Increases in temperature may lead to an increase of 1-2 hardiness zones and 2-4 heat zones in the Chicago area.

Chicago and areas right around Lake Michigan in Indiana are in hardiness zone 6a (mean annual lowest temperature -10 to -5 °F; -21 to -23 °C), and most of the surrounding area is currently in hardiness zone 5b (mean annual lowest temperature -15 to -10 ° F; -26 to -23 °C). By the end of the century, hardiness zones are expected to shift to between 6a and 6b under a low emissions scenario up to potentially 7b under a high emissions scenario. However, it is important to keep in mind that hardiness zones are based on a 30-year average and some very cold winters could still be possible.
Heat zones are determined by the average number of days above 86 °F (30 °C). Much of the Chicago Wilderness region is in heat zone 5 (30 to 45 days above 86 °F). Lake Michigan helps moderate temperatures in areas along the coast, so the greatest effects of the Chicago area’s urban heat island during the day is actually in the western suburbs in Dupage and Will counties (which are in heat zone 6; 45 to 60 days). Lake and Porter counties in Indiana are also heat zone 6. Lake County, Illinois, and southeastern Wisconsin have cooler summer high temperatures and are in heat zone 4 (14 to 30 days). As summer high temperatures rise, heat zones will also increase in the area, meaning that some areas will be too hot to be suitable for some species. Under the PCM B1 scenario, heat zones are not expected to change much from current conditions, but could shift up to zone 8 under the GFDL A1FI scenario (90 to 120 days above 86° F).