Average annual temperature has increased by about 1°F since the beginning of the 20th century. There has been seasonal variation in this warming, with average spring temperature increasing by about 2°F and average summer temperature increasing very little. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Precipitation in spring and summer has generally been above average over the past two decades, affecting agriculture in both positive (adequate soil moisture) and negative (delays in spring planting) ways. Precipitation in winter and spring is projected to increase, which poses a continuing risk of spring planting delays.s.
Severe flooding and drought have occurred periodically in recent years. Future increases in extreme precipitation events and in evaporation rates may increase the intensity of both floods and droughts.
Rows and Trails
Photo by H. Michael Miley
Illinois’s location in the interior of the North American continent exposes it to a climate with large ranges in temperature with warm, humid summers and cold winters. The lack of mountains to the north or south allows very cold air masses from the Arctic in the winter and warm, humid air masses from the Gulf of Mexico in the summer to move into the state, further increasing the range of conditions that affect Illinois. Temperature varies widely across the state, with a range of about 10°F from north to south. In northeastern Illinois, Lake Michigan moderates the temperature, causing cooler summers and warmer winters. Topography and urban areas also have local impacts on climate.
Since the beginning of the 20th century, temperatures in Illinois have risen approximately 1°F (Figure 1). Temperatures in the 2000s have been higher than any other historical period, with the exception of the early 1930s “Dust Bowl” era. Warming has been concentrated in winter and spring while summers have not warmed substantially in the state, a feature characteristic of much of the Midwest (Figure 2). The lack of summer warming is reflected in a below average occurrence of very hot days (days with maximum temperature above 95°F) since the mid 1950s (Figure 3a) and no overall trend in very warm nights (minimum temperature above 75°F) since the beginning of the 20th century (Figure 3b). The winter warming trend is reflected in a below average number of very cold nights (minimum temperature below 0°F) over the past 25 years (Figure 3c).
Statewide annual precipitation has ranged from a low of 25.52 inches in 1901 to a high of 51.18 inches in 1993. The driest multi-year periods occurred in the majority of years in the first half of the 1900s, and the wettest periods have been observed since the 1990s and into the 2000s. (Figure 3d). The driest 5-year period was 1952–1956 and the wettest was 2007–2011. However, annual precipitation varies widely across the state, ranging from more than 48 inches in the south to less than 32 inches in the north. For snowfall, the pattern is reversed, with the northeastern part of the state averaging 40 inches of snowfall annually, compared to only 10 inches in the southernmost section. In the Chicago Metropolitan area, the proximity to Lake Michigan occasionally results in heavy winter precipitation from lake-effect snows.
Agriculture is an important component of the state’s economy, and the agricultural sector is particularly vulnerable to extreme precipitation conditions. On average, Illinois has experienced above average precipitation in spring and summer over the past two decades (Figure 4). While precipitation during these critical growth months is important for adequate soil moisture, it is also vital for proper planting and root development. Poor root development in important state crops, such as corn and soybeans, can lead to reduced plant absorption of nutrients and water from the soil, increased soil erosion, and loss of nutrients from the fields into the rivers and streams. Both flooding and droughts have resulted in billions of dollars in losses in recent years. In 2012, a large drought across the Midwest had severe impacts on Illinois. Rainfall totals for May, June, and July were several inches below average and ranked as the third driest period (after 1936 and 1988) in 120 years of record. By early August, much of the state was in extreme drought. The drought caused major damage to crops, particularly in the southern third of the state.
llinois has experienced a dramatic increase in the number of extreme precipitation events (more than 2 inches of precipitation), which can cause severe flooding in the state (Figure 5). In the summer of 1993, persistent heavy rainfall over the upper Midwest caused severe flooding along the Mississippi River. The 1993 flood was one of the greatest natural disasters in U.S. history, causing billions of dollars in damage to homes, businesses, agriculture, and infrastructure. Recently, from April 16 to 19, 2013, heavy rainfall from a slow-moving storm system caused severe flooding across parts of northern and central Illinois, with some areas receiving up to 10 inches of rain. This event, along with the wettest January–June on record in the state, caused planting to be delayed and resulted in diminished revenue for many farmers. In addition, Illinois has struggled with urban flooding caused by heavy rains falling on impervious surfaces (e.g., roads, sidewalks, and driveways) with inadequate infrastructure. A recent report found that more than 90% of urban flooding damage claims from 2007 to 2014 were outside the mapped floodplain.
Illinois experiences storms during all seasons. From February 1 to 3, 2011, Illinois was hit by one of the most powerful winter storms in history. The greatest snow accumulation associated with the storm was in Antioch where 27 inches of snow was measured; this area averages only one snowfall greater than 6 inches annually. Chicago O’Hare International Airport recorded wind gusts of more than 60 mph and 20.2 inches of snow, the third largest snowfall accumulation reported for the city in 120 years of record. More than 9.8 million Illinois residents were in areas with 12 or more inches of snow. Severe thunderstorms occur frequently during late spring and early summer. These storms can occasionally cause tornadoes, which sometimes result in major damage and loss of life.
Water levels in the Great Lakes have fluctuated over a range of three to six feet since the late 19th century (Figure 6). Higher lake levels were generally noted in the latter part of the 19th century and early 20th century, the 1940s and 1950s, and the 1980s. Lower lake levels were observed in the 1920s and 1930s and again in the 1960s. For Lake Michigan-Huron, the first decade of the 21st century has also seen lower levels. Overall, Lake Michigan-Huron has shown a statistically significant downward trend over the past 150 years. The trend is largely due to the high levels early in the period and the extremely low levels in the past 10 years.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records (Figure 1). From July 12 to 16, 1995, Chicago experienced a severe heat wave— the worst weather-related disaster in the city’s history. Over a five-day period, more than 700 people died in Chicago. In addition to daytime highs of greater than 90°F (including two days greater than 100°F), nighttime temperatures only dropped into the 80s. Furthermore, the heat index, which considers both temperature and humidity, reached values of 105°F or more for 42 hours during the event. Values of 105°F are considered dangerous by the National Weather Service. An analysis of hourly data in Chicago from the 1930s onward (Figure 7) shows that conditions were the most severe on record in terms of the number of hours above critical thresholds of the heat index. Future heat waves are likely to be more intense if temperature increases continue, coupled with periods of high humidity. This will pose risks to human health, particularly in the Chicago and St. Louis metro areas. Cold wave intensity is projected to decrease.
Precipitation is projected to increase in Illinois, with increases most likely during the winter and spring (Figure 8). Extreme precipitation is also projected to increase, potentially increasing the frequency and intensity of floods. Springtime flooding in particular could pose a threat to Illinois’s important agricultural economy by delaying planting and resulting in loss of yield.
The intensity of future droughts is projected to increase.Even if precipitation increases in the future, increases in temperature will increase evaporation rates and the rate of loss of soil moisture. Thus, future summer droughts, a natural part of the Illinois climate, are likely to be more intense.