Global climate is changing and this change is apparent across a wide range of observations. The global warming of the past 50 years is primarily due to human activities.

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 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 changes in global climate arises from multiple analyses of data from in-situ, satellite, and other records undertaken by many groups over several decades.25578196-95d0-4ac7-b889-0e863985423d Changes in the mean state have been accompanied by changes in the frequency and nature of extreme events.bc4e302f-0956-4bb0-b345-e84dfb03223f A substantial body of analysis comparing the observed changes to a broad range of climate simulations consistently points to the necessity of invoking human-caused changes to adequately explain the observed climate system behavior.926c4c11-d896-488e-8c7c-3930fe978424 6eb4d004-0634-413c-bfda-a997348fdec7 The influence of human impacts on the climate system has also been observed in a number of individual climate variables.f813f663-0919-4050-a458-47fdfcbe2f5f ab75a74f-274f-4996-99b4-d009caa68641 a5d1165d-0287-4f99-ad7d-75d8dcd3b32e bee16192-914f-4561-98e6-6cc3e33e2e41 6e726151-1805-4e14-9210-0b81a99844ac 4693d80b-2e2f-4a8c-8fca-bfe353270894 aaab80c9-5bef-4072-ab5a-2cea29a7a0a9 A discussion of the slowdown in temperature increase with associated references (for example, Balmaseda et al. 2013; Easterling and Wehner 2009be8d377c-8b89-4839-8a00-0f8240450472 119a732c-f113-4c1b-bc77-e584d52f5505) is included in the chapter. The Climate Science Supplement Appendix provides further discussion of types of emissions or heat-trapping gases and particles, and future projections of human-related emissions. Supplemental message 4 of the Appendix provides further details on attribution of observed climate changes to human influence.

New information and remaining uncertainties: Key remaining uncertainties relate to the precise magnitude and nature of changes at global, and particularly regional, scales, and especially for extreme events and our ability to simulate and attribute such changes using climate models. Innovative new approaches to climate data analysis, continued improvements in climate modeling, and instigation and maintenance of reference quality observation networks such as the U.S. Climate Reference Network (http://www.ncdc.noaa.gov/crn/) all have the potential to reduce uncertainties.

Assessment of confidence based on evidence: There is very high confidence that global climate is changing and this change is apparent across a wide range of observations, given the evidence base and remaining uncertainties. All observational evidence is consistent with a warming climate since the late 1800s. There is very high confidence that the global climate change of the past 50 years is primarily due to human activities, given the evidence base and remaining uncertainties. Recent changes have been consistently attributed in large part to human factors across a very broad range of climate system characteristics.

References :

• Attribution of observed surface humidity changes to human influence (6e726151)
• Identifying human influences on atmospheric temperature (f813f663)
• Improved constraints on 21st-century warming derived using 160 years of temperature observations (926c4c11)
• Identification of human-induced changes in atmospheric moisture content (bee16192)
• Global observed changes in daily climate extremes of temperature and precipitation (bc4e302f)
• State of the Climate in 2009 (25578196)
• Variability of ocean heat uptake: Reconciling observations and models (aaab80c9)
• Attribution of anthropogenic influence on seasonal sea level pressure (4693d80b)
• Human contribution to more-intense precipitation extremes (ab75a74f)
• Is the climate warming or cooling? (be8d377c)
• Distinctive climate signals in reanalysis of global ocean heat content (119a732c)
• Detection and attribution of climate change: a regional perspective (6eb4d004)
• Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000 (a5d1165d)
• 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)