The rising incidence of weather extremes will have increasingly negative impacts on crop and livestock productivity because critical thresholds are already being exceeded.

This finding is from chapter 6 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: A central component of the process was the development of a foundational technical input report (TIR), “Climate Change and Agriculture in the United States: An Assessment of Effects and Potential for Adaptation”.3baf471f-751f-4d68-9227-4197fdbb6e5d A public session conducted as part of the Tri-Societies (https://www.acsmeetings.org/home) meeting held in San Antonio, Texas, on Oct. 16-19, 2011, provided input to this report. The report team engaged in multiple technical discussions via teleconference, which included careful review of the foundational TIR3baf471f-751f-4d68-9227-4197fdbb6e5d and of approximately 56 additional technical inputs provided by the public, as well as other published literature and professional judgment. Discussions were followed by expert deliberation of draft key messages by the authors and targeted consultation with additional experts by the lead author of each message.

Description of evidence base: The key message and supporting text summarizes extensive evidence documented in the Agriculture TIR, “Climate Change and Agriculture in the United States: An Assessment of Effects and Potential for Adaptation”.3baf471f-751f-4d68-9227-4197fdbb6e5d Additional Technical Input Reports (56) on a wide range of topics were also received and reviewed as part of the Federal Register Notice solicitation for public input. Numerous peer-reviewed publicationsc54b9473-cdc3-4f22-97a8-4df5253f9682 bc4e302f-0956-4bb0-b345-e84dfb03223f 9f976ed6-c1dd-437d-aaf9-062bca25bbc9 e3415970-4962-4b46-93e3-6918000d0596 provide evidence that the occurrence of extreme events is increasing, and exposure of plants or animals to temperatures and soil water conditions (drought, water-logging, flood) outside of the biological range for the given species will cause stress and reduce production.c54b9473-cdc3-4f22-97a8-4df5253f9682 bc4e302f-0956-4bb0-b345-e84dfb03223f 9f976ed6-c1dd-437d-aaf9-062bca25bbc9 e3415970-4962-4b46-93e3-6918000d0596 The direct effects of an extreme event will depend upon the timing of the event relative to the growth stage of the biological system.

New information and remaining uncertainties: Important new evidence (cited above) confirmed many of the findings in the past Synthesis and Assessment Product on agriculture 76db17ce-354b-4f0c-ad10-3e701c0387fc, which informed the 2009 National Climate Assessment.e251f590-177e-4ba6-8ed1-6f68b5e54c8a One key area of uncertainty is the timing of extreme events during the phenological stage of the plant or the growth stage of the animal. For example, plants are more sensitive to extreme high temperatures during the pollination stage compared to vegetative growth stages.a2704ef3-5be4-41ee-8dfa-4c82e416a292 A parallel example for animals is relatively strong sensitivity to high temperatures during the conception phase.f2d47bcb-0b5f-468c-b574-a9f155403ed7 Milk and egg production are also vulnerable to temperature extremes. The effects of extreme combinations of weather variables must be considered, such as elevated humidity in concert with high temperatures.f2d47bcb-0b5f-468c-b574-a9f155403ed7 Other key uncertainties include inadequate precision in simulations of the timing of extreme events relative to short time periods of crop vulnerability, and temperatures close to key thresholds such as freezing.41452674-2b8f-472c-8388-02c670541943 The uncertainty is amplified by the rarity of extreme events; this rarity means there are infrequent opportunities to study the impact of extreme events. In general, a shift of the distribution of temperatures can increase the frequency of threshold exceedance.3baf471f-751f-4d68-9227-4197fdbb6e5d The information base can be enhanced by improving the forecast of extreme events, given that the effect of extreme events on plants or animals is known.70e31ee1-6e73-45f1-b993-cf25d371651a bc4e302f-0956-4bb0-b345-e84dfb03223f

Assessment of confidence based on evidence: There is high confidence in the effects of extreme temperature events on crops and livestock, and the agreement in the literature is good.

References :

• Projected change in climate thresholds in the Northeastern U.S.: implications for crops, pests, livestock, and farmers (41452674)
• Effects of global climate change on world agriculture: an interpretive review (70e31ee1)
• SAP 4.3. The Effects of Climate Change on Agriculture, Land Resources, Water Resources, and Biodiversity. (76db17ce)
• Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (c54b9473)
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