1. Air Temperature

The Great Lakes region has a humid continental climate, characterized by large variations in temperatures driven by air masses from the Arctic, Pacific, and Gulf of Mexico.22 The Great Lakes themselves moderate seasonal temperatures to some degree but do not have as large of an effect as do the oceans along the Atlantic, Pacific, and Gulf coasts. That said, along the lakeshores, the water bodies act to absorb heat in summers and radiate heat during winters. This works to buffer extreme temperatures relative to locations further inland, away from the moderating effects of the lakes.

Since 1900, instruments indicate that on average, the climate in the Great Lakes region has been relatively stable. While the climate, defined as the average of long-term variations in weather, has been stable over the past century, there has been a high degree of inter-annual variability; annual temperatures can regularly flux by up to -15°C from the long-term mean.23 However, over the past few decades, average temperatures have been increasing relative to historical norms, especially in the winter.24

Most temperature observations around the Great Lakes region evidence a general, region-wide warming trend since the 1950s. Average temperatures in Chicago have increased by 0.25°C per decade from 1945-2009.25 Across all of Illinois, average temperatures have increased by 0.16°C per decade. Average annual temperatures in Minnesota have increased by 0.27°C per decade.26 Over the coming century, temperatures are expected to continue to increase throughout the Great Lakes region due to anthropogenic climate change. By 2100, computer ensemble models indicate that average temperatures could increase by an estimated 1.5-7°C depending upon the emissions scenario applied.27 The projected temperatures are a direct function of greenhouse gas emissions; higher emissions equate to higher temperatures.

Rates of temperature change will vary with seasons and geographic location. Initially, temperature increases are projected to be most apparent for winter months in the more northerly states. Currently in the Northeast United States, wintertime temperatures are rising twice the rate of the annual average.28 After the middle of the century, however, models indicate that temperature increases will be greater during summer in the more southerly states such as Illinois and Indiana. By the end of the century, mean wintertime temperatures are projected to rise by 2-4°C and the temperature of the coldest day of the year is projected to warm by 4-8°C. The number of frost days per year will be significantly decreased and the date of the last frost will be 20-30 days earlier than historic norms. Summers will experience an increase in the frequency, duration, and intensity of heat waves. Some models indicate that heat waves could occur every other year to several each summer. The number of hot days over 32.2°C will increase and there will be a larger proportion of extremely hot days with temperatures over 37.8°C by the end of the century.

One way scientists have been visioning the future is by projecting “climate migrations” of states and cities.29 To do this analysis, a city/state’s current climate conditions are quantified and classified using historic data. Then, using global climate models, scientists identify regions where the climate is projected to mimic a given city/state’s conditions. Therefore it would appear that the city/state’s climate has ‘migrated’ due to climate change. The Union of Concerned Scientists has an interactive Migrating Climates feature on its website; for example, by the end of the century, Illinois summers will become more like those in present-day Texas and Indiana winters will feel more like those in present-day Washington, D.C./Virginia.30

Secondary Impacts

  • Increased frequency, duration, and intensity of heat waves: A warmer future will naturally lend itself to having a higher frequency of heat waves. In the future, hot days will generally feel hotter due to an increase in humidity. By the end of the century, heat waves could occur approximately three times each year under higher emissions scenarios.31 From 1961-1990, Chicago experienced an average heat wave season length of 68.6 days; in the future, the season could extend to 108.0/137.7 days under low/high emissions scenarios.32
  • Reduced air quality: Models indicate that summer weather patterns could arrive earlier and last longer in the Great Lakes region. The more intense and extended heat could cause an increase in ground level ozone of 10-25% and particulate matter. This would cause regional air quality to decrease and affect incidence of respiratory illness and disease.33
  • Longer growing season: Warmer temperatures will increase the length of the growing season mainly due to earlier dates from the last spring frost and later dates for the first fall freeze. It has been estimated that across the Northern Hemisphere, growing seasons have advanced by one to 1.5 days per decade during the past 50 years.34 Also, more CO2 availability in the atmosphere may stimulate growth rates of plants but other nutrients, such as nitrogen, may become limiting factors.
  • Increased agricultural pests: Warmer winters and warmer, drier summers could increase growth rates and wintertime survival of known agricultural pests. For example, the gypsy moth, a generalist defoliator, may experience increased survival rates during warm winters, enabling population sizes to overwinter and increase.35
  • Increased invasives establishment: Many plants and animals will adapt to warmer than normal water and air temperatures. In some cases, an increase in temperature may make some invasive species more aggressive, causing current invasive species to expand their geographic spread and population size. Further, new invasive species can be opportunistic, thriving where landscapes have been altered or disturbed. Climate change will stress many native plants and animals, providing a window within which invasive species may establish themselves.36
  • Shifting landscapes: Many plants and animals have preferred thermal regimes that coincide with their historic ranges. As air temperatures rise due to climate change, plants and animals may exhibit a northward movement from their historic ranges. However, the predicted rates of change may exceed some species’ natural rates of migration or habitat fragmentation may prevent the ability of some animals and plants to migrate. Climate change may therefore cause some species to die off in regions in which they are currently found and, without active management, may not relocate to more tolerant thermal regimes.
  • Shifting zones: Shifts in plant hardiness zones, a metric based on the average annual minimum temperature in which a plant species can be cultivated, are expected to migrate northward.37 By the end of the century, the Chicago region is expected to have a plant hardiness zone similar to current day conditions in Southern Illinois under a low emissions scenario or Northern Alabama under a high emissions scenario. This will have implications for the nearly 100 tree species and shrubs found within the region. The new plant hardiness zones may be unsuitable for regionally important species such as the northern red oak, black cherry, white oak, sugar maple, and red maple. Instead, the habitat may become more suitable for silver maple, bur oak, post oak, sweetfum, Kentucky coffee tree, black hickory and wild plum.38,39 The establishment of novel plants will alter the region’s landscape ecology and biology.
  • Shifts in phenology: Phenology refers to the general timing of natural events; the timing of tree budding and butterfly migrations are well known indicators of phenology. In Sauk County, Wisconsin, a 65-year long record of 55 different seasonal events started by Aldo Leopold indicates that 19 of the 55 processes recorded have steadily occurred earlier in the region since the 1930s.40 As the rhythms of nature respond to the changing climate, there will be unforeseen consequences to existing trophic structures. For example, if the timing of insect emergence and migratory bird patterns no longer overlap, the region could experience an increase in pests due the absence of a top-down predator control on insect populations. This explosion of pests may adversely affect agriculture, natural ecosystems, and urban heat islands.

22 Government of Canada - Toronto, Ontario and United States Environmental Protection Agency - Great Lakes National Program Office. (1995). The Great Lakes: An Environmental Atlas and Resource Book. Third Edition, Chicago, Illinois.

23 Kling et al. 2003

24 Wuebbles, D.J., K. Hayhoe, & J. Parzen. (2010). Introduction: Assessing the effects of climate change on Chicago and the Great Lakes. Journal of Great Lakes Research 36: 1-6.

25 Hayhoe, K., J. VanDorn, T. Croley II, N. Schlegal, & D. Wuebbles. (2010a). Regional climate change projections for Chicago and the US Great Lakes. Journal of Great Lakes Research 36: 7-21.

26 NCDC Climate at a Glance in Hayhoe et al. 2010a

27 Angel, J.R. & K.E. Kunkel. (2010). The response of Great Lakes water levels to future climate scenarios with an emphasis on Lake Michigan-Huron. Journal of Great Lakes Research 36: 51-58.

28 Hayhoe, K., C. Wake, T. Huntington, L. Luo, M. Schwartz, J. Sheffield, E. Wood, B. Anderson, J. Bradbury, A. DeGaetano, T. Troy, & D. Wolfe. (2006). Past and future changes in climate and hydrological indicators in the U.S. Northeast. Climate Dynamics 28: 381-407.

29 Kling et al. 2003

30 Union of Concerned Scientists. Migrating Climates.

31 Hayhoe, K., S. Sheridan, L. Kalkstein, & S. Greene. (2010b). Climate change, heat waves, and mortality projections for Chicago. Journal of Great Lakes Research 36:65-73.

32 Vavrus. S. & J.V. Dorn. (2010). Projected future temperature and precipitation extremes in Chicago. Journal of Great Lakes Research 36: 22-32.

33 Lin, J.-T., D.J. Wuebbles, H.-C. Huang, Z. Tao, M. Caughey, X.-Z.Liang, J.-H.Zhu, & T. Holloway. (2010). Potential effects of climate and emissions changes on surface ozone in the Chicago area. Journal of Great Lakes Research 36 (Supplement 2). 59-64.

34 Schwartz, M.D., R. Ahas, & A. Aasa. (2006). Onset of spring starting earlier across the Northern Hemisphere. Glob. Change Biol. 12: 343-351.

35 Williams, R.S., D.E. Lincoln, & R.J. Norby. (2003). Development of gypsy moth larvae feeding on red maple saplings at elevated CO2 and temperature. Oecologia 137: 114-122.

36 Hellmann, J.J., K.J. Nadelhoffer, L.R. Iverson, L.F. Ziska, S.N. Matthews, P. Myers, A.M. Prasad, & M.P. Peters. (2010). Climate change impacts on terrestrial ecosystems in metropolitan Chicago and its surrounding, multi-state region. Journal of Great Lakes Research 36: 74-85.

37 Hellmann et al. 2010

38 Iverson, L.R., A.M. Prasad, S.N. Matthews, & M. Peters. (2008). Estimating potential habitat for 134 eastern US tree species under six climate scenarios. For. Ecol. Manag. 254: 390-406.

39 Prasad, A.M., L.F. Iverson, S. Matthews, & M. Peters. (2007-ongoing). A climate change atlas for 134 forest tree species of the Eastern United States [database]. Northern Research Station, USDA Forest Service, Delaware, Ohio.

40 Bradley, N.L., A.B. Leopold, J. Ross, & W. Huffaker. (1999). Phenological changes reflect climate change in Wisconsin. Proc. Natil. Acad. Sci. 96: 9701-9704.