Jim Angel, Prairie Research Institute, University of Illinois
Barbara Mayes Boustead, National Oceanic and Atmospheric Administration
Kathryn C. Conlon, Centers for Disease Control and Prevention
Kimberly R. Hall, The Nature Conservancy
Jenna L. Jorns, University of Michigan, Great Lakes Integrated Sciences and Assessments
Kenneth E. Kunkel, North Carolina State University
Maria Carmen Lemos, University of Michigan, Great Lakes Integrated Sciences and Assessments
Brent Lofgren, National Oceanic and Atmospheric Administration
Todd A. Ontl, USDA Forest Service, Northern Forests Climate Hub
John Posey, East West Gateway Council of Governments
Kim Stone, Great Lakes Indian Fish and Wildlife Commission (through January 2018)
Eugene Takle, Iowa State University
Dennis Todey, USDA, Midwest Climate Hub
Thomas Bonnot, University of Missouri
Katherine Browne, University of Michigan
Melonee Montano, Great Lakes Indian Fish and Wildlife Commission
Hannah Panci, Great Lakes Indian Fish and Wildlife Commission
Jason Vargo, University of Wisconsin
Madeline R. Magee, University of Wisconsin-Madison
Kristin Lewis, Senior Scientist
Allyza Lustig, Program Coordinator
Katie Reeves, Engagement and Communications Lead
<b>Angel</b>, J., C. Swanston, B.M. Boustead, K.C. Conlon, K.R. Hall, J.L. Jorns, K.E. Kunkel, M.C. Lemos, B. Lofgren, T.A. Ontl, J. Posey, K. Stone, G. Takle, and D. Todey, 2018: Midwest. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 872–940. doi: 10.7930/NCA4.2018.CH21
The Midwest is a major producer of a wide range of food and animal feed for national consumption and international trade. Increases in warm-season absolute humidity and precipitation have eroded soils, created favorable conditions for pests and pathogens, and degraded the quality of stored grain. Projected changes in precipitation, coupled with rising extreme temperatures before mid-century, will reduce Midwest agricultural productivity to levels of the 1980s without major technological advances.
Recent Agriculturally Important Trends
The two main commodity crops in the Midwest are corn and soybeans, which are grown on 75% of the arable land. Wheat and oats are important crops grown on fewer acres. An increasing number of niche but higher-value crops (such as apples, grapes, cherries, cranberries, blueberries, and pumpkins) also are grown in the region.43
Over the past 30 years, increased rainfall from April to June has been the most impactful climate trend for agriculture in the Midwest,3 providing a favorable supply of soil moisture while also reducing flexibility for timing of spring planting and increasing soil erosion.44 In addition, wet conditions at the end of the growing season can create elevated levels of mold, fungus, and toxins.45 The last spring frost has occurred earlier, causing the frost-free season to increase by an average of nine days since 1901.46 However, daily maximum temperatures in summer in the Midwest have not followed the upward global trend, in part due to higher early summer rainfall on deep, water-holding soils,47 thereby avoiding plant stress detrimental to crops. The avoidance of heat stress and longer growing seasons have favored production in some parts of and some years in the Midwest.
Daily minimum temperatures have increased in all seasons due to increasing humidity.48,49 Elevated growing-season minimum daily temperatures are considered a factor in reducing grain weight in corn due to increased nighttime plant respiration.50 Warming winters have increased the survival and reproduction of existing insect pests51 and already are enabling a northward range expansion of new insect pests and crop pathogens into the Midwest.52
A contributing factor underpinning Midwest growing-season trends in both temperature and precipitation is the increase in water vapor (absolute humidity):49,53 higher humidity decreases the day–night temperature range and increases warm-season precipitation. Rising humidity also leads to longer dew periods and high moisture conditions that favor many agricultural pests and pathogens for both growing plants and stored grain.
Projected Trends and Agricultural Impacts
Warm-season temperatures are projected to increase more in the Midwest than any other region of the United States.54 The frost-free season is projected to increase 10 days by early this century (2016–2045), 20 days by mid-century (2036–2065), and possibly a month by late century (2070–2099) compared to the period 1976–2005 according to the higher scenario (RCP8.5).46
By the middle of this century (2036–2065), 1 year out of 10 is projected to have a 5-day period that is an average of 13°F warmer than a comparable period at the end of last century (1976–2005).54 Current average annual 5-day maximum temperature values range from about 88°F in Northern Minnesota to 97°F in Southern Missouri. Tables 21.1 and 21.2 show that by mid-century under the higher scenario (RCP8.5), 5-day maximum temperatures are projected to have moved further above optimum conditions for many crops and closer to the reproductive failure temperature, especially for corn in the southern half of the Midwest. Higher growing-season temperatures also shorten phenological stages in crops (for example, the grain fill period for corn).35,50 Under these temperatures, overall yield trends will be reduced because of periodic pollination failures and reduced grain fill during other years.
Table 21.1: Average Annual 5-Day Maximum Temperature
Modeled Historical (1976–2005)
Mid-21st Century (2036–2065) for Lower Scenario (RCP4.5)
Mid-21st Century (2036–2065) for Higher Scenario (RCP8.5)
Table 21.1: These modeled historical and projected average annual 5-day maximum temperatures illustrate the temperature increases projected for the middle of this century across the Midwest. Sources: NOAA NCEI and CICS-NC.
Table 21.2: Optimum and Failure Temperatures for Vegetative Growth and Reproduction
Failure for Growth
Failure for Reproduction
Table 21.2: This table shows the temperatures at which corn and soybeans reach optimum growth and reproduction as well as the temperatures at which growth and reproduction fail.50
Increases in humidity in spring through mid-century3,4 are expected to increase rainfall, which will increase the potential for soil erosion5,6 and further reduce planting-season workdays due to waterlogged soil.7 As an example, for the Cedar River Basin in Iowa, the 100-year flood (1% chance of occurring in a given year) of the 20th century is projected to be a 25-year flood (4% chance per year) in the 21st century,55 with associated increased frequency of flooding of agricultural land. Increased spring precipitation and higher temperatures and humidity are expected to increase the number and intensity of fungus and disease outbreaks56,57 and the prevalence of bacterial plant diseases,58 such as bacterial spot in pumpkin and squash.59 Increased precipitation and soil moisture in a warmer climate also lead to increased loss of soil carbon60 and degraded surface water quality due to loss of soil particles and nutrients.61,62 Transitions from extremes of drought to floods, in particular, increase nitrogen levels in rivers63 and lead to harmful algal blooms.
Current understanding of drought in the Midwest is that human activity has not been a major component in historical droughts, and it remains uncertain how droughts will behave in the future. However, future projections show that Midwest surface soil moisture likely will transition from excessive levels in spring due to increased precipitation to insufficient levels in summer driven by higher temperatures, causing more moisture to be lost through evaporation.64
Projections of mid-century yields of commodity crops65,66 show declines of 5% to over 25% below extrapolated trends broadly across the region for corn (also known as maize) and more than 25% for soybeans in the southern half of the region, with possible increases in yield in the northern half of the region. Increases in growing-season temperature in the Midwest are projected to be the largest contributing factor to declines in the productivity of U.S. agriculture.2 In particular, heat stress in maize during the reproductive period is projected by crop models to reduce yields in the second half of the 21st century.67 These losses may be mitigated by enhanced photosynthesis and reduced crop water use, although the magnitude is uncertain.68,69 Elevated atmospheric CO2 is expected to partially, but not completely, offset yield declines caused by climate extremes, with effects on soybeans less than on maize.70
Non-commodity crops produced in the Midwest include tree fruits, sweet corn, and vegetables for farmers markets and canning. While the general impacts of climate change on specialty crops are similar to commodity crops, the more intense heat waves, excessive rain interspersed with drought, and higher humidity of a future climate likely will degrade market quality as well as yield by mid-century.71 Although data on climate-related losses are sparse, excess moisture is emerging as a major cause of crop loss.72 Wild rice is an annual plant harvested by tribes and others in shallow wetlands of northern Minnesota, Wisconsin, and Michigan. Stable production depends on a stable climate that maintains ecosystem diversity. Declines in production are expected, related to increases in climate extremes and climate-related disease and pest outbreaks as well as northward shifts of favorable growing regions.73
Longer growing seasons and the introduction of hoop buildings (low, translucent, fabric-covered structures that protect plants from extreme weather) have allowed local growers of annual vegetable crops to extend the fresh produce season. However, unsheltered perennial crops such as tree fruits may be subjected increasingly to untimely budbreak followed by cold pulses due to earlier and longer occurrences of warm conditions in late winter.
Most animal agriculture in the region is in confinement, rather than range-based without shelter, and therefore offers an opportunity for mitigating some of the effects of climate change. Without adaptive actions, breeding success and production of milk and eggs will be reduced due to projected temperature extremes by mid-century.74,75,76
Conservation Practices Reduce Impact of Heavy Rains
Figure 21.2: Integrating strips of native prairie vegetation into row crops has been shown to reduce sediment and …
Soil-erosion suppression methods in row-crop agriculture subjected to more intense rains include use of cover crops, grassed waterways, water management systems, contour farming, and prairie strips.6,40 More diversity in planting dates, pollination periods, chemical use, and crop and cultivar selection reduces vulnerability of overall production to specific climate extremes or the changes in pests and pathogens that they cause. An example of a highly successful program is the Iowa State Science-based Trials of Rowcrops Integrated with Prairie Strips (STRIPS) program that demonstrates that replacing 10 percent of cropland with prairie grasses reduced sediment loss 20-fold while total nitrogen concentrations were 3.3 times lower (Figure 21.2).33 An example of a private–public response is the National Corn Growers Association’s Soil Health Partnership (SHP),77 a network of working farms across the Midwest engaged in refining techniques for growing cover crops, implementing conservation tillage, and using science-based nutrient management to reduce erosion and nutrient loss while increasing organic matter.
Acreage under irrigation has expanded modestly since 2002,78 mostly in the northern part of the Midwest where coarse soils of lower water-holding capacity are more vulnerable to drying under increased temperature. No strategies currently are available for maintaining historical trends in commodity agriculture production to cope with increases in spring rainfall and summer heat waves projected for mid-century.2,65