finding 15.1 : key-finding-15-1

Positive feedbacks (self-reinforcing cycles) within the climate system have the potential to accelerate human-induced climate change and even shift the Earth’s climate system, in part or in whole, into new states that are very different from those experienced in the recent past (for example, ones with greatly diminished ice sheets or different large-scale patterns of atmosphere or ocean circulation). Some feedbacks and potential state shifts can be modeled and quantified; others can be modeled or identified but not quantified; and some are probably still unknown. (Very high confidence in the potential for state shifts and in the incompleteness of knowledge about feedbacks and potential state shifts).

This finding is from chapter 15 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The key finding is based on NRC3dcd5a73-de83-4b37-884a-5236407c170e and IPCC AR5 WG1 Chapter 12 section 12.5.5,b3bbc7b5-067e-4c23-8d9b-59faee21e58e which made a thorough assessment of the relevant literature.

Description of evidence base: This key finding is based on a large body of scientific literature recently summarized by Lenton et al.,d64a3dbf-d45e-49de-98b9-b4ea32da888f NRC,3dcd5a73-de83-4b37-884a-5236407c170e and Kopp et al.08bc6610-586b-421c-a788-f5e18781ac52 As NRC3dcd5a73-de83-4b37-884a-5236407c170e (page vii) states, “A study of Earth’s climate history suggests the inevitability of ‘tipping points’—thresholds beyond which major and rapid changes occur when crossed—that lead to abrupt changes in the climate system” and (page xi), “Can all tipping points be foreseen? Probably not. Some will have no precursors, or may be triggered by naturally occurring variability in the climate system. Some will be difficult to detect, clearly visible only after they have been crossed and an abrupt change becomes inevitable.” As IPCC AR5 WG1 Chapter 12, section 12.5.5b3bbc7b5-067e-4c23-8d9b-59faee21e58e further states, “A number of components or phenomena within the Earth system have been proposed as potentially possessing critical thresholds (sometimes referred to as tipping points) beyond which abrupt or nonlinear transitions to a different state ensues.” Collins et al.b3bbc7b5-067e-4c23-8d9b-59faee21e58e further summarizes critical thresholds that can be modeled and others that can only be identified.

New information and remaining uncertainties: The largest uncertainties are 1) whether proposed tipping elements actually undergo critical transitions; 2) the magnitude and timing of forcing that will be required to initiate critical transitions in tipping elements; 3) the speed of the transition once it has been triggered; 4) the characteristics of the new state that results from such transition; and 5) the potential for new tipping elements to exist that are yet unknown.

Assessment of confidence based on evidence: There is very high confidence in the likelihood of the existence of positive feedbacks, and the tipping elements statement is based on a large body of literature published over the last 25 years that draws from basic physics, observations, paleoclimate data, and modeling.

There is very high confidence that some feedbacks can be quantified, others are known but cannot be quantified, and others may yet exist that are currently unknown.

This finding was derived from figure -.2: Confidence / Likelihood

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