Average annual temperature has increased about 0.5°F since the early 20th Century. Winter warming has been characterized by a below average occurrence of extremely cold days since 1990. Under a higher emissions pathway, historically unprecedented future warming is projected.
Missouri has experienced an increase in heavy rain events, a trend which is projected to continue. Future increases in winter precipitation will pose a continued risk of spring planting delays and increased flooding along rivers and streams.
Severe drought, a natural part of Missouri’s climate, is a risk to this agriculture-dependent state. Future increases in evaporation rates due to higher temperatures may increase the intensity of naturally-occurring droughts.
Missouri’s location in the interior of the North American continent exposes it to a climate with large ranges in temperature with hot, humid summers and cold winters. The lack of mountain barriers both to the north and to the south, and the state’s inland location away from the moderating effects of the oceans, allow it to be influenced by both cold Arctic air masses and warm, moist air masses from the Gulf of Mexico. Average annual temperatures across the state vary over a range of about 10°F from north to south. The year 2012 was the hottest on record, with an average annual temperature of 58.6°F, 4.1°F higher than the long-term average.
Since the beginning of the 20th century, temperatures in Missouri have risen approximately 0.5°F (Figure 1) and temperatures in the 2000s have been higher than any other historical period with the exception of the early 1930s Dust Bowl era. This warming has been concentrated in the winter and spring while average summer temperatures have not increased substantially in the state until the most recent 5 years, a feature characteristic of much of the Midwest (Figure 2). Due to extreme drought and poor land management practices, the summers of the 1930s remain the warmest on record. The recent summer warming has been characterized by much warmer nights (above “Dust Bowl” levels) while daytime highs have only increased a little. The state has also experienced a below average occurrence of extremely hot days (maximum temperature above 100°F) (Figure 3a). In addition to the overall trend of higher average temperatures, the state has experienced an above average number of very warm nights (minimum temperature above 75°F) (Figures 2 and 4). Since 1950, the annual number of these very warm nights has increased by about 2 days per decade at St. Louis Lambert Airport. Also, there is an upward trend in summer humidity since the mid-20th Century. 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 3b).
Average annual precipitation varies widely across the state, from a low of 34 inches in the northwest to a high of 50 inches in the southeast. The northern part of the state receives more snowfall, with an annual average of 18–24 inches in the northern counties, compared to only 8–12 inches in the south. Annual statewide average precipitation has ranged from a low of 25.12 inches in 1953 to a high of 57.14 inches in 1973. The driest 5-year period was the early 1930s while the wettest 5-year period was the early 1990s (Figure 3c). Summer precipitation exhibits no trend (Figure 3d). For large portions of the state, more than 40% of the total annual precipitation occurs on the 10 wettest days of the year.
Agriculture is an important component of Missouri’s economy; therefore, the state is particularly vulnerable to extreme precipitation conditions. Both floods and droughts can result in billions of dollars in losses. In 2012, a severe drought across the Midwest had large impacts on Missouri. Rainfall totals for the critical growth months of May, June, and July were several inches below average with only 5.76 inches of rain, the third driest such period (after 1901 and 1936) in 120 years of record. The drought was the worst Missouri had seen in 30 years; by the end of July, all 114 counties had been declared disaster areas.
Missouri has experienced an increase in the number of heavy rain events, and the state’s position in the lower river basins of several large Midwestern rivers makes downstream flooding an extreme hazard in this state (Figure 5). Missouri is ranked fourth in state losses due to flooding for the period of 1955–1997. One of the most severe climate events in the state’s history was the 1993 Mississippi River flood. Near St. Louis, the Mississippi River crested at 49.6 feet, almost 20 feet above flood stage, and 6 feet higher than the previous peak in April 1973. The flooding resulted in billions of dollars in damage to homes, businesses, agriculture, and infrastructure. In 2011, the state experienced flooding along both the Mississippi and Missouri Rivers. A wet April along the Ohio River Valley and record snowmelt in the upper Mississippi River basin caused record swelling along both the Ohio and Mississippi Rivers. To save the town of Cairo, Illinois, a levee was destroyed near Birds Point, flooding hundreds of thousands of acres of Missouri farmland. Property and crop damages were estimated at around $320 million. In June of that year, runoff from a record winter snowpack in the northern Rockies, combined with heavy rains, caused major flooding along the entire length of the Missouri River.
Severe thunderstorms are common in Missouri. During the summer, the state’s lack of geographic barriers allows cold, dry air from the north to collide with warm moist air from the Gulf of Mexico, triggering severe thunderstorms, which can produce high winds, heavy rain, tornadoes, and hail. On April 10, 2001, a strong thunderstorm produced catastrophic hail damage across the I-70 corridor. The storm produced hail as large as three inches in diameter, and damages in the Kansas City and St. Louis areas were estimated at around $1.5 billion. Missouri has a long and deadly history of tornadic storms. On May 22, 2011, an EF-5 tornado with winds exceeding 200 mph hit the city of Joplin, killing more than 150 people and causing billions of dollars in damages. This was the deadliest tornado in Missouri history. On March 18, 1925 the Tri-State Tornado, the deadliest tornado in U.S. history, tracked more than 200 miles from southeastern Missouri, across southern Illinois, and into Indiana. In Missouri, the storm killed at least 11 people and caused extensive property damage, including near complete destruction of the town of Annapolis.
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. In southern Missouri, the annual maximum number of consecutive days with temperatures exceeding 95°F is projected to increase by up to 20 days. Temperature increases will cause future heat waves to be more intense, a concern for this region which already experiences hot and humid conditions. Extreme heat is of particular concern for urban areas, such as St. Louis and Kansas City, where the urban heat island effect raises summer temperatures. In 1966, St. Louis experienced a severe heat wave from July 9 to 14 which caused many deaths. In addition to daytime highs between 101°F and 106°F at St. Louis Lambert International Airport, nighttime temperatures never dropped below 77°F, and on July 12th, the low was only 84°F. If the warming trend continues, future heat waves are likely to be more intense, and cold wave intensity is projected to decrease.
Although projections of overall annual precipitation are uncertain, winter and spring precipitation are projected to increase, while summer precipitation may decrease (Figure 6). Additionally, extreme precipitation is projected to increase, potentially increasing the frequency and intensity of floods. Springtime flooding in particular could pose a threat to Missouri’s important agricultural economy by delaying planting and resulting in loss of yield.
The intensity of droughts is projected to increase. Even if precipitation increases in the future, rising temperatures will increase evaporation rates, resulting in more rapid loss of soil moisture. Thus, future summer droughts, a natural part of the Missouri climate, are likely to become more intense.