finding 14.2 : key-finding-14-2

Stabilizing global mean temperature to less than 3.6°F (2°C) above preindustrial levels requires substantial reductions in net global CO2 emissions prior to 2040 relative to present-day values and likely requires net emissions to become zero or possibly negative later in the century. After accounting for the temperature effects of non-CO2 species, cumulative global CO2 emissions must stay below about 800 GtC in order to provide a two-thirds likelihood of preventing 3.6°F (2°C) of warming. Given estimated cumulative emissions since 1870, no more than approximately 230 GtC may be emitted in the future to remain under this temperature threshold. Assuming global emissions are equal to or greater than those consistent with the RCP4.5 scenario, this cumulative carbon threshold would be exceeded in approximately two decades. (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: Fundamental scientific understanding of the climate system provides a framework for considering potential pathways for achieving a target of preventing 3.6°F (2°C) of warming. There are uncertainties about cumulative CO2 emissions compatible with this goal, in large part because of uncertainties about the role of non-CO2 species, but it appears, based on past emissions and future projections, that the cumulative carbon threshold for this goal could be reached or exceeded in about two decades.

Description of evidence base: Key Finding 2 is a case study, focused on a pathway associated with 3.6°F (2°C) of warming, based on the more general concepts described in the chapter. As such, the evidence for the relationship between cumulative CO2 emissions and global mean temperature responseb3bbc7b5-067e-4c23-8d9b-59faee21e58e a8b2d884-2bfb-4a5a-bfa4-a28bd4d68ca9 44137105-aa0e-4e33-afc5-0d6c84115504 also supports Key Finding 3.

Numerous studies have provided best estimates of cumulative CO2 compatible with 3.6°F (2°C) of warming above preindustrial levels, including a synthesis by the IPCC.b3bbc7b5-067e-4c23-8d9b-59faee21e58e Sanderson et al.bf91f878-c82a-409b-8d96-e21320366a69 provide further recent evidence to support the statement that net CO2 emissions would need to approach zero or become negative later in the century in order to avoid this level of warming. Rogelj et al. 201517451eb2-574e-4bc3-9196-c4ffe4ba23af and the IPCCb3bbc7b5-067e-4c23-8d9b-59faee21e58e demonstrate that the consideration of non-CO2 species has the effect of further constraining the amount of cumulative CO2 emissions compatible with 3.6°F (2°C) of warming.

Table 14.1 shows the IPCC estimates associated with different probabilities (66% [the one highlighted in Key Finding 2], 50%, and 33%) of cumulative CO2 emissions compatible with warming of 3.6°F (2°C) above preindustrial levels, and the cumulative CO2 emissions compatible with 2.7°F (1.5°C) are in turn linearly derived from those, based on the understanding that cumulative emissions scale linearly with global mean temperature response. The IPCC estimates take into account the additional radiative forcing effects—past and future—of non-CO2 species based on the emissions pathways consistent with the RCP scenarios (available here: https://tntcat.iiasa.ac.at/RcpDb/dsd?Action=htmlpage&page=about#descript).

The authors calculated the dates shown in Table 14.1, which supports the last statement in Key Finding 2, based on Le Quéré et al.e10cd7fc-e821-474a-bbde-74426a52b1d0 and the publicly available RCP database. Le Quéré et al.e10cd7fc-e821-474a-bbde-74426a52b1d0 provide the widely used reference for historical global, annual CO2 emissions from 1870 to 2015 (land-use change emissions were estimated up to year 2010 so are assumed to be constant between 2010 and 2015). Future CO2 emissions are based on the lower and higher scenarios (RCP4.5 and RCP8.5, respectively); annual numbers between model-projected years (2020, 2030, 2040, etc.) are linearly interpolated.

New information and remaining uncertainties: There are large uncertainties about the course of future CO2 and non-CO2 emissions, but the fundamental point that CO2 emissions need to eventually approach zero or possibly become net negative to stabilize warming below 3.6°F (2°C) holds regardless of future emissions scenario. There are also large uncertainties about the magnitude of past (since 1870 in this case) CO2 and non-CO2 emissions, which in turn influence the uncertainty about compatible cumulative emissions from the present day forward. Further uncertainties regarding non-CO2 species, including aerosols, include their radiative forcing effects. The uncertainty in achieving the temperature targets for a given emissions pathway is, in large part, reflected by the range of probabilities shown in Table 14.1.

Assessment of confidence based on evidence: There is very high confidence in the first statement of Key Finding 2 because it is based on a number of sources with a high level of agreement. The role of non-CO2 species in particular introduces uncertainty in the second statement of Key Finding 2 regarding compatible cumulative CO2 emissions that take into account past and future radiative forcing effects of non-CO2 species; though this estimate is based on a synthesis of numerous studies by the IPCC. The last statement of Key Finding 2 is straightforward based on the best available estimates of historical emissions in combination with the widely used future projections of the RCP scenarios.

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

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