finding 18.1 : decreased-agricultural-productivity

In the next few decades, longer growing seasons and rising carbon dioxide levels will increase yields of some crops, though those benefits will be progressively offset by extreme weather events. Though adaptation options can reduce some of the detrimental effects, in the long term, the combined stresses associated with climate change are expected to decrease agricultural productivity.



This finding is from chapter 18 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: The assessment process for the Midwest Region began with a workshop was that was held July 25, 2011, in Ann Arbor, Michigan. Ten participants discussed the scope and authors for a foundational Technical Input Report (TIR) report entitled “Midwest Technical Input Report.”2626b5ca-ec04-4e41-8405-9f582c779a7a The report, which consisted of nearly 240 pages of text organized into 13 chapters, was assembled by 23 authors representing governmental agencies, non-governmental organizations (NGOs), tribes, and other entities. The Chapter Author Team engaged in multiple technical discussions via teleconferences that permitted a careful review of the foundational TIR2626b5ca-ec04-4e41-8405-9f582c779a7a and of approximately 45 additional technical inputs provided by the public, as well as the other published literature, and professional judgment. The Chapter Author Team convened teleconferences and exchanged extensive emails to define the scope of the chapter for their expert deliberation of input materials and to generate the chapter text and figures. Each expert drafted key messages, initial text and figure drafts and traceable accounts that pertained to their individual fields of expertise. These materials were then extensively discussed by the Author Team and were approved by the Chapter Team members.

Description of evidence base: The key message and supporting text summarize extensive evidence documented in the Technical Input Report.2626b5ca-ec04-4e41-8405-9f582c779a7a Technical input reports on a wide range of topics were also received and reviewed as part of the Federal Register Notice solicitation for public input. Evidence for altered growing seasons across the U.S. are discussed in Chapter 2 (Our Changing Climate, Key Message 4) and its Traceable Accounts. “Climate Trends and Scenarios for the U.S. National Climate Assessment”95f2ea7d-12e3-4ed5-9247-7cf139db91a9 and its references provide specific details for the Midwest. Evidence for longer growing seasons in the Midwest is based on regional temperature records and is incontrovertible, as is evidence for increasing carbon dioxide concentrations. U.S. Department of Agriculture data tables provide evidence for the importance of the eight Midwest states for U.S. agricultural production.e215c640-ec14-4909-8bb5-380267560f3e Evidence for the effect of future elevated carbon dioxide concentrations on crop yields is based on scores of greenhouse and field experiments that show a strong fertilization response for C3 plants such as soybeans and wheat and a positive but not as strong a response for C4 plants such as corn. Observational data, evidence from field experiments, and quantitative modeling are the evidence base of the negative effects of extreme weather events on crop yield: early spring heat waves followed by normal frost events have been shown to decimate Midwest fruit crops; heat waves during flowering, pollination, and grain filling have been shown to significantly reduce corn and wheat yields; more variable and intense spring rainfall has delayed spring planting in some years and can be expected to increase erosion and runoff; and floods have led to crop losses.a2704ef3-5be4-41ee-8dfa-4c82e416a292 713ca06b-894a-44a1-9e02-448faa3ec810 e08bf6f4-f64e-4f0d-9655-a69d0f5f0470 fd4ce342-e150-4493-933e-4b4e1dc14ed0

New information and remaining uncertainties: Key issues (uncertainties) are: a) the rate at which grain yield improvements will continue to occur, which could help to offset the overall negative effect of extreme events at least for grain crops (though not for individual farmers); and b) the degree to which genetic improvements could make some future crops more tolerant of extreme events such as drought and heat stress. Additional uncertainties are: c) the degree to which accelerated soil carbon loss will occur as a result of warmer winters and the resulting effects on soil fertility and soil water availability; and d) the potential for increased pest and disease pressure as southern pests such as soybean rust move northward and existing pests better survive milder Midwest winters.

Assessment of confidence based on evidence: Because nearly all studies published to date in the peer-reviewed literature agree that Midwest crops benefit from CO2 fertilization and some benefit from a longer growing season, there is very high confidence in this component of the key message. Studies also agree that full benefits of climate change will be offset partly or fully by more frequent heat waves, early spring thaws followed by freezing temperatures, more variable and intense rainfall events, and floods. Again, there is very high confidence in this aspect. There is less certainty (high) about pest effects and about the potential for adaptation actions to significantly mitigate the risk of crop loss.

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