Climate Change in Illinois

Climate change is a serious issue, and has and is projected to continue to have direct and indirect impacts on Illinoisans.

What is Climate Change?

Climate change refers to a change in the usual weather of a region. Illinois’ climate has changed in the past due to natural forces such as changes in the amount of solar radiation, ocean circulation, volcanoes, etc. However humans can influence the climate on local, regional, and global scales, through increases in greenhouse gasses, changing aerosols (small particles in the air such as dust or sulfates from coal burning), and land use changes (change from prairie to agriculture to cities).

The changes observed in Illinois’ climate since the early 20th century are primarily driven by human activities, particularly increased emissions of heat-trapping greenhouse gases including carbon dioxide and methane. These gases change the Earth’s energy balance, and increased emissions have resulted in a consistent increase in global average temperature (i.e., global warming). Regional climate change, such as that experienced here in Illinois, is primarily a consequence of human-caused warming, although natural processes including ocean patterns, volcanic activity, and changes in solar radiation, can contribute to these climate changes.

Recent Changes in Illinois’ Climate

Overall, Illinois’ climate has gotten warmer and wetter since the start of the 20th Century. According to high quality climate monitoring data from the NOAA Centers for Environmental Information, over the past 120 years:

  • Average daily temperature in Illinois has increased by 1 to 2 degrees Fahrenheit. Overnight minimum temperatures have increased more than daytime maximum temperatures.
  • There has been seasonal variation in warming. Winter and spring temperatures in Illinois have increased by 2 to 3 degrees Fahrenheit, while summer temperatures have increased very little since the beginning of 20th century.

The maps below show the change in average daily temperature between 1900 and 2020 in each season by county in Illinois. The units of change are in degrees Fahrenheit per decade.

Maps of average temperature trends by season and county in Illinois.
Maps show trends in daily average temperature in degrees Fahrenheit per decade by season and county in Illinois. Trends are calculated using observations between 1900 and 2020.

  • Precipitation has increased across Illinois. Total annual precipitation has increased by 5 inches over the past 120 years. This is equivalent to a 12 to 15% increase in total annual precipitation.
  • All seasons have gotten wetter across the state, but the majority of the increase in precipitation in southern Illinois has come in the spring (March to May), while most of the change has come in summer (June to August) precipitation in central and northern Illinois.
  • Precipitation has also gotten more intense, as the number of 2-inch rain days in Illinois has increased by 40% since the beginning of the 20th century

The figure below shows the number of days with precipitation of 2 inches or greater in Illinois between 1900 and 2018, averaged over 5-year periods. Dots show annual values, and the horizontal black line shows the long-term annual average of approximately 1.7 events per station between 1900 and 2018. Source: taken from Wuebbles et al. (2021)

Extreme precipitation day frequency in Illinois between 1900 and 2018.
The number of days with precipitation of 2 inches or greater in Illinois between 1900 and 2018, averaged over 5-year periods. Dots show annual values, and the horizontal black line shows the long-term annual average of approximately 1.7 events per station between 1900 and 2018. Source: taken from Wuebbles et al. (2021).

Projections of Illinois Climate Change in the 21st Century

Scientists use global climate models to assess the potential for climate change, and impacts thereof, in the near and distant future. Typically, many models are run under a number of assumptions of socioeconomic conditions between now and the end of the 21st century. These assumptions are referred to as scenarios, and differ in many ways but primarily in the net and cumulative emissions of greenhouse gases between now and the end of the century. Using these scenarios for projections of future climate change help us understand (1) the potential magnitude of future change and (2) how sensitive that change is to changes in human greenhouse gas emissions. Assessment of projections of future climate change in Illinois based on many global climate models (Wuebbles et al. 2021) found:

  • Illinois is expected to continue to see increasing air temperatures and increasing precipitation throughout the 21st century
  • By the end of the 21st century, average daily temperatures are projected to increase between 4 and 9 degrees Fahrenheit under the lower emissions scenario and between 8 and 14 degrees under the higher emissions scenario. These increases are expected to coincide with increased risk of extreme high temperatures in Illinois, and reduced risk of extreme cold temperatures.
  • Illinois is expected to see an overall increase in precipitation, but projections show a change in the distribution and seasonality of precipitation with increases in both heavy rain and length of dry spells
  • Projected increases in summer temperatures are expected to increase the severity and rate of intensification of naturally occurring droughts in Illinois

Changes in extreme heat are shown in the figure below. Specifically, global climate models project the number of very hot days, those with a high temperature at or above 95 degrees Fahrenheit, will increase significantly throughout the 21st century in all parts of Illinois. For example, the number of very hot days in northern Illinois is projected to increase from between 1 and 2 per year over the historical period to between 10 and 60 per year by 2090 under the lower greenhouse gas emissions scenario and between 25 and 90 per year by 2090 under the higher greenhouse gas emissions scenario.

Projected annual number of extreme heat days in Illinois.
The plots show the simulated historical and projected (future) annual number of days with daily maximum temperature of 95 degrees Fahrenheit or higher for the state of Illinois (left panel), northern Illinois (top right panel), central Illinois (middle right panel), and southern Illinois (lower right panel). The observed climatological values averaged for the period 1960 to 2013 are shown by the black line. Historical simulations (gray shading) are shown for 1960 to 2004. Projected changes for 2006 to 2100 are shown for the higher greenhouse gas emissions scenario (red shading) and the lower greenhouse gas emissions scenario (teal shading). The shaded ranges indicate the 10% to 90% confidence intervals of 20-year running averages from the set of climate models. Source: Wuebbles et al. 2021. 

This assessment concludes that changes in extreme heat, heavy downpours, and flooding have already affected and will continue to affect infrastructure, human health, agriculture, forestry, transportation, and air and water quality. Climate change will also exacerbate a range of risks to the Great Lakes.

These findings are similar to those from the 2018 National Climate Assessment, which reviewed climate trends across the Midwest, including Illinois. You can see, based on the National Climate Assessment, how climate change has and is expected to affect your area using the U.S. Climate Resilience Toolkit.