|
 |
|
Please see EPA's Climate Change site for current information on climate change and global warming. EPA no longer updates EPA's Global Warming Site, but is maintaining this archive for historical purposes. Thank you for visiting the archive of EPA's Global Warming Site.
Great Lakes
| |
What Can We Expect?
Global warming could cause lake levels in all the region's lakes, including the Great Lakes, to drop substantially. Climate change probably will reduce the habitat available for cold water fish, such as trout and salmon, in lakes and streams. Wetlands also may be vulnerable to changes in climate. Forests and other ecosystems in the Great Lakes region may be transformed as the climate warms. Depending on the magnitude of changes in temperature and precipitation, northern tree species such as balsam fir may migrate northward or die out, with other tree species such as maple moving in to take their place. Habitat for endangered species such as the Kirtland's warbler could be lost as forests change.
Changes in the Lakes
The Great Lakes, their connecting channels, and the St. Lawrence River form the largest fresh surface water system on the planet. The lakes cover more than 94,000 square miles and hold an estimated 6 quadrillion gallons of water, 95 percent of the U.S. freshwater supply. The Great Lakes and other lakes and streams in the region are famous for their recreational fishing and boating opportunities.
 With global warming, the water temperatures of the Great Lakes and smaller lakes in the region could increase because of the warmer summer air temperatures and longer ice-free season. Warmer temperatures could degrade water quality by decreasing dissolved oxygen in the water and increasing the growth of algae.
Warmer waters in the region's lakes and streams would reduce the size of favorable habitat for trout, whitefish, and other cold water fish species. A recent EPA study found that a warming of 4.5°F over the next 70 years could cut the habitat of brook, rainbow, cutthroat, and brown trout by one-fourth to one-third nationwide. A 4.5°F warming is slightly below the midpoint of the 2-8°F range predicted by climate models for the year 2100; the actual temperature change that occurs could be smaller or greater. Chum, chinook, pink, and coho salmon would experience similar habitat losses. According to the study, Pennsylvania, New York, Ohio, Indiana, and Illinois would collectively lose 86 percent of their habitat for rainbow trout. These changes are expected to occur gradually over the decades ahead as the climate shifts.
Would the Great Lakes basin be as popular a fishing destination if classic northern cold water species like pike, muskellunge, trout, and salmon became less common? Recreational fishing certainly would continue, but the experience might change.
Warmer waters also could affect the timing and frequency of "overturning," in which oxygen-rich surface waters sink and mix with other water layers in the lake. The lakes currently turn over in the spring and fall of every year. In a warmer climate, the overturning may not occur every year in all lakes. Turnover is the main way for deeper lake waters to become replenished with oxygen. Without enough dissolved oxygen, cold water lake fish and other species will be unable to survive in their deepwater habitats.
If the climate warms, ice cover on lakes and streams would not last as long as it does today. Streamflows could peak sooner in the spring because of earlier snowmelt and ice breakup. There already is evidence that the annual rising and falling of some of the Great Lakes occurs nearly a month earlier than it did 140 years ago. Changes in the timing and volume of peak streamflow also may affect fish and other creatures that live in the streams.
 A warmer climate would lead to increased evapotranspiration. (Evapotranspiration is the water lost to the atmosphere by evaporation and transpiration combined. Evaporation is the loss from open bodies of water; transpiration is the loss from living plants.) Summer streamflows probably would decrease, reducing the water quality. Freshwater flow into the Great Lakes could decrease by 20 percent with a 4°F warming (slightly below the current mid-range estimate projected by climate models), potentially reducing lake levels by a foot or more. Because lake levels respond to hydrologic changes in their drainage basins, the Great Lakes would respond to global warming very differently than the oceans would. Global warming will cause the oceans to rise as warm water expands and freshwater from melting glaciers and ice sheets enters the sea. Water levels in the Great Lakes, on the other hand, are likely to fall.
Lower lake levels would reduce inputs to hydroelectric power facilities, increase the concentration of water pollutants, and require more dredging to maintain ship channels.
Lake levels in the Great Lakes have varied dramatically in the past due to natural variations in the region's climate. For example, very low levels occurred in the late 1920s, mid-1930s, and mid-1960s, while very high levels occurred in the 1830s, 1880s, early 1950s, early 1970s, mid-1980s, and mid-1990s. The low water levels in the 1960s caused significant losses in hydroelectric power output, economic hardships for marina operators and other lakeside recreational facilities, loss of wetlands, and reductions in municipal water quality. Global warming could increase the frequency and severity of droughts in the Midwest, causing low lake levels to occur more frequently.
 In the region's smaller lakes, prolonged drought from climate change could decrease the number of lakes with suitable habitat for organisms such as crayfish and snails. Drought also could decrease groundwater supplies of silica, an essential nutrient for freshwater sponges and diatoms.
On the positive side, flood damages could be reduced by lower lake levels. Global warming also would lengthen the ice-free season for the Great Lakes and St. Lawrence Seaway, with benefits for ship navigation. Warm-water fish, both native and introduced, could experience longer growing seasons and flourish in a warmer climate.
Forests TransformedDespite nearly 400 years of forest clearing, agricultural and industrial development, and other human impacts, the Great Lakes basin still contains vast tracts of forest. Boreal and northern mixed forestlands such as those in the Boundary Waters Canoe Area Wilderness, a million-acre wilderness in the Superior National Forest, provide habitat for spectacular wildlife species such as moose, black bears, and wolves. The region's forests are a popular destination for backpackers, naturalists, hunters, anglers, and wildlife photographers.
Trees and forests are adapted to specific climatic conditions. As these conditions are altered, forests will change. Global warming could cause forests to undergo changes in species, geographic extent, health, and productivity. If conditions become drier, the current range and density of forests could be reduced and replaced by grasslands and pasture. Even a warmer and wetter climate would lead to changes; trees that are better adapted to these conditions, such as oaks, would thrive. Under wetter conditions, forests could become more dense. These changes could occur during the lifetimes of today's children, particularly if the changes are accelerated by other stresses, such as fire, pests, and diseases, that could themselves be worsened by a warmer and drier climate.
In Michigan, for example, changes in climate could cause the extent of forested areas to change little — or decline by as much as 50-70 percent. The uncertainties depend on many factors, including whether soil becomes drier and, if so, by how much. Hotter, drier weather could increase the frequency and intensity of naturally caused wildfires. The mixed aspen, birch, beech, maple, and pine forests found in the northern part of the state could be replaced over time by a combination of grasslands, savanna, and hardwood forests of oak, elm, and ash. The predominant hardwood forests in southern Michigan could give way to pine and oak forests. These changes could affect the character of Michigan forests and the activities that depend on them.
In northern Minnesota, the boreal forests in Voyageurs National Park and in the Boundary Waters Canoe Area Wilderness could be replaced by mixed forests better adapted to warmer conditions. Boreal forests are what most people think of when they envision Minnesota's remote north woods: small northern conifers such as tamarack, black spruce, and balsam fir, in a landscape that typically includes wetlands, bogs, and carpets of lichens and mosses. According to one climate study, changes in the Great Lakes' boreal forests could be apparent in the relatively near future, between 2010 and 2040.
The world's boreal forests cover a vast area 29 times the size of Texas, but they are rare in the lower 48 states. The Great Lakes region contains mostly transitional forests that include species typical of both the Canadian boreal forests to the north and the broadleaf deciduous woodlands to the south. But a few regions of boreal or boreal-like forests occur near the Canadian border. If climate change causes boreal species to shift their range northward out of the United States, some of the romance and evocative character of the northern borderlands will be lost.
Whether we consider these changes to the forests "good" or "bad" depends on our individual perspectives. Some plants and animals may benefit from climate change while others may be harmed. An increase in wildfires might create new habitats for some populations while eliminating habitats for others. A decline in one species may make way for another species to expand its range.
|
|
|