Average annual temperature has increased about 1°F since the early 20th century, with warming concentrated during the winter and spring. Winter warming has been characterized by a much below average occurrence of very cold nights since 1990. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Precipitation can vary greatly from year to year in this region of east-to-west transition from humid to semi-arid conditions. Projected increases in winter precipitation may result in both beneficial and negative impacts.
The agricultural economy of Nebraska makes the state particularly vulnerable to droughts, several of which have occurred in recent years, although the impacts can be mitigated where irrigation is possible. Projected increases in temperature and evaporation rates may increase the intensity of future naturally-occurring droughts.
Nebraska lies in the central Great Plains, straddling the east-to-west transition from relatively abundant precipitation (averaging over 34 inches annually at Falls City) in the far southeast to semi-arid conditions (averaging 17 inches at Harrison) in the panhandle. The state is far from the moderating effects of oceans and experiences warm to hot summers and cold winters. Temperatures vary widely across seasons, averaging 22.1°F in January and 74.5°F in July. The hottest year on record was 2012, with an average temperature of 52.7°F, 4.3°F higher than the long-term average.
Since the beginning of the 20th century, temperatures in Nebraska have risen approximately 1°F (Figure 1). Temperatures in the 2000s have been warmer than the long-term average and comparable to the previous record warmest period of the early 1930s Dust Bowl era, when drought and poor land management likely exacerbated the hot summer temperatures. The recent warming has been concentrated in the winter and spring while summers have not warmed substantially in the state, a characteristic of much of the Great Plains and Midwest (Figure 2). This is reflected in a below average occurrence of extremely hot days (Figure 3a) and no overall trend in the number of warm nights (Figure 3b) since the 1960s. The winter warming trend is reflected in a below average number of very cold nights since 1990 (Figure 4).
Precipitation is highly variable from year to year, with the statewide annual average ranging from a low of 13.36 inches in 2012 to a high of 35.50 inches in 1915. The driest multi-year period was in the 1930s, and the wettest in the 1900s and late 2000s (Figure 3c). The driest 5-yr period was 1936–1940 and the wettest was 2007–2011. The majority of precipitation falls during the spring and summer months, but seasonal precipitation varies widely (Figure 3d).
Agriculture is a vital component of Nebraska’s economy, and the state is particularly vulnerable to both high and low extreme precipitation. The frequency of heavy rain events has increased in recent years, with Nebraska experiencing an above average number of 2-inch rain events over the last decade (Figure 5). Nebraska also experiences periodic episodes of severe drought, which can sometimes last for several years. One of the worst droughts in the state’s history was the 1930s drought of the Dust Bowl era, when the impacts of the dry conditions were exacerbated by extreme heat. Nebraska’s hottest summers on record occurred in 1934 and 1936; they were also among the top four driest summers (1934—fourth, 1936—second). Conditions in July 1936 were particularly extreme, with Omaha experiencing 16 days with temperatures above 100°F and one day with temperatures exceeding 110°F. This combination of heat and dryness, along with the close temporal proximity of these two extreme summers and exacerbated by poor land management practices, is unique in the record and contributed to the severe impacts of the Dust Bowl era. Nebraska’s driest year on record, however, was 2012. Statewide precipitation averaged only 3.74 inches during the summer months, well below the historical average of 9.42 inches. By the end of September, over 75% of the state was experiencing exceptional drought conditions. The drought, combined with the extreme summer heat, had significant negative impacts on non-irrigated crop yields and pasture conditions, and the state did not see substantial relief from drought conditions for months.
Thousands of miles of rivers flow through Nebraska, and the state is bordered by the Missouri River to the east. With many cities and farmlands located along these waterways, flooding from both heavy precipitation and snowmelt can cause problems in the state. In the summer of 1993, heavy rains throughout the central U.S. caused record flooding along the Missouri (and Mississippi) River. For the three-month period of June–August, Nebraska precipitation totaled 16.77 inches on average statewide, more than 7 inches above the long-term summer average. The flooding caused millions of dollars in damages to crops and infrastructure. In June 2011, runoff from the record winter snowpack in the Rocky Mountains along with heavy rains, particularly in the upper Missouri River basin, caused major flooding along the entire length of the Missouri River. In Omaha, the river crested at 36.29 feet on July 2, 2011.
Nebraska experiences damaging storms during all seasons. During the winter months, snowstorms and ice storms are a frequent hazard. Western Nebraska, along with the Dakotas, has the highest probability of blizzards in the nation, with a greater than 50% probability of a blizzard occurring in any given year. For example, during December 23–27, 2009, a storm produced heavy snowfall in the eastern part of the state, with some long-term stations reporting snow depths in excess of 20 inches. Wind gusts of up to 60 mph on December 25th reduced visibility and produced whiteout conditions. Convective storms are common in the warmer months, including flash flood-producing rainstorms and severe thunderstorms capable of producing hail, damaging winds, and tornadoes. The southwestern part of the Nebraska Panhandle lies in “Hail Alley,” the most hail prone area in the entire country, and averages 7–9 hail days each year. Nebraska averages 57 tornadoes annually—the fifth highest number of any state—and these tornadoes can be violent. On May 6, 1975, an F4 tornado struck Omaha, killing three people and causing damages of over $1 billion. On June 16, 2014, a supercell thunderstorm produced four EF4 tornadoes (including a set of rare “twin tornadoes”) in the northeastern part of the state, killing two and destroying large portions of the town of Pilger.
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. Increases in heat wave intensity are projected, but the intensity of cold waves is projected to decrease.
Although projections of overall annual precipitation are uncertain, winter and spring precipitation is projected to increase across the state (Figure 6). Heavier winter precipitation could have both positive and negative effects on Nebraska’s important agricultural economy, improving soil moisture for winter wheat but potentially delaying planting for summer crops. Heavy precipitation events are also projected to increase, leading to increased runoff and flooding which can reduce water quality and erode soils.
The intensity of droughts is projected to increase. Although projections of overall precipitation are uncertain, and droughts are a natural part of the climate system, higher temperatures will increase evaporation rates and decrease soil moisture, leading to more intense future droughts. This would have negative impacts on dryland farming, although the impacts could be mitigated where irrigation is possible.