Average annual temperature has increased about 1°F over the last two decades. Although there has been little change in the occurrence of high summer daytime extremes, winter warming is evident, with a generally below average occurrence of extremely cold days since 1990. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Spring precipitation has generally been above average in the past two decades, affecting agriculture in both positive (adequate soil moisture) and negative (delays in spring planting) ways. Future increases in winter and spring precipitation will pose a continued risk of spring planting delays.
Severe flooding and drought have occurred periodically in recent years with major impacts in several communities across the state. Future increases in extreme precipitation events and in evaporation rates may increase the intensity of both floods and droughts as well as the frequency of floods.
After the Rain
Photo by TumblingRun
Iowa’s location in the interior of North America and the lack of mountains to the north or south exposes the state to incursions of bitterly cold air masses from the Arctic, as well as warm and humid air masses from the Gulf of Mexico. As a result, its climate is characterized by large ranges in temperature with cold winters and warm, humid summers.
Since the beginning of the 20th century, temperatures in Iowa have risen approximately 1°F (Figure 1). Temperatures in the 2000s have been higher than any other historical period with the exception of the 1930s “Dust Bowl” era. The warming is due to increases in nighttime minimum temperatures while daytime maximum temperatures have not changed. Increases in humidity may be one cause of this asymmetric warming between night and day. The year 2012 was the hottest on record with an average annual temperature of 52.1°F, 4.6°F above the long-term average. This warming has been concentrated in the winter and spring while summers have not warmed substantially in the state, a feature characteristic of much of the Midwest (Figure 2a). This is reflected in a below average occurrence of very hot days (maximum temperature above 95°F) (Figure 2b) and no overall trend in warm nights (minimum temperature above 70°F) (Figure 2c). The winter warming trend is reflected in a below average number of very cold nights (minimum temperature below 0°F) over the past two decades (Figure 2d).
Precipitation varies widely across Iowa, with the southeastern portion of the state receiving around 38 inches annually compared to only 26 inches in the northwest. Much of Iowa’s precipitation falls during the summer months, averaging more than 14 inches in the central part of the state. Recent years have seen above average precipitation in the spring which can make it difficult for farmers to plant crops (Figure 3). Summer and annual precipitation have also been above average (Figures 3 and 4), which has been beneficial for crop production but has also increased flooding. April and June have been particularly wet in recent years, averaging more than 40% above average since 2008. Statewide annual precipitation has ranged from a low of 20.21 inches in 1910 to a high of 47.88 inches in 1993. Snowfall also varies across the state, ranging from 40 inches in the northeast to about 20 inches in the southeast. For most of the state, more than 40% of the annual precipitation occurs on the 10 wettest days of the year, a percentage which rises to over 48% in the west. The frequency of extreme precipitation events has increased, with the highest number of 2-inch rain events occurring during the last decade (Figure 5).
Agriculture is a vital component of Iowa’s economy, and the state is particularly vulnerable to extreme precipitation conditions. Both flooding and droughts have resulted in billions of dollars in losses in recent years. Following abnormally dry conditions in 2011, Iowa experienced severe drought conditions in 2012. Rainfall totals for the critical growth months of July and August were several inches below average (average statewide July-August rainfall is 7.74 inches for 1895–2014) with observed summer rainfall totaling 6.42 inches, 4.05 inches, and 3.23 inches in 2011, 2012, and 2013, respectively. Below normal July-August rainfall during this three-year period was unlike any other three-year period on record dating back to 1893, superseding the dry period of the Dust Bowl years. By the end of September 2012, much of the state was in extreme drought, with portions in the northwest experiencing exceptional drought conditions extending into 2013.
Thousands of miles of rivers flow through Iowa and the state is bordered by the Mississippi River to the east and the Big Sioux and Missouri Rivers to the west. With many of these waterways located alongside cities and farmland, flooding is a severe hazard to the state. For the period of 1955–1997, Iowa was ranked first in state losses due to flooding. During the first two weeks of June 2008, heavy rainfall on soil already saturated from unusually wet conditions caused record flooding along multiple rivers in the state. Multiple long-term stations reported more than 10 inches of rain during the two-week period, and levels on the Cedar River exceeded the previous record by more than 11 feet. Of the state’s 99 counties, 83 were declared disaster areas and damages were estimated at almost $10 billion. Snowmelt, as well as ice jams, can also cause flooding. In June 2011, runoff from a record winter snowpack in the Rocky Mountains along with heavy rains caused major flooding along the entire length of the Missouri River. The region around Hamburg was particularly hard hit, where levee failures forced evacuation of the town and flooding of the farmland caused extensive agricultural losses.
Iowa experiences damaging storms during all seasons. During the winter months, snowstorms and ice storms are a frequent hazard. Over December 8–9, 2009, a strong storm produced heavy snowfall across the state, with multiple long-term stations reporting more than 15 inches of snow. Wind gusts over 50 mph produced large snow drifts and caused widespread whiteout conditions. The blizzard conditions were compounded by bitter cold on December 9, with large portions of the state experiencing temperatures below 10°F and wind chills below 0°F. Convective storms are common in the warmer months, including thunderstorms (capable of producing hail and tornadoes), and flood-producing rainstorms. On May 25, 2008, an EF-5 tornado killed 8 people and destroyed nearly 200 homes in Parkersburg. This was the strongest tornado to hit the state since June 13, 1976.
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. Intense heat waves can occur in Iowa and these are often accompanied by high humidity. Heat waves are projected to become more intense and impacts on human health could be significant. However, cold waves are projected to be less intense.
Precipitation is projected to increase in Iowa, with the largest increases expected in winter and spring (Figure 6). In addition, extreme precipitation is projected to increase, potentially increasing the frequency and intensity of floods. Springtime flooding in particular could pose a threat to Iowa’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 the rate of loss of soil moisture. Thus, the periodic summer droughts, a natural part of Iowa’s climate, are likely to be more intense in the future.