The city of Chicago, with a population of over three million people, is the third largest city in the United States. Its economy is primarily based on manufacture and transportation of goods, although in recent years increasing revenue is also being generated by real estate and tourism. As the financial, industrial, and cultural capital of the Midwest, Chicago is responsible for roughly 34.6 million metric tons of heat-trapping or “greenhouse” gases, in CO2-equivalent terms. Adding in the six surrounding counties in the Chicago area increases this to about 103 million metric tons per year. This region accounts for nearly half the total emissions of the state of Illinois, with emissions that are greater than the state totals of more than 30 individual states.

Emissions of greenhouse gases from human activities over the last century have already begun to alter climate around the world. Chicago is no exception; while recently observed changes are still within the range of natural variability for the region, the patterns of change are very similar to those seen elsewhere that are being caused by human-driven climate change. Climate changes that have already been seen in Chicago over the last few decades include:

• Increases of 2.3°F (1.3°C) in annual average temperatures since 1980, with the greatest increases of 6oF (3.4oC) occurring in winter

• A longer growing season, with earlier spring flowering of trees and plants, and fall frosts occurring later in the year

• Several major heat waves, particularly those in 1995 and 1999

• Decreases in winter ice coverage on Lake Michigan and smaller lakes in the area

• A doubling in the frequency of heavy rainfall events over the last century

The climate of Chicago is expected to warm further over coming decades, as human emissions of heat-trapping gases continue. The extent of this warming and the resulting impacts depends strongly on the magnitude of those future emissions. For that reason, in estimating potential impacts from climate change, we focus on two alternative futures. In the “higher” emission scenario, Chicago and the rest of the world continues to depend on fossil fuels as their primary energy source, and atmospheric carbon dioxide levels rise from their present-day levels of 385 parts per million (ppm) to almost 1000 ppm by the end of the century. Under the “lower” emission scenario, transitioning to alternative energy sources results in atmospheric carbon dioxide levels of about 550 ppm by the end of the century. Combining observations, climate models, and statistical downscaling techniques, we evaluate the potential changes in climate in the Chicago area and their impacts on Chicago's public health system, water supply, ecosystems, infrastructure, and other key sectors.