U.S. average temperature has increased by 1.3°F to 1.9°F since record keeping began in 1895; most of this increase has occurred since about 1970. The most recent decade was the nation’s warmest on record. Temperatures in the United States are expected to continue to rise. Because human-induced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.

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. Evidence for the long-term increase in temperature is based on analysis of daily maximum and minimum temperature observations from the U.S. Cooperative Observer Network (http://www.nws.noaa.gov/om/coop/). With the increasing understanding of U.S. temperature measurements, a temperature increase has been observed, and temperature is projected to continue rising.42b449ef-418d-4f0c-8e9c-9f5c16fc2367 c0160553-45a8-4e6f-be7f-711a32c8d0f8 fe074063-3e08-4a10-b184-a3f946b9715c 9a18ff0d-ad2f-4176-856c-d54aea0a92e1 32bec5d2-97fe-41c5-8eed-6920bbf096f4 8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c 66ccff5f-4828-4e03-be08-ee6f49296f34 Observations show that the last decade was the warmest in over a century. A number of climate model simulations were performed to assess past, and to forecast future, changes in climate; temperatures are generally projected to increase across the United States. The section entitled “Quantifying U.S. Temperature Rise” explains the rational for using the range 1.3°F to 1.9°F in the key message. All peer-reviewed studies to date satisfying the assessment process agree that the U.S. has warmed over the past century and in the past several decades. Climate model simulations consistently project future warming and bracket the range of plausible increases.

New information and remaining uncertainties: Since the previous National Climate Assessment,e251f590-177e-4ba6-8ed1-6f68b5e54c8a there have been substantial advances in our understanding of the U.S. temperature record (Climate Science Appendix, Supplemental Message 7).42b449ef-418d-4f0c-8e9c-9f5c16fc2367 c0160553-45a8-4e6f-be7f-711a32c8d0f8 fe074063-3e08-4a10-b184-a3f946b9715c 9a18ff0d-ad2f-4176-856c-d54aea0a92e1 32bec5d2-97fe-41c5-8eed-6920bbf096f4 8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c 66ccff5f-4828-4e03-be08-ee6f49296f34 A potential uncertainty is the sensitivity of temperature trends to adjustments that account for historical changes in station location, temperature instrumentation, observing practice, and siting conditions. However, quality analyses of these uncertainties have not found any major issues of concern affecting the conclusions made in the key message (Appendix 3: Climate Science, Supplemental Message 7). (for example, Williams et al. 201266ccff5f-4828-4e03-be08-ee6f49296f34). While numerous studies (for example, c0160553-45a8-4e6f-be7f-711a32c8d0f8 8243ec9e-5b70-4c53-a6bd-a8f41adb2d9c 66ccff5f-4828-4e03-be08-ee6f49296f34) verify the efficacy of the adjustments, the information base can be improved in the future through continued refinements to the adjustment approach. Model biases are subject to changes in physical effects on climate; for example, model biases can be affected by snow cover and hence are subject to change as a warming climate changes snow cover.

Assessment of confidence based on evidence: Given the evidence base and remaining uncertainties, confidence is very high in the key message. Because human-induced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.

References :

• Benchmarking the performance of pairwise homogenization of surface temperatures in the United States (66ccff5f)
• Impacts of land use land cover on temperature trends over the continental United States: assessment using the North American Regional Reanalysis (42b449ef)
• Homogenization of Temperature Series via Pairwise Comparisons (32bec5d2)
• Global Climate Change Impacts in the United States (e251f590)
• On the reliability of the U.S. surface temperature record (fe074063)
• An intercomparison of temperature trends in the U.S. Historical Climatology Network and recent atmospheric reanalyses (8243ec9e)
• The U.S. Historical Climatology Network Monthly Temperature Data, Version 2 (9a18ff0d)
• Analysis of the impacts of station exposure on the U.S. Historical Climatology Network temperatures and temperature trends (c0160553)
• CMIP5 Climate Model Analyses: Climate Extremes in the United States (b91893b4)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)
• Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge (e15600d0)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)