finding 4.3 : key-finding-4-3

Beyond the next few decades, the magnitude of climate change depends primarily on cumulative emissions of greenhouse gases and aerosols and the sensitivity of the climate system to those emissions (high confidence). Projected changes range from 4.7°–8.6°F (2.6°–4.8°C) under the higher scenario (RCP8.5) to 0.5°–1.3°F (0.3°–1.7°C) under the much lower scenario (RCP2.6), for 2081–2100 relative to 1986–2005 (medium confidence).

This finding is from chapter 4 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The estimated warming presented in this key finding is based on calculations reported by Collins et al.b3bbc7b5-067e-4c23-8d9b-59faee21e58e The key finding that human emissions and climate sensitivity are the most important sources of uncertainty over the long-term is based on both basic physics regarding the radiative properties of greenhouse gases, as well as a large body of peer reviewed publications.

Description of evidence base: The estimate of projected long-term warming under continued emissions of carbon dioxide and other greenhouse gases and aerosols under the RCP scenarios was obtained directly from IPCC AR5 WG1.b3bbc7b5-067e-4c23-8d9b-59faee21e58e

All credible climate models assessed in Chapter 9 of the IPCC WG1 AR5a46eaad1-5c17-46f7-bba6-d3fee718a092 from the simplest to the most complex respond with elevated global mean temperature, the simplest indicator of climate change, when atmospheric concentrations of greenhouse gases increase. It follows then that an emissions pathway that tracks or exceeds the higher scenario (RCP8.5) would lead to larger amounts of climate change.

The statement regarding the sources of uncertainty in long-term projections is based on Hawkins and Sutton.c247acf0-2a87-4337-b6d7-77686d049eed cbb67d95-422d-4ffc-9fbd-dec79795f437

New information and remaining uncertainties: As stated in the key finding, the magnitude of climate change over the long term is uncertain due to human emissions of greenhouse gases and climate sensitivity.

Assessment of confidence based on evidence: The first statement regarding additional warming and its dependence on human emissions and climate sensitivity has high confidence, as understanding of the radiative properties of greenhouse gases and the existence of both positive and negative feedbacks in the climate system is basic physics, dating to the 19th century. The second has medium confidence in the specific magnitude of warming, due to the uncertainties described in the key finding.

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