This paper introduces the analyses of the potential impacts of climate change on the city of Chicago and the Great Lakes region and potential response options that provide the basis for this special issue. Covering projected changes in climate and hydrology, this collection of studies first estimates the potential impacts of climate change on human health, natural ecosystems, water resources, energy, and infrastructure in the city of Chicago and the surrounding Great Lakes region. A consistent set of future climate projections have been used as the basis for each analysis, which together provide a vivid impression of the consequences likely to result under the SRES higher (A1FI) as compared to the lower (B1) emission scenarios. These findings highlight the importance of the next set of analyses, which focus on quantifying Chicago's greenhouse gas emissions and prioritizing emissions reduction and adaptation options in light of the projected impacts. Together, these analyses form the basis for the City of Chicago's Climate Action Plan, the first such plan to be based on a thorough climate change impact assessment exploring the implications of a range of future climate scenarios. Although focused on Chicago and the Great Lakes region, the methods described here are generally applicable across North America and many other parts of the world, serving as a pattern for future regional assessments to directly inform adaptation and mitigation policy at the local to regional scale.

Assessing regional impacts of climate change begins with development of climate projections at relevant temporal and spatial scales. Here, proven statistical downscaling methods are applied to relatively coarse-scale atmosphere—ocean general circulation model (AOGCM) output to improve the simulation and resolution of spatial and temporal variability in temperature and precipitation across the US Great Lakes region. The absolute magnitude of change expected over the coming century depends on the sensitivity of the climate system to human forcing and on the trajectory of anthropogenic greenhouse gas emissions. Annual temperatures in the region are projected to increase 1.4 ± 0.6 °C over the near-term (2010–2039), by 2.0 ± 0.7 °C under lower and 3 ± 1 °C under higher emissions by midcentury (2040–2069), and by 3 ± 1 °C under lower and 5.0 ± 1.2°C under higher emissions by end-of-century (2070–2099), relative to the historical reference period 1961–1990. Simulations also highlight seasonal and geographical differences in warming, consistent with recent trends. Increases in winter and spring precipitation of up to 20% under lower and 30% under higher emissions are projected by end-of-century, while projections for summer and fall remain inconsistent. Competing effects of shifting precipitation and warmer temperatures suggest little change in Great Lake levels over much of the century until the end of the century, when net decreases are expected under higher emissions. Overall, these projections suggest the potential for considerable changes to climate in the US Great Lakes region; changes that could be mitigated by reducing global emissions to follow a lower as opposed to a higher emissions trajectory over the coming century.

Weather extremes have profound societal impacts, and their characteristics are expected to change in the future due to greenhouse forcing. In Chicago, heat waves and cold waves cause more than 100 fatalities per year, while extremely heavy rainfall can trigger disease outbreaks via contaminant discharge of storm water and sewage overflows. Here we analyze statistically downscaled climate model projections of extreme heat, cold, and precipitation in Chicago, based on higher (SRES A1FI) and lower (SRES B1) greenhouse gas emissions scenarios. The frequency, duration, and intensity of heat waves in Chicago are likely to increase substantially, and the heat-wave “season” extended (time during the year when extreme heat occurs). The simulated frequency of hot days—daily maximum temperature (T_max) >32 °C—increases from 15 days/year in the late 20th century to 36 days (B1) to 72 days (A1FI) by the end of this century. Proportionally, a much larger increase (a factor of 4 to 15) is projected in very hot days (T_max>38 °C). Conversely, the frequency and intensity of extreme cold is likely to decline considerably during this century. The coldest night of the year is projected to warm by 4–8 °C, while the simulated occurrence of very cold conditions (daily minimum temperature<-18 °C) declines by ∼50% (5 days, B1) to nearly 90% (8 days, A1F1) relative to the late 20th century. Simulated extreme precipitation events generally increase, especially during winter and spring, consistent with the seasonal changes in total precipitation. The projected seasonal changes in atmospheric circulation generally resemble the synoptic weather patterns associated with current extreme events, particularly during spring and summer, suggesting that some of the modeled response of extremes may be driven by mean dynamical changes.

The Great Lakes are an important source of fresh water, recreation resource and transportation corridor for the Midwestern United States and Canada. The timing and quantity of fresh water inputs and how those may change under projections of future climate change are important for understanding how conditions, including river flows, and lake levels, within the region may be affected. Water quality and the density and diversity of in-stream habitats are responsive to changes in the distribution of daily streamflow, something not typically included in studies of climate change impacts. Projections of precipitation and air temperature changes in the four states surrounding Lake Michigan from the IPCC AR4 were downscaled and biascorrected before being used to drive a large-scale hydrology model and produce maps of surface runoff and baseflow. These were then routed along drainage networks for regional rivers, and hydrologic metrics describing aspects of the distribution of daily flows important for hydrology and in-stream ecology were computed. The impact of regional climate change projections on early- (water years 2010–2039) and midcentury (water years 2040–2069) streamflow was highly variable; however, by the late-century period (water years 2070–2099) annual streamflow was found to have increased in all rivers. Seasonally, winter and spring flows increased significantly by the late-century period, but summer flows become more variable with a decrease in low-flows and an increase in peak-flows. The number of days with flows above the annual mean-flow (T_Qmean) decreased in summer, but flashiness (R-B Index) increased.

Future climate change and its impact on Lake Michigan is an important issue for water supply planning in Illinois. To estimate possible future levels of the Great Lakes due to climate change, the output of 565 model runs from 23 Global Climate Models were applied to a lake-level model developed by the Great Lakes Environmental Research Laboratory (GLERL). In this study, three future emission scenarios were considered: the B1, A1B, and A2 emission scenarios representing relatively low, moderate, and high emissions, respectively. The results showed that the A2 emission scenario yielded the largest changes in lake levels of the three emission scenarios. Of the three periods examined, lake levels in 2080–2094 exhibited the largest changes. The response of Lake Superior was the smallest of the Great Lakes, while lakes Michigan-Huron, Erie, and Ontario were similar in their response over time and between emission scenarios. For Lake Michigan-Huron, the median changes in lake levels at 2080–2094 were - 0.25, - 0.28, and - 0.41 m for the B1, A1B, and A2 emission scenarios, respectively. However, the range in lake levels was considerable. The wide range of results is due to the differences in emission scenarios and the uncertainty in the model simulations. Selecting model simulations based on their historical performance does little to reduce the uncertainty. The wide range of lake-level changes found here make it difficult to envision the level of impacts that change in future lake levels would cause.

Future changes in climate and precursor emissions will likely have important consequences on ground-level ozone concentrations for the City of Chicago and its surrounding suburban/rural areas. Here we use a regional climate-air quality modeling system to evaluate the combined and individual effects of climate warming (and resulting biogenic emissions increases) and anthropogenic emissions perturbations from 1996–2000 to 2048– 2052 and 2095–2099 in this region. Two scenarios are considered, including A1FI (higher warming with increasing anthropogenic emissions) and B1 (less warming with reduced anthropogenic emissions). Relative to 1996–2000, projected changes in climate and anthropogenic emissions together lead to little ozone change for the City of Chicago under A1FI but 5.0–7.8 ppb increases under B1 by 2048–2052 and 2095–2099. For A1FI, the decreasing ratio of volatile organic compounds (VOCs) to nitrogen oxides (NOx) reduces ozone concentrations over Chicago, despite the increasing emissions for both NOx and VOCs. Averaged over the Chicago urban and surrounding suburban area, however, surface ozone increase 2.3–7.1 ppb under A1FI by 2095–2099. Additionally, the seasonal ozone variation is projected to increase 84–127% under A1FI but decrease 23–30% under B1 over the Chicago area. By comparison, projected climate warming alone increases the surface ozone by 2.1–8.7 ppb and its seasonal variation by 22–89% over the Chicago area from 1996–2000 to 2095–2099 under both scenarios. Therefore, effective emission regulation and climate considerations are both important to pollution mitigation in the Chicago area.

Over the coming century, climate change is projected to increase both mean and extreme temperatures as heat waves become more frequent, intense, and long-lived. The city of Chicago has already experienced a number of severe heat waves, with a 1995 event estimated to be responsible for nearly 800 deaths. Here, future projections under SRES higher (A1FI) and lower (B1) emission scenarios are used to estimate the frequency of 1995-like heat wave events in terms of both meteorological characteristics and impacts on heat-related mortality. Before end of century, 1995-like heat waves could occur every other year on average under lower emissions and as frequently as three times per year under higher. Annual average mortality rates are projected to equal those of 1995 under lower emissions and reach twice 1995 levels under higher. An “analog city” analysis, transposing the weather conditions from the European Heat Wave of 2003 (responsible for 70,000 deaths across Europe) to the city of Chicago, estimates that if a similar heat wave were to occur over Chicago, more than ten times the annual average number of heat-related deaths could occur in just a few weeks. Climate projections indicate that an EHW-type heat wave could occur in Chicago by mid-century. Between mid- and end-of-century, there could be as many as five such events under lower, and twenty-five under higher emissions. These results highlight the importance of both preventive mitigation and responsive adaptation strategies in reducing the vulnerability of Chicago's population to climate change-induced increases in extreme heat.

This paper describes the potential impacts of warming temperatures and changing precipitation on plants, wildlife, invasive species, pests, and agricultural ecosystems across the multi-state region centered on Chicago, Illinois. We examine a geographic area that captures much of Lake Michigan, including a complex mosaic of urbanization and agriculture surrounding southern Lake Michigan. We consider species currently present within this broad region as well as species that are expected to move into or out of the area as climate zones shift northward through the coming century. Our analysis draws upon disparate data sources to compile projections. We conclude that a complex mixture of land use poses particular challenges to natural ecosystems in this region under climate change. Dispersal is likely to be limited for some species, and some populations of native taxa may already be reduced due to habitat loss. Other species can persist, even thrive, within a mixed landscape mosaic, provided natural areas and green spaces are available. If such spaces are somehow connected, they can provide opportunities for native species to inhabit and move through the metropolitan region (perhaps even better than the landscapes previously dominated by agriculture). Strategies for adapting regional agriculture and minimizing pest outbreaks also call for creative management intervention. With additional research, Chicago and its surrounding environs have an opportunity to provide leadership on effective management of natural resources under climate change.

Range distributions for many animal species across North America are shifting, many in directions consistent with anthropogenic climate change over the last century. Assuming climate change continues to act as a driver for geographic range shifts, theoretical northern range shift movements can be calculated using a bioclimatic envelope approach that assume the need to maintain current temperatures and determine whether these would be attainable given projected temperature changes over the coming century. We focus on historically present small mammals in northern Indiana whose range shifts from their 1930s capture locations are attributed to be a hypothetical “response” to changes in recorded and projected average January temperature from 1914–1944 to 1961–1990, 2020–2049, and 2070–2099. Over the mid-20th century, the theoretical distance a mammal must move to remain at the same temperature (temperature-maintaining distance, TMD) ranged from 18.6 to 97.3 km (0.40–2.07 km/year) and appears to have been attainable. However, based on future temperature changes projected under the SRES higher (A1FI) and lower (B1) emissions scenarios, we found significantly larger increases in TMDs by both mid- and end-of-century relative to the historical period. Given recognized barriers to northern range extension, future small mammal TMDs greater than 4 km/year in some scenarios appear less viable than those experienced in the past in this region. As differences between higher and lower emissions scenarios may ultimately influence the ability of mammals to move TMDs, particularly by the end of the century, future emissions have the potential to markedly affect the resulting range shifts.

We use a quantitative modeling framework capable of translating increasing stress on energy demand and costs, infrastructure maintenance, and capital investments into economic impacts to estimate future climate change effects on urban infrastructure and economy. This framework enables quantitative estimates of the economic impacts of climate change based on observed relationships between key climate thresholds and their impacts on energy and infrastructure. Although the version presented here is based on information specific to city departments, the generalized modeling framework can be applied across entire urban and metro areas. For the City of Chicago, energy and infrastructure impacts, including both costs and savings, are driven primarily by increases in mean annual temperature and secondarily by increases in the frequency of extreme-heat events and decreases in cold days. With more frequent, severe, and longer periods of extreme-heat, annual average and peak electricity demands will increase. Aggregated costs for Chicago's maintenance, labor, and capital investments could be as much as 3.5 times greater under a higher (A1FI) emissions scenario as compared to the lower (B1) scenario. These differences highlight how even partial success at reducing emissions could produce a disproportionately large reduction in economic costs for the City, the Great Lakes Region, and the nation at large. At the same time, since a single city's mitigation efforts represent only a small proportion of what is required at the global scale, adaptation to anticipated changes is also essential.

A greenhouse gas emissions inventory was conducted for Chicago and its metropolitan region for the years 2000 and 2005. Emissions of carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride totaled 34.7 million metric tons of carbon dioxide equivalents (MMTC0₂e) in Chicago in 2000 with 91% of emissions attributable to the indirect emissions associated with electricity consumption, the direct emissions of natural gas use, and the direct emissions of the transportation sector. A portfolio of 33 potential emissions reduction strategies was analyzed that, implemented together, could meet Chicago's target of reducing greenhouse gas emissions to 25% below 1990 levels by 2020. The largest potential for reduction is found in the areas with the largest emissions—energy use in buildings and transport. Compared to its metropolitan region, Chicago is found to have existing transportation efficiencies on a per household basis that can be an example for other communities.

The Chicago Climate Action Plan (CCAP), Chicago's roadmap for reducing climate change impacts and adapting to the changes already occurring, relied on rigorous analysis to formulate policy decisions through stakeholder coordination and public engagement. Three key pieces of analysis contributed to Chicago's adaptation strategy: an evaluation of Chicago's higher and lower greenhouse gas emissions scenarios; an assessment of Chicago's economic risk under both emissions scenarios; and a prioritization of potential impacts using a scoring system that included likelihood of occurrence and local consequences of occurrence.

Potential adaptation tactics were categorized according to their expected benefits and costs and led to the creation of working groups to develop action plans that will include primary actors, timelines, budgets, and performance measures that the City will monitor. While not essential for all cities, the impacts analysis was of high value to the adaptation strategy. However, a strategy for stakeholder engagement is crucial in ensuring that the implications of climate impacts are properly understood.