil and seed dispersal (Watson et al. 1997, USDA 2001, Parmesan 2006).
Under either forest expansion or contraction, relative mixtures of species in forest communities will change. This may cause some animal species to be at risk. Greater rates of change are associated with greater disequilibrium between the habitat needs of the species and the habitat realities (i.e., temperature and precipitation) (Watson et al. 1997, Parmesan and Yohe 2003).
Disturbance
Changes in climate are having significant effects on breeding and winter distribution of birds in North America (Watson et al. 1997). Changing runoff patterns result in changing stream channel erosion and deposition patterns (Pruski and Nearing 2002).
Increasing summer air and surface water temperatures coupled with a reduction in the temperature gradient between air and water are destabilizing the atmospheric surface layer above Lake Superior, resulting in a 5% per decade increase in surface wind speeds above the lake (Desai et al. 2009).
In Minnesota, recent mild winters have led to a rapid population growth of eastern larch beetle, which has resulted in significant mortality for Minnesota larch stands (Frelich and Reich 2009).
A model based on historical voltinism (number of generations bred per year) for grape berry moth under varying scenarios of climate change in Lake Erie and other large lakes shows that increases in mean surface temperatures >2°C can significantly affect insect voltinism (Tobin et al. 2008).
A coastal vulnerability assessment of 22 coastal national parks found that Great Lakes shorelines are considered moderately vulnerable (Pendleton et al. 2010).
Cultural Resources
Historic structures are vulnerable to changes in temperature, wind, and moisture as well as infestation of pests (UNESCO 2007).
Preservation of archeological resources in the earth depends on a delicate balance of conditions. Changes to these conditions may reduce the change of artifacts' survival (UNESCO 2007).
Land use areas that are fixed in place, like national parks and Native American reservations, are particularly vulnerable to the effects of climate change because they cannot adapt by relocating in response to changes.
Visitor Experience
The winter recreation season in the region is becoming shorter and less reliable overall, although some areas have experienced an increase in seasonal snowfall over the past few decades.
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