finding 2.1 : key-message-2-1

Global climate is changing rapidly compared to the pace of natural variations in climate that have occurred throughout Earth’s history. Global average temperature has increased by about 1.8°F from 1901 to 2016, and observational evidence does not support any credible natural explanations for this amount of warming; instead, the evidence consistently points to human activities, especially emissions of greenhouse or heat-trapping gases, as the dominant cause. (Very High Confidence)

This finding is from chapter 2 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

This chapter is based on the collective effort of 32 authors, 3 review editors, and 18 contributing authors comprising the writing team for the Climate Science Special Report (CSSR),75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 a featured U.S. Global Change Research Project (USGCRP) deliverable and Volume I of the Fourth National Climate Assessment (NCA4). An open call for technical contributors took place in March 2016, and a federal science steering committee appointed the CSSR team. CSSR underwent three rounds of technical federal review, external peer review by the National Academies of Sciences, Engineering, and Medicine, and a review that was open to public comment. Three in-person Lead Authors Meetings were conducted at various stages of the development cycle to evaluate comments received, assign drafting responsibilities, and ensure cross-chapter coordination and consistency in capturing the state of climate science in the United States. In October 2016, an 11-member core writing team was tasked with capturing the most important CSSR key findings and generating an Executive Summary. The final draft of this summary and the underlying chapters was compiled in June 2017.

The NCA4 Chapter 2 author team was pulled exclusively from CSSR experts tasked with leading chapters and/or serving on the Executive Summary core writing team, thus representing a comprehensive cross-section of climate science disciplines and supplying the breadth necessary to synthesize CSSR content. NCA4 Chapter 2 authors are leading experts in climate science trends and projections, detection and attribution, temperature and precipitation change, severe weather and extreme events, sea level rise and ocean processes, mitigation, and risk analysis. The chapter was developed through technical discussions first promulgated by the literature assessments, prior efforts of USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 e-mail exchanges, and phone consultations conducted to craft this chapter and subsequent deliberations via phone and e-mail exchanges to hone content for the current application. The team placed particular emphasis on the state of science, what was covered in USGCRP,75cf1c0b-cc62-4ca4-96a7-082afdfe2ab1 and what is new since the release of the Third NCA in 2014.dd5b893d-4462-4bb3-9205-67b532919566

Description of evidence base:

The Key Message and supporting text summarize extensive evidence documented in the climate science literature and are similar to statements made in previous national (NCA3)dd5b893d-4462-4bb3-9205-67b532919566 and internationalf03117be-ccfe-4f88-b70a-ffd4351b8190 assessments. The human effects on climate have been well documented through many papers in the peer-reviewed scientific literature (e.g., see Fahey et al. 20170615b4ff-d185-4e14-9d4d-5bea1ce6ca51 and Knutson et al. 20170725eae6-7458-4ec2-8f66-880d88118148 for more discussion of supporting evidence).

The finding of an increasingly strong positive forcing over the industrial era is supported by observed increases in atmospheric temperatures (see Wuebbles et al. 2017666daffe-2c3b-4e2d-9157-16b989860618) and by observed increases in ocean temperatures.666daffe-2c3b-4e2d-9157-16b989860618,3bae2310-7572-47e2-99a4-9e4276764934,c66bf5a9-a6d7-4043-ad99-db0ae6ae562c The attribution of climate change to human activities is supported by climate models, which are able to reproduce observed temperature trends when radiative forcing from human activities is included and considerably deviate from observed trends when only natural forcings are included (Wuebbles et al. 2017; Knutson et al. 2017, Figure 3.1666daffe-2c3b-4e2d-9157-16b989860618,0725eae6-7458-4ec2-8f66-880d88118148).

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. The exact effects from land-use changes relative to the effects from greenhouse gas emissions need to be better understood.

The largest source of uncertainty in radiative forcing (both natural and anthropogenic) over the industrial era is quantifying forcing by aerosols. This finding is consistent across previous assessments (e.g., IPCC 2007, IPCC 2013f03117be-ccfe-4f88-b70a-ffd4351b8190,c54b9473-cdc3-4f22-97a8-4df5253f9682).

Recent work has highlighted the potentially larger role of variations in ultraviolet solar irradiance, versus total solar irradiance, in solar forcing. However, this increase in solar forcing uncertainty is not sufficiently large to reduce confidence that anthropogenic activities dominate industrial-era forcing.

Assessment of confidence based on evidence:

There is very high confidence for a major human influence on climate.

Assessments of the natural forcings of solar irradiance changes and volcanic activity show with very high confidence that both forcings are small over the industrial era relative to total anthropogenic forcing. Total anthropogenic forcing is assessed to have become larger and more positive during the industrial era, while natural forcings show no similar trend.

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