Editing IPCC:AR6/SR15/Chapter-2 (section)

==== 2.2.2.2 CO <sub>2</sub> and non-CO <sub>2</sub> contributions to the remaining carbon budget ====

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A remaining carbon budget can be estimated from calculating the amount of CO <sub>2</sub> emissions consistent (given a certain value of TCRE) with an allowable additional amount of warming. Here, the allowable warming is the 1.5°C warming threshold minus the current warming taken as the 2006–2015 average, with a further amount removed to account for the estimated non-CO <sub>2</sub> temperature contribution to the remaining warming (Peters, 2016; Rogelj et al., 2016b) <sup>[[#fn:r98|98]]</sup> . This assessment uses the TCRE range from AR5 WGI (Collins et al., 2013) <sup>[[#fn:r99|99]]</sup> supported by estimates of non-CO <sub>2</sub> contributions that are based on published methods and integrated pathways (Friedlingstein et al., 2014a; Allen et al., 2016, 2018; Peters, 2016; Smith et al., 2018) <sup>[[#fn:r100|100]]</sup> . Table 2.2 and Figure 2.3 show the assessed remaining carbon budgets and key uncertainties for a set of additional warming levels relative to the 2006–2015 period (see Supplementary Material 2.SM.1.1.2 for details). With an assessed historical warming of 0.87°C ± 0.12°C from 1850–1900 to 2006–2015 (Chapter 1, Section 1.2.1), 0.63°C of additional warming would be approximately consistent with a global mean temperature increase of 1.5°C relative to pre-industrial levels. For this level of additional warming, remaining carbon budgets have been estimated (Table 2.2, Supplementary Material 2.SM.1.1.2).

The remaining carbon budget calculation presented in the Table 2.2 and illustrated in Figure 2.3 does not consider additional Earth system feedbacks such as permafrost thawing. These are uncertain but estimated to reduce the remaining carbon budget by an order of magnitude of about 100 GtCO <sub>2</sub> and more thereafter. Accounting for such feedbacks would make the carbon budget more applicable for 2100 temperature targets, but would also increase uncertainty (Table 2.2 and see below). Excluding such feedbacks, the assessed range for the remaining carbon budget is estimated to be 840, 580, and 420 GtCO <sub>2</sub> for the 33rd, 50th and, 67th percentile of TCRE, respectively, with a median non-CO <sub>2</sub> warming contribution and starting from 1 January 2018 onward. Consistent with the approach used in the IPCC Fifth Assessment Report (IPCC, 2013b) <sup>[[#fn:r101|101]]</sup> , the latter estimates use global near-surface air temperatures both over the ocean and over land to estimate global surface temperature change since pre-industrial. The global warming from the pre-industrial period until the 2006–2015 reference period is estimated to amount to 0.97°C with an uncertainty range of about ±0.1°C (see Chapter 1, Section 1.2.1). Three methodological improvements lead to these estimates of the remaining carbon budget being about 300 GtCO <sub>2</sub> larger than those reported in Table 2.2 of the IPCC AR5 SYR (IPCC, 2014a) <sup>[[#fn:r102|102]]</sup> ( ''medium confidence'' ). The AR5 used 15 Earth System Models (ESM) and 5 Earth-system Models of Intermediate Complexity (EMIC) to derive an estimate of the remaining carbon budget. Their approach hence made implicit assumptions about the level of warming to date, the future contribution of non-CO <sub>2</sub> emissions, and the temperature response to CO <sub>2</sub> (TCRE). In this report, each of these aspects are considered explicitly. When estimating global warming until the 2006–2015 reference period as a blend of near-surface air temperature over land and sea-ice regions, and sea-surface temperature over open ocean, by averaging the four global mean surface temperature time series listed in Chapter 1 Section 1.2.1, the global warming would amount to 0.87°C ±0.1°C. Using the latter estimate of historical warming and projecting global warming using global near-surface air temperatures from model projections leads to remaining carbon budgets for limiting global warming to 1.5°C of 1080, 770, and 570 GtCO <sub>2</sub> for the 33rd, 50th, and 67th percentile of TCRE, respectively. Note that future research and ongoing observations over the next years will provide a better indication as to how the 2006–2015 base period compares with the long-term trends and might affect the budget estimates. Similarly, improved understanding in Earth system feedbacks would result in a better quantification of their impacts on remaining carbon budgets for 1.5°C and 2°C.

After TCRE uncertainty, a major additional source of uncertainty is the magnitude of non-CO <sub>2</sub> forcing and its contribution to the temperature change between the present day and the time of peak warming. Integrated emissions pathways can be used to ensure consistency between CO <sub>2</sub> and non-CO <sub>2</sub> emissions (Bowerman et al., 2013; Collins et al., 2013; Clarke et al., 2014; Rogelj et al., 2014b, 2015a; Tokarska et al., 2018) <sup>[[#fn:r103|103]]</sup> . Friedlingstein et al. (2014a) <sup>[[#fn:r104|104]]</sup> used pathways with limited to no climate mitigation to find a variation due to non-CO <sub>2</sub> contributions of about ±33% for a 2°C carbon budget. Rogelj et al. (2016b) <sup>[[#fn:r105|105]]</sup> showed no particular bias in non-CO <sub>2</sub> radiative forcing or warming at the time of exceedance of 2°C or at peak warming between scenarios with increasing emissions and strongly mitigated scenarios (consistent with Stocker et al., 2013) <sup>[[#fn:r106|106]]</sup> . However, clear differences of the non-CO <sub>2</sub> warming contribution at the time of deriving a 2°C-consistent carbon budget were reported for the four RCPs. Although the spread in non-CO <sub>2</sub> forcing across scenarios can be smaller in absolute terms at lower levels of cumulative emissions, it can be larger in relative terms compared to the remaining carbon budget (Stocker et al., 2013; Friedlingstein et al., 2014a; Rogelj et al., 2016b) <sup>[[#fn:r107|107]]</sup> . Tokarska and Gillett (2018) <sup>[[#fn:r108|108]]</sup> find no statistically significant differences in 1.5°C-consistent cumulative emissions budgets when calculated for different RCPs from consistent sets of CMIP5 simulations.

The mitigation pathways assessed in this report indicate that emissions of non-CO <sub>2</sub> forcers contribute an average additional warming of around 0.15°C relative to 2006–2015 at the time of net zero CO <sub>2</sub> emissions, reducing the remaining carbon budget by roughly 320 GtCO <sub>2</sub> . This arises from a weakening of aerosol cooling and continued emissions of non-CO <sub>2</sub> GHGs (Sections 2.2.1, 2.3.3). This non-CO <sub>2</sub> contribution at the time of net zero CO <sub>2</sub> emissions varies by about ±0.1°C across scenarios, resulting in a carbon budget uncertainty of about ±250 GtCO <sub>2</sub> , and takes into account marked reductions in methane emissions (Section 2.3.3). If these reductions are not achieved, remaining carbon budgets are further reduced. Uncertainties in the non-CO <sub>2</sub> forcing and temperature response are asymmetric and can influence the remaining carbon budget by −400 to +200 GtCO <sub>2</sub> , with the uncertainty in aerosol radiative forcing being the largest contributing factor (Table 2.2). The MAGICC and FAIR models in their respective parameter setups and model versions used to assess the non-CO <sub>2</sub> warming contribution give noticeable different non-CO <sub>2</sub> effective radiative forcing and warming for the same scenarios while both being within plausible ranges of future response (Figure 2.2 and Supplementary Material 2.SM.1.1, 2.SM.1.2). For this assessment, it is premature to assess the accuracy of their results, so it is assumed that both are equally representative of possible futures. Their non-CO <sub>2</sub> warming estimates are therefore averaged for the carbon budget assessment and their differences used to guide the uncertainty assessment of the role of non-CO <sub>2</sub> forcers. Nevertheless, the findings are robust enough to give ''high confidence'' that the changing emissions of non-CO <sub>2</sub> forcers (particularly the reduction in cooling aerosol precursors) cause additional near-term warming and reduce the remaining carbon budget compared to the CO <sub>2</sub> -only budget.

TCRE uncertainty directly impacts carbon budget estimates (Peters, 2016; Matthews et al., 2017; Millar and Friedlingstein, 2018) <sup>[[#fn:r109|109]]</sup> . Based on multiple lines of evidence, AR5 WGI assessed a ''likely'' range for TCRE of 0.2°–0.7°C per 1000 GtCO <sub>2</sub> (Collins et al., 2013) <sup>[[#fn:r110|110]]</sup> . The TCRE of the CMIP5 Earth system models ranges from 0.23°C to 0.66°C per 1000 GtCO <sub>2</sub> (Gillett et al., 2013) <sup>[[#fn:r111|111]]</sup> . At the same time, studies using observational constraints find best estimates of TCRE of 0.35°–0.41°C per 1000 GtCO <sub>2</sub> (Matthews et al., 2009; Gillett et al., 2013; Tachiiri et al., 2015; Millar and Friedlingstein, 2018) <sup>[[#fn:r112|112]]</sup> . This assessment continues to use the assessed AR5 TCRE range under the working assumption that TCRE is normally distributed (Stocker et al., 2013) <sup>[[#fn:r113|113]]</sup> . Observation-based estimates have reported log-normal distributions of TCRE (Millar and Friedlingstein, 2018) <sup>[[#fn:r114|114]]</sup> . Assuming a log-normal instead of normal distribution of the assessed AR5 TCRE range would result in about a 200 GtCO <sub>2</sub> increase for the median budget estimates but only about half at the 67th percentile, while historical temperature uncertainty and uncertainty in recent emissions contribute ±150 and ±50 GtCO <sub>2</sub> to the uncertainty, respectively (Table 2.2).

Calculating carbon budgets from the TCRE requires the assumption that the instantaneous warming in response to cumulative CO <sub>2</sub> emissions equals the long-term warming or, equivalently, that the residual warming after CO <sub>2</sub> emissions cease is negligible. The magnitude of this residual warming, referred to as the zero-emission commitment, ranges from slightly negative (i.e., a slight cooling) to slightly positive for CO <sub>2</sub> emissions up to present-day (Chapter 1, Section 1.2.4) (Lowe et al., 2009; Frölicher and Joos, 2010; Gillett et al., 2011; Matthews and Zickfeld, 2012) <sup>[[#fn:r115|115]]</sup> . The delayed temperature change from a pulse CO <sub>2</sub> emission introduces uncertainties in emission budgets, which have not been quantified in the literature for budgets consistent with limiting warming to 1.5°C. As a consequence, this uncertainty does not affect our carbon budget estimates directly but it is included as an additional factor in the assessed Earth system feedback uncertainty (as detailed below) of roughly 100 GtCO <sub>2</sub> on decadal time scales presented in Table 2.2.

Remaining carbon budgets are further influenced by Earth system feedbacks not accounted for in CMIP5 models, such as the permafrost carbon feedback (Friedlingstein et al., 2014b; MacDougall et al., 2015; Burke et al., 2017; Lowe and Bernie, 2018) <sup>[[#fn:r116|116]]</sup> , and their influence on the TCRE. Lowe and Bernie (2018) <sup>[[#fn:r117|117]]</sup> used a simple climate sensitivity scaling approach to estimate that Earth system feedbacks (such as CO <sub>2</sub> released by permafrost thawing or methane released by wetlands) could reduce carbon budgets for 1.5°C and 2°C by roughly 100 GtCO <sub>2</sub> on centennial time scales. Their findings are based on an older understanding of Earth system feedbacks (Arneth et al., 2010) <sup>[[#fn:r118|118]]</sup> . This estimate is broadly supported by more recent analysis of individual feedbacks. Schädel et al. (2014) <sup>[[#fn:r119|119]]</sup> suggest an upper bound of 24.4 PgC (90 GtCO <sub>2</sub> ) emitted from carbon release from permafrost over the next forty years for a RCP4.5 scenario. Burke et al. (2017) <sup>[[#fn:r120|120]]</sup> use a single model to estimate permafrost emissions between 0.3 and 0.6 GtCO <sub>2</sub> y <sup>-1</sup> from the point of 1.5°C stabilization, which would reduce the budget by around 20 GtCO <sub>2</sub> by 2100. Comyn-Platt et al. (2018) <sup>[[#fn:r121|121]]</sup> include carbon and methane emissions from permafrost and wetlands and suggest the 1.5°C remaining carbon budget is reduced by 116 GtCO <sub>2</sub> . Additionally, Mahowald et al. (2017) <sup>[[#fn:r122|122]]</sup> find there is possibility of 0.5–1.5 GtCO <sub>2</sub> y <sup>-1</sup> being released from aerosol-biogeochemistry changes if aerosol emissions cease. In summary, these additional Earth system feedbacks taken together are assessed to reduce the remaining carbon budget applicable to 2100 by an order of magnitude of 100 GtCO <sub>2,</sub> compared to the budgets based on the assumption of a constant TCRE presented in Table 2.2 ( ''limited evidence, medium agreement'' ), leading to overall ''medium confidence'' in their assessed impact. After 2100, the impact of additional Earth system feedbacks is expected to further reduce the remaining carbon budget ( ''medium confidence'' ).

The uncertainties presented in Table 2.2 cannot be formally combined, but current understanding of the assessed geophysical uncertainties suggests at least a ±50% possible variation for remaining carbon budgets for 1.5°C-consistent pathways. By the end of 2017, anthropogenic CO <sub>2</sub> emissions since the pre-industrial period are estimated to have amounted to approximately 2200 ±320 GtCO <sub>2</sub> ( ''medium confidence'' ) (Le Quéré et al., 2018) <sup>[[#fn:r123|123]]</sup> . When put in the context of year-2017 CO <sub>2</sub> emissions (about 42 GtCO <sub>2</sub> yr <sup>-1</sup> , ±3 GtCO <sub>2</sub> yr <sup>-1</sup> , ''high confidence'' ) (Le Quéré et al., 2018) <sup>[[#fn:r124|124]]</sup> , a remaining carbon budget of 580 GtCO <sub>2</sub> (420 GtCO <sub>2</sub> ) suggests meeting net zero global CO <sub>2</sub> emissions in about 30 years (20 years) following a linear decline starting from 2018 (rounded to the nearest five years), with a variation of ±15–20 years due to the geophysical uncertainties mentioned above ( ''high confidence'' ).

The remaining carbon budgets assessed in this section are consistent with limiting peak warming to the indicated levels of additional warming. However, if these budgets are exceeded and the use of CDR (see Sections 2.3 and 2.4) is envisaged to return cumulative CO <sub>2</sub> emissions to within the carbon budget at a later point in time, additional uncertainties apply because the TCRE is different under increasing and decreasing atmospheric CO <sub>2</sub> concentrations due to ocean thermal and carbon cycle inertia (Herrington and Zickfeld, 2014; Krasting et al., 2014; Zickfeld et al., 2016) <sup>[[#fn:r125|125]]</sup> . This asymmetrical behaviour makes carbon budgets path-dependent in the case of a budget and/or temperature overshoot (MacDougall et al., 2015) <sup>[[#fn:r126|126]]</sup> . Although potentially large for scenarios with large overshoot (MacDougall et al., 2015) <sup>[[#fn:r127|127]]</sup> , this path-dependence of carbon budgets has not been well quantified for 1.5°C- and 2°C-consistent scenarios and as such remains an important knowledge gap. This assessment does not explicitly account for path dependence but takes it into consideration for its overall confidence assessment.

This assessment finds a larger remaining budget from the 2006–2015 base period than the 1.5°C and 2°C remaining budgets inferred from AR5 from the start of 2011, which were approximately 1000 GtCO <sub>2</sub> for the 2°C (66% of model simulations) and approximately 400 GtCO <sub>2</sub> for the 1.5°C budget (66% of model simulations). In contrast, this assessment finds approximately 1600 GtCO <sub>2</sub> for the 2°C (66th TCRE percentile) and approximately 860 GtCO <sub>2</sub> for the 1.5°C budget (66th TCRE percentile) from 2011. However, these budgets are not directly equivalent as AR5 reported budgets for fractions of CMIP5 simulations and other lines of evidence, while this report uses the assessed range of TCRE and an assessment of the non-CO <sub>2</sub> contribution at net zero CO <sub>2</sub> emissions to provide remaining carbon budget estimates at various percentiles of TCRE. Furthermore, AR5 did not specify remaining budgets to carbon neutrality as we do here, but budgets until the time the temperature limit of interest was reached, assuming negligible zero emission commitment and taking into account the non-CO <sub>2</sub> forcing at that point in time.

In summary, although robust physical understanding underpins the carbon budget concept, relative uncertainties become larger as a specific temperature limit is approached. For the budget, applicable to the mid-century, the main uncertainties relate to the TCRE, non-CO <sub>2</sub> emissions, radiative forcing and response. For 2100, uncertain Earth system feedbacks such as permafrost thawing would further reduce the available budget. The remaining budget is also conditional upon the choice of baseline, which is affected by uncertainties in both historical emissions, and in deriving the estimate of globally averaged human-induced warming. As a result, only ''medium confidence'' can be assigned to the assessed remaining budget values for 1.5°C and 2.0°C and their uncertainty.

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'''Table 2.2:'''

<span id="the-assessed-remaining-carbon-budget-and-its-uncertainties."></span>
'''The assessed remaining carbon budget and its uncertainties.'''

The assessed remaining carbon budget and its uncertainties. Shaded blue horizontal bands illustrate the uncertainty in historical temperature increase from the 1850–1900 base period until the 2006–2015 period as estimated from global near-surface air temperatures, which impacts the additional warming until a specific temperature limit like 1.5°C or 2°C relative to the 1850–1900 period. Shaded grey cells indicate values for when historical temperature increase is estimated from a blend of near-surface air temperatures over land and sea ice regions and sea-surface temperatures over oceans.
<!-- TABLE -->
{| class="wikitable"
|-
! Additional Warming since<br />
2006–2015 [°C] <sup>(1.)</sup>
! Approximate Warming since<br />
1850–1900 [°C] <sup>(1.)</sup>
! colspan="3"| Remaining Carbon Budget<br />
(Excluding Additional<br />
Earth System Feedbacks <sup>(5.)</sup> )[G t CO <sub>2</sub> from 1.1.2018] <sup>(2.)</sup>
! colspan="6"| Key Uncertainties and Variations <sup>(4.)</sup>
|-
|
| colspan="3"| Percentiles of TCRE <sup>(3.)</sup>
| Earth System Feedbacks <sup>(5.)</sup>
| Non-CO <sub>2</sub> scenario variation <sup>(6.)</sup>
| Non-CO <sub>2</sub> forcing and response uncertainty
| TCRE<br />
distribution uncertainty <sup>(7.)</sup>
| Historical temperature uncertainty <sup>(1.)</sup>
| Recent emissions uncertainty <sup>(8.)</sup>
|-
|
| 33rd
| 50th
| 67th
| [GtCO <sub>2</sub> ]
|-
| 0.3
|
| 290
| 160
| 80
| rowspan="15"| Budgets on the left are reduced by about –100 on centennial time scales
|
|-
| 0.4
|
| 530
| 350
| 230
|
|-
| 0.5
|
| 770
| 530
| 380
|
|-
| 0.53
| ~1.5°C
| 840
| 580
| 420
| ±250
| –400 to +200
| +100 to +200
| ±250
| ±20
|-
| 0.6
|
| 1010
| 710
| 530
|
|-
| 0.63
|
| 1080
| 770
| 570
|
|-
| 0.7
|
| 1240
| 900
| 680
|
|-
| 0.78
|
| 1440
| 1040
| 800
|
|-
| 0.8
|
| 1480
| 1080
| 830
|
|-
| 0.9
|
| 1720
| 1260
| 980
|
|-
| 1
|
| 1960
| 1450
| 1130
|
|-
| 1.03
| ~2°C
| 2030
| 1500
| 1170
|
|-
| 1.1
|
| 2200
| 1630
| 1280
|
|-
| 1.13
|
| 2270
| 1690
| 1320
|
|-
| 1.2
|
| 2440
| 1820
| 1430
|
|}
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# Chapter 1 has assessed historical warming between the 1850–1900 and 2006–2015 periods to be 0.87°C with a ±0.12°C likely (1-standard deviation) range, and global near-surface air<br />
temperature to be 0.97°C. The temperature changes from the 2006–2015 period are expressed in changes of global near-surface air temperature.
# Historical CO <sub>2</sub> emissions since the middle of the 1850–1900 historical base period (mid-1875) are estimated at 1940 GtCO <sub>2</sub> (1640–2240 GtCO <sub>2</sub> , one standard deviation range) until end 2010. Since 1 January 2011, an additional 290 GtCO <sub>2</sub> (270–310 GtCO <sub>2</sub> , one sigma range) has been emitted until the end of 2017 (Le Quéré et al., 2018).
# TCRE: transient climate response to cumulative emissions of carbon, assessed by AR5 to fall likely between 0.8–2.5°C/1000 PgC (Collins et al., 2013), considering a normal distribution consistent with AR5 (Stocker et al., 2013). Values are rounded to the nearest 10 GtCO <sub>2</sub> .
# Focussing on the impact of various key uncertainties on median budgets for 0.53°C of additional warming.
# Earth system feedbacks include CO <sub>2</sub> released by permafrost thawing or methane released by wetlands, see main text.
# Variations due to different scenario assumptions related to the future evolution of non-CO <sub>2</sub> emissions.
# The distribution of TCRE is not precisely defined. Here the influence of assuming a lognormal instead of a normal distribution shown.
# Historical emissions uncertainty reflects the uncertainty in historical emissions since 1 January 2011.

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