Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.5.2.3 Remaining Carbon Budget ====

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The combination of the five components assessed in Sections 5.5.2.2.1–5.5.2.2.5 allows for an overall assessment of the remaining carbon budget in line with different levels of global average warming, as documented in SR1.5 ([[#Rogelj--2018b|Rogelj et al., 2018b]]). The overall assessment of remaining carbon budgets (Table 5.8) reflects the uncertainty in TCRE quantification and provides estimates of the uncertainties surrounding the contributions of each of the respective further components. A formal combination of all uncertainties is not possible because they are not all independent, or because they represent choices rather than probabilistic uncertainties ([[#Matthews--2021|Matthews et al., 2021]]). In light of all uncertainties related to TCRE, non-CO <sub>2</sub> forcing and response, the level of non-CO <sub>2</sub> mitigation, and historical warming, there is a small probability that the remaining carbon budget for limiting warming to 1.5°C since the 1850–1900 period is effectively zero. However, applying best estimate values for all but uncertainties in Earth system feedbacks and TCRE, the remaining carbon budgets in line with the Paris Agreement are generally small yet not zero (see Table 5.8).

There is ''robust evidence'' supporting the concept of TCRE as well as ''high confidence'' in the range of historical human-induced warming. Combined with the assessed uncertainties in the Earth system’s response to non-CO <sub>2</sub> emissions and less well-established quantification of some of the effect of non-linear Earth system feedbacks, this leads to ''medium confidence'' being assigned to the assessed remaining carbon budget estimates while noting the identified and assessed uncertainties and potential variations. The reported values are applicable to warming and cumulative emissions over the 21st century. For climate stabilization beyond the 21st century, this confidence would decline to ''very low confidence'' due to uncertainties in Earth system feedbacks and the ZEC. For estimates of total carbon budgets in line with limiting global warming to a specific level, an estimate of historical CO <sub>2</sub> emissions should be added to the remaining carbon budget values reported in Table 5.8. Historical CO <sub>2</sub> emissions between 1850 and 2019 have been estimated at about 655 ± 65 PgC (1-sigma range, or 2390 ± 240 GtCO <sub>2</sub> , see Table 5.1), while since 1 January 2015, an additional 57 PgC (210 GtCO <sub>2</sub>) has been emitted until the end of 2019 ([[#Friedlingstein--2020|Friedlingstein et al., 2020]]).

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