finding 1.3 : key-finding-1-3

Many lines of evidence demonstrate that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. Formal detection and attribution studies for the period 1951 to 2010 find that the observed global mean surface temperature warming lies in the middle of the range of likely human contributions to warming over that same period. We find no convincing evidence that natural variability can account for the amount of global warming observed over the industrial era. For the period extending over the last century, there are no convincing alternative explanations supported by the extent of the observational evidence. Solar output changes and internal variability can only contribute marginally to the observed changes in climate over the last century, and we find no convincing evidence for natural cycles in the observational record that could explain the observed changes in climate. (Very high confidence)



This finding is from chapter 1 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. The analyses described in the NCA3 and IPCC assessments support our findings, and new observations and modeling studies have further substantiated these conclusions.

Description of evidence base: The Key Finding and supporting text summarizes 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 Ch. 2: Physical Drivers of Climate Change and Ch. 3: Detection and Attribution for more discussion of supporting evidence).

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.

Assessment of confidence based on evidence: There is very high confidence for a major human influence on climate.

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