There have been changes in some types of extreme weather events over the last several decades. Heat waves have become more frequent and intense, especially in the West. Cold waves have become less frequent and intense across the nation. There have been regional trends in floods and droughts. Droughts in the Southwest and heat waves everywhere are projected to become more intense, and cold waves less intense everywhere.

This finding is from chapter 2 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. Key message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b The Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection.

Description of evidence base: The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input. Analysis of U.S. temperature records indicates that record cold events are becoming progressively less frequent relative to record high events.62bcd3f7-a6f1-4b0b-85a8-29cbf7412579 0ebef171-4903-4aa6-b436-2936da69f84e There is evidence for the corresponding trends in a global framework.6eb4d004-0634-413c-bfda-a997348fdec7 714ae627-5acd-4f06-8d37-748adae46d4e A number of publications have explored the increasing trend of heat waves.6eb4d004-0634-413c-bfda-a997348fdec7 43d596ef-081b-4929-92fe-a1fb4e41b16f 3a244268-5014-4ad9-943a-c3361454c25b Additionally, heat waves observed in the southern Great Plains,e251f590-177e-4ba6-8ed1-6f68b5e54c8a Europe,6eb4d004-0634-413c-bfda-a997348fdec7 43d596ef-081b-4929-92fe-a1fb4e41b16f and Russia62bcd3f7-a6f1-4b0b-85a8-29cbf7412579 714ae627-5acd-4f06-8d37-748adae46d4e 7de011b1-2d12-48e0-b3e1-0d81e4a8a7d6 have now been shown to have a higher probability of having occurred because of human-induced climate change. Some parts of the U.S. have been seeing changing trends for floods and droughts over the last 50 years, with some evidence for human influence.a5d1165d-0287-4f99-ad7d-75d8dcd3b32e e15600d0-290f-44e2-9b58-9ffd295ee6d2 43d596ef-081b-4929-92fe-a1fb4e41b16f In the areas of increased flooding in parts of the Great Plains, Midwest, and Northeast, increases in both total precipitation and extreme precipitation have been observed and may be contributing to the flooding increases. However, when averaging over the entire contiguous U.S., there is no overall trend in flood magnitudes.a7f8dbf5-3ec8-4ee1-8740-014006b72bfd fcd12450-81ff-4322-8a50-09c0662512eb a639de52-b0d2-4580-a27c-5039d036d210 c25726e8-4e99-42fc-b35c-4d3164ff1aba A number of publications project drought as becoming a more normal condition over much of the southern and central U.S. (most recent references: Dai 2012; Hoerling et al. 2012; Wehner et al. 20110b3b2ff4-9ee7-45fe-8d0c-895076013715 09d97b0a-79d9-4803-a83d-b5982e4b3371 948ffa58-24f3-4129-90c3-8d49f3172f74). Analyses of U.S. daily temperature records indicate that low records are being broken at a much smaller rate than high records, and at the smallest rate in the historical record.62bcd3f7-a6f1-4b0b-85a8-29cbf7412579 0ebef171-4903-4aa6-b436-2936da69f84e However, in certain localized regions, natural variations can be as large or larger than the human induced change.

New information and remaining uncertainties: The key uncertainty regarding projections of future drought is how soil moisture responds to precipitation changes and potential evaporation increases. Most studies indicate that many parts of the U.S. will experience drier soil conditions but the amount of that drying is uncertain. Natural variability is also an uncertainty affecting projections of extreme event occurrences in shorter timescales (several years to decades), but the changes due to human influence become larger relative to natural variability as the timescale lengthens. Stakeholders should view the occurrence of extreme events in the context of increasing probabilities due to climate change. Continuation of long term temperature and precipitation observations is critical to monitoring trends in extreme weather events.

Assessment of confidence based on evidence: Given the evidence base and uncertainties, confidence is high for the entire key message. Heat waves have become more frequent and intense, and confidence is high that heat waves everywhere are projected to become more intense in the future. Confidence is high that cold waves have become less frequent and intense across the nation. Confidence is high that there have been regional trends in floods and droughts. Confidence is high that droughts in the Southwest are projected to become more intense.

References :

• Flood peak distributions for the eastern United States (a639de52)
• Has the Magnitude of Floods Across the USA Changed with Global CO2 Levels? (a7f8dbf5)
• Changes in precipitation with climate change (43d596ef)
• SAP 3.3. Weather and Climate Extremes in a Changing Climate (3a244268)
• Increasing prevalence of extreme summer temperatures in the U.S. (7de011b1)
• Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge (e15600d0)
• Global Climate Change Impacts in the United States (e251f590)
• Relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S. (62bcd3f7)
• Examining Flood Frequency Distributions in the Midwest U.S. (c25726e8)
• On the stationarity of annual flood peaks in the continental United States during the 20th century (fcd12450)
• Projections of Future Drought in the Continental United States and Mexico (948ffa58)
• The Role of Human Activity in the Recent Warming of Extremely Warm Daytime Temperatures (714ae627)
• Increasing drought under global warming in observations and models (0b3b2ff4)
• Detection and attribution of climate change: a regional perspective (6eb4d004)
• Regional Climate Trends and Scenarios for the U.S. National Climate Assessment: Part 9. Climate of the Contiguous United States. NOAA Technical Report NESDIS 142-9 (0ebef171)
• Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000 (a5d1165d)
• Is a Transition to Semipermanent Drought Conditions Imminent in the U.S. Great Plains? (09d97b0a)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)
• CMIP5 Climate Model Analyses: Climate Extremes in the United States (b91893b4)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)