Global Warming and the Great Lakes
The Great Lakes are a crown jewel of North America, holding nearly one-fifth of the planet's surface freshwater. They have nearly 11,000 miles of shoreline and harbor, more than 530,000 acres of coastal wetlands and the world's largest freshwater delta (in Lake St. Clair). They range from the cold and deep waters of Lake Superior to the relatively warm and shallow waters of Lake Erie.
The watershed drains more than 200,000 square miles ranging from heavily forested areas to mixed urban and agricultural development, and supports approximately 6,000 species. The lakes are home to numerous fish, including species undergoing restoration efforts such as lake trout and lake sturgeon, and species that are popular in commercial or recreational fisheries, including lake whitefish, walleye, muskellunge and several introduced salmon species.
The Great Lakes are important sources of drinking water, economic livelihood and recreation opportunities for millions of Americans and Canadians. Recreational boating in the eight Great Lakes states produces more than $35 billion in economic activity annually, and fishing, hunting and wildlife watching amount to more than $18 billion in annual economic activity in these states.
Threats from Global Warming
Global warming adds yet another stress to a Great Lakes system already struggling with aquatic invasive species, deleterious land use changes, nonpoint source pollution, toxic chemical contamination, and coastal habitat degradation/wetlands loss. Potential global warming impacts include reduced water levels (due in particular to decreased winter ice cover allowing more evaporation), increased frequency of intense storm events (altering the timing of inflows), and warmer water temperatures.
Already, Lake Superior has increased water temperatures and an earlier onset of summer stratification by about two weeks in just the past 30 years. Within another 30 years Lake Superior may be mostly ice-free in a typical winter.
Lake Erie water levels, already below average, could drop 4-5 feet by the end of this century, significantly altering shoreline habitat. Global warming could change internal water cycling in the Great Lakes with longer summer stratification potentially leading to larger dead zones (lacking in oxygen). Other potential consequences include less habitat for coldwater fish, more suitable temperatures for aquatic invasive species and hazardous algal blooms, and more mobilization of contaminated sediments as well as nutrients and toxic chemicals from urban and agricultural runoff.
Threats to specific wildlife habitats include:
- Declining Moose Populations in Minnesota - In a recent study of moose at the southern edge of their range in northwest Minnesota, researchers found that over the past 40 years, declines in population growth are related to increases in mean summer temperature with winter and summer temperatures increasing by an average of 12 and 4 degrees F (6.8 and 2.1 degrees C), respectively over this period. Lack of food resources and increased exposure to deer parasites associated with warmer summer temperatures appear to be the primary cause of their decline. The authors suggest that the northwest Minnesota moose population likely would not persist over the next 50 years and that the southern distribution of moose may become restricted in areas where climate and habitat conditions are marginal, especially where deer are abundant and act as reservoir hosts for parasites.
- Physical Changes to Lakes Will Impact Aquatic Organisms - In future scenarios for a doubled CO2 climate, researchers projected significant changes to characteristics of the Great Lakes. Physical changes, such as decreases in water level (0.65-8.2 ft or 0.2-2.5 m) and ice cover, and increases in water temperature 2-12 degrees F (or 1-7 degrees C) at surface, up to 14 degrees F or 8 degrees C at depth) will in turn affect phytoplankton, zooplankton and fishes. Some warm-water fish species could move north by 300-400 miles (500-600 km); invasions of warmer water fishes and disappearances of colder water fishes should increase. Climate change effects interact strongly with effects of other human-caused stresses such as eutrophication, acid precipitation, toxic chemicals and the spread of exotic organisms.
- Disappearing Cold-Water Fish Species - Scientists projected changes to the distribution of fish species under a 2xCO2 climate scenario at 209 locations in the contiguous United States. Cold-water fish habitat is projected to persist in deep lakes near the northern border of the United States, but is likely to be eliminated from most shallow lakes in the contiguous states, reducing the number of lakes that have suitable coldwater and cool-water fish habitat by up to 45 percent and 30 percent, respectively. On the other hand, warm-water fish habitat is likely to increase. Good-growth periods are projected to increase on average by 37 days for cool-water fishes and by 40 days for warm-water fishes.
- Potential Changes to Fish Populations - Researchers used historical data to predict how growth of warm-water (e.g. smallmouth bass and yellow perch) and cold-water (e.g. lake trout) fish species may change under changing climatic conditions. In years with warmer air temperatures and early on-set of warm surface waters, smallmouth bass and yellow perch grew bigger and faster than normal. Lake trout growth was poorer likely due to early on-set of water stratification –trout fry had fewer days to feed on prey species in surface waters due to rapid warming.
- Yellow-Headed Blackbirds - Researchers found that during a dry period in the Prairie Pothole region in Iowa, yellow-headed blackbirds (Xanthocephalus xanthocephalus), which solely breed in wetlands, nested later and laid fewer eggs, in part due to lack of food. Far fewer chicks hatched and fledged during these years primarily due to nest predation – more predators found and fed on more eggs and chicks during years with low water levels.
- Zebra Mussels Respond Favorably to Warmer Waters - Growth and survival of zebra mussels (Dreissena polymorpha) in southwestern Lake Erie and the Ohio River (KY) were studied in different water temperatures (within expected range of temperature increase for lakes of 3-7 degrees F or 2-4 degrees C). Experimental increases in temperature significantly enhanced growth rates in fall and early winter and increased mortality in the summer-fall season. Based on these experiments and related laboratory studies, the authors predicted northern populations of zebra mussels will probably benefit from predicted climatic change and may extend their range to higher latitudes and altitudes.
Conservation Investments to Minimize Global Warming Impacts
Global warming-induced water level declines increase the need to adopt the Great Lakes-St. Lawrence River Basin Water Resources Compact to protect against large-scale out-of-basin diversions, and implement programs to reduce agricultural and urban water use through conservation. As lake levels decline and shoreline wetlands are lost, efforts will be needed to enhance protection, restoration, and development of wetlands at lower water levels to retain their many functions including wildlife habitat and water purification.
The potential for new exotic species to take hold as the water warms could exacerbate an already serious aquatic invasive species problem. Adequate programs to prevent the introduction of new species (e.g., through adequate screening, early detection and rapid response, and treatment protocols) and restrict movement for those invasive species already in the region are essential.
Global warming has the potential to profoundly influence water supply and its quality for the Great Lakes from the surrounding watershed. To maintain healthy lakes it will be important to monitor and manage impacts in the watershed, such as storm surge inputs and erosion. The altering of hydrological cycles by global warming may even require that stormwater and wastewater treatment infrastructures are redesigned or upgraded.
The Great Lakes Regional Collaboration process recommended major restoration of the Great Lakes at a cost of about $20 billion over five years. If implemented, this could result in $80-$100 billion in short and long-term economic benefits to the regional and national economies and is a worthy cause. However, to be effective, these assessments and the restoration efforts must take into account global warming.