The burning of fossil fuels has emitted an estimated 500 billion metric tons of CO2 into the atmosphere since the Industrial Revolution. The increased amount of atmospheric CO2 and other greenhouse gases (e.g., methane, nitrous oxide) has altered the radiative balance of Earth and is changing temperatures, precipitation patterns, and water chemistry relative to pre-industrial times. These changes will have cascading and unforeseen consequences to regional freshwater resources and those that depend upon them.

While climate change is a global issue, the ramifications are often most detectable at regional and local scales. Unique regional features, such as the large, dynamic water bodies of the Great Lakes system, can make it difficult for scientists to construct exact predictions or establish empirical relationships between global changes and the local ramifications therein. In order to understand the consequences of climate change, impacts necessarily need to be considered at the regional or local level.

In addition, climate change and other regional stressors, such as pollution and urban development, will have independent, synergistic, additive, or antagonistic effects with each other; for example, nutrient runoff from agricultural lands and urban areas has been identified as a leading cause of increased harmful algal blooms in the Great Lakes.³ Climate change is expected to increase water temperatures and enhance seasonal stratification in the Great Lakes; coupled with nutrient influx, these climatic changes will have additive effects on the occurrence of harmful algal blooms in the region.

There is an increasing awareness that climate change is impacting the Great Lakes region. However, direct attribution of regional-scale climate trends to anthropogenic influences is difficult due to the relative dearth of long-term records available to statistically delineate between natural variability and climate-induced change. Recent trends, both in the Great Lakes region and globally, strongly suggest that human-driven climate change is altering long-term climate patterns relative to historic norms. As atmospheric CO2 accumulation continues to accelerate into the coming century and global change accelerates the many manifestations of climate change, the pace of change and associated impacts are expected to become more apparent. Many studies suggest that the positive impacts of climate change in the Great Lakes region, such as longer growing seasons or reduced wintertime energy consumption due to increased temperatures, will be largely outweighed by adverse effects, such as increased numbers of invasive species or more frequent heat waves.

In this section, we briefly examine Great Lakes characteristics (geography, climate, terrain, economy, stressors) before outlining some of the primary impacts expected in the region due to climate change. Changes considered include air temperature, lake temperature, precipitation patterns, lake level, water chemistry, and associated secondary impacts.

Historic patterns are reviewed when possible and future trends are highlighted. Ultimately, the resultant future will be a complex interaction of fossil fuel emissions intensities, management strategies, biological responses, and any combination and interaction with other environmental stressors. Non-linear trends and extremes in climatic changes are important to consider. Predictive models and assessment reports such as this one can help to frame the future in a static world; in the end, however, the future will be an evolving process, continually painted and colored by daily activities and long-term strategies developed now and used to better inform our future.

1. Geography

Over millions of years, the Great Lakes region has been carved out by glaciers. Repeated glaciations scoured the landscape, creating huge basins that would later be filled with freshwater and become collectively known as the Great Lakes. Lakes Superior, Michigan, Huron, Erie, and Ontario combined have an estimated surface area of 95,000 square miles and hold roughly 90% of the U.S. supply of liquid freshwater. For perspective, the entire volume of the Great Lakes, six quadrillion gallons, is enough water to flood North America to an average depth of one meter.⁴

Today, the thousands of small, networked streams and lakes that are scattered throughout the 770,000 km2 drainage basin feed the five Great Lakes with freshwater. In sum, the entire Great Lakes drainage basin is roughly two times the size of the Great Lakes themselves, resulting in relatively long water retention times of a minimum of 2.6 years for Lake Erie to a maximum of 191 years for Lake Superior.⁵ Water levels in the region fluctuate due to extraction, precipitation, evaporation, and outflow to the ocean through a series of canals via the St. Lawrence Seaway. The Great Lakes are integrators of change in the upland freshwater ecosystems and surrounding areas; as a result, the Great Lakes are relatively slow to respond to short-term change, but, conversely, once a change is initiated, it is difficult to slow its momentum.

2. Climate

Owing to its mid-continent location, far away from the moderating effects of the ocean and with an absence of any significant mountain barriers, the Great Lakes region typically experiences large swings in seasonal temperatures. During winter, cooler temperatures average around -9 oC and stormy, windy, and snowy conditions arrive as air from the Arctic descends south; springtime temperatures average around 5oC⁶; in summer, warmer temperatures and humid conditions arrive when the subtropical Atlantic forces warm, humid air into the region.⁷ Summer in the Great Lakes region tends to be the rainiest season with short-lived but intense thunderstorms and rainfall.

3. Terrain

The diverse landscapes bordering the Great Lakes range from forested shores, marshes and wetlands, and prairie lands, to metropolitan cities. The U.S. portion of the Great Lakes basin can be divided into two ecoprovinces of nearly equal size – a Laurentian Mixed Forest is primarily found in the northwestern portion of the basin and an Eastern Broadleaf Forest is found in the southeast.⁸ Much of the land in the southern portion of the Great Lakes region has been highly modified during the past two centuries to support agricultural interests and the development of large urban and suburban areas. It has been estimated that conversion rates of undeveloped lands to modified landscapes in Illinois has rivaled or outpaced any tropical deforestation rates witnessed in recent decades.^9,10

4. Economy

The Great Lakes regional economy is dominated by agriculture, shipping, fisheries, and tourism and recreation. During the last glacial recession 9,000 years ago, large quantities of sediments and fertile soils were deposited by the glaciers throughout the basin. Today, the northern portion of the Great Lakes remains largely forested but the southern region has experienced metropolitan and agricultural development. The agricultural industry grosses an estimated $15 billion dollars annually, accounting for seven percent of total U.S. production and 25% of Canadian production.^11,12 Water diversions and withdrawals for agricultural needs will likely increase as climate-driven changes in temperature and precipitation patterns increase.

One of the other major industries for the Great Lakes is shipping and cargo transport along the 1,270-mile long Great Lakes-St. Lawrence Seaway. This seaway connects the Great Lakes to the Atlantic Ocean through a series of manmade canals and locks. Bulk cargo carriers, ocean-going vessels, and smaller cruise ships can all safely transit the St. Lawrence Seaway, stopping at any of the 15 major international ports or 50 of the smaller regional ports. Approximately two billion tons of commercial shipping – mostly iron ore, coal, grain, or steel – passes through the St. Lawrence Seaway annually.¹³ Shipping and its associated industries support more than 225,000 jobs and gross an average of more than $35 billion per year for the region.¹⁴ Lake level changes are therefore one of the most important concerns for regional managers. Future water levels will affect shipping canals, ports, and shoreline development, as well as residential water supply and quality.

Commercial and recreational fisheries are important in the Great Lakes. Commercial fisheries generated approximately $17.8 million in 2010¹⁵ and the recreational industry generates around $4 billion annually.¹⁶ Species and habitats are at risk from changes in temperature and precipitation patterns, as well as changes in lake level and pH and the introduction of invasive and non-native species (some of which have been introduced to enhance the recreational industry¹⁷).

5. Compounding Stressors

The impacts of climate change have the potential to amplify and counteract existing environmental stressors in the Great Lakes region, potentially undermining environmental advances made to protect lake conditions. Danz et al. (2007)¹⁸ attempted to systematically isolate the key environmental stressors in the Great Lakes basin using a principle component analysis methodology. Stressors were classified into one of the following categories – agriculture, atmospheric deposition, human population, land cover, or point source pollution – and compared using a multi-variate approach.

The key stressor from agriculture was identified to be related to nitrogen, phosphorus, and herbicide application and subsequent runoff; atmospheric deposition was related to chloride, sulfate, nitrate, sodium, and inorganic nitrogen deposition; human population stressors were most related to population density, road density, and proportion of developed land; land cover stressors were most related to the amount of non-native landscapes (i.e. agricultural land, grazing land, urban development); and point source pollution was most related to overall discharge of chemical pollutants from point sources of wastewater.¹⁹ The Great Lakes have also been polluted over the past 200 years with mercury deposited from the burning of fossil fuels, waste incineration, chlorine production, and mining²⁰; elevated mercury levels have been detected in the pelagic food web.²¹

In addition, human alterations to the landscape, such as dams, levees, water diversions for agriculture and development, and removal or destruction of vegetation in coastal and riparian zones, all combine to decrease the resilience of freshwater systems. The impacts of climate change will act as another layer of stress to systems in the Great Lakes region. Biological assemblages will also be subjected to multiple stressors and may become more sensitive to cumulative stress over time as climate change impacts become more prevalent.