finding 14.1 : key-finding-14-1

Reducing net emissions of CO2 is necessary to limit near-term climate change and long-term warming. Other greenhouse gases (for example, methane) and black carbon aerosols exert stronger warming effects than CO2 on a per ton basis, but they do not persist as long in the atmosphere; therefore, mitigation of non-CO2 species contributes substantially to near-term cooling benefits but cannot be relied upon for ultimate stabilization goals. (Very high confidence)

This finding is from chapter 14 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The qualitative statements contained in Key Finding 1 reflect aspects of fundamental scientific understanding, well grounded in the literature, that provide a relevant framework for considering the role of CO2 and non-CO2 species in mitigating climate change.

Description of evidence base: Joos et al.16610639-a18d-4e02-b55e-b82a41005e9e and Ciais et al. (see Box 6.1 in particular)7316c70f-6b67-4a68-a23a-c9d9f604c003 describe the climate response of CO2 pulse emissions, and Solomon et al.,640b9ae9-3351-48d1-b897-557db63a6cfd NRC,a8b2d884-2bfb-4a5a-bfa4-a28bd4d68ca9 and Collins et al.b3bbc7b5-067e-4c23-8d9b-59faee21e58e describe the long-term warming and other climate effects associated with CO2 emissions. Paltsev et al.8bfa125a-bd59-4ae1-9aa0-a0b0cf5d8cbb and Collins et al.b3bbc7b5-067e-4c23-8d9b-59faee21e58e describe the near-term vs. long-term nature of climate outcomes resulting from GHG mitigation. Myhre et al.6c7c285c-8606-41fe-bf93-100d80f1d17a synthesize numerous studies detailing information about the radiative forcing effects and atmospheric lifetimes of all GHGs and aerosols (see in particular Appendix 8A therein). A recent body of literature has emerged highlighting the particular role that non-CO2 mitigation can play in providing near-term cooling benefits (e.g., Shindell et al. 2012;a65e5260-d143-49dd-b20e-c0fefddbef70 Zaelke and Borgford-Parnell 2015;6247384e-71f2-4d68-870e-e382cec05137 Rogelj et al. 201517451eb2-574e-4bc3-9196-c4ffe4ba23af). For each of the individual statements made in Key Finding 1, there are numerous literature sources that provide consistent grounds on which to make these statements with very high confidence.

New information and remaining uncertainties: The Key Finding is comprised of qualitative statements that are traceable to the literature described above and in this chapter. Uncertainties affecting estimates of the exact timing and magnitude of the climate response following emissions (or avoidance of those emissions) of CO2 and other GHGs involve the quantity of emissions, climate sensitivity, some uncertainty about the removal time or atmospheric lifetime of CO2 and other GHGs, and the choice of model carrying out future simulations. The role of black carbon in climate change is more uncertain compared to the role of the well-mixed GHGs (see Bond et al. 2013c024a923-aedd-4e72-8555-3a37ccc41e14).

Assessment of confidence based on evidence: Key Finding 1 is comprised of qualitative statements based on a body of literature for which there is a high level of agreement. There is a well-established understanding, based in the literature, of the atmospheric lifetime and warming effects of CO2 vs. other GHGs after emission, and in turn how atmospheric concentration levels respond following the emission of CO2 and other GHGs.

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

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