18


EMISSIONS GAP REPORT 2018 – THE EMISSIONS GAP


Box 3.1 Uncertainties surrounding carbon budgets and corresponding temperature limits


global mean temperature (Gillett et al., 2013; Matthews et al., 2017) and the contribution of non-CO2


Carbon budgets in line with a specific temperature limit have an uncertainty range to which various factors contribute, including the proportionality factor between cumulative CO2


to this, IPCC’s 5th


ongoing emissions continue reducing the remaining carbon budget, these uncertainties become more important in relative terms (Peters, 2018).


of 33–66 percent, with a median estimate of slightly above 3,000 GtCO2


2014) indicated a variation in the carbon budget (from 1870 onwards) for limiting warming to 2°C of about 1,250 GtCO2


Recent studies have attempted to reduce these uncertainties, for example, by using a more recent reference period to express carbon budget estimates, such as 2006– 2015 instead of pre-industrial times (see, for example, Millar et al., 2017; Tokarska and Gillett, 2018). Although this approach eliminates some uncertainties that have accumulated over the historical period, it comes with caveats and limitations, which result in higher carbon budgets compared with estimated provided in the IPCC 5th Assessment Report (Pfleiderer et al., 2018). The choice of method to estimate warming until the present is an important contributor to these higher budget estimates. Recent research also showed that Earth system feedbacks could cause additional warming, both during (Comyn-Platt et al., 2018; Lowe et al., 2018) and beyond the 21st


century


(Fischer et al., 2018). This new literature on additional feedbacks therefore suggests that carbon budgets consistent with warming of


1.5°C or 2°C may be approximately 100 GtCO2 smaller.


uses the climate model and set-up used in the IPCC 5th


Although this report groups scenarios based on their cumulative CO2


emissions, it also Assessment Report (Meinshausen et


al., 2011; Clarke et al., 2014) for comparability with earlier reports.


(range ). As past and


Box 3.2 Technical comparison with the IPCC Special Report on Global Warming of 1.5°C


emissions and the increase in


forcers (Rogelj et al., 2016b). Due Assessment Report (IPCC,


The analysis included in this chapter is consistent with the latest assessment of the IPCC Special Report on Global Warming of 1.5°C (IPCC, 2018). The range of Kyoto-GHG emissions in 2030 consistent with limiting warming to 1.5°C used in this report (24 GtCO2


e/year with a range of 22–30 GtCO2 year) is consistent with the 25–30 GtCO2


e/ e/


year range reported in the recent IPCC Special Report (IPCC, 2018) for scenarios limiting global warming to 1.5°C with no or limited overshoot. Differences are attributed to the exclusive use of scenarios that start emissions reductions from 2020 onwards in this report, compared with a wider set in the IPCC Special Report. Overall, these minor changes do not affect the assessment of the adequacy of current NDCs for limiting warming to 1.5°C or well below 2°C.


From 2018 to the time of reaching net zero, the below 1.5°C by 2100 scenario has cumulative CO2 GtCO2


emissions of less than 600 . This broadly corresponds to the


remaining carbon budget for limiting warming to 1.5°C with 50 percent probability (580 GtCO2


from 2018 to the time of reaching net


zero) of the IPCC Special Report, limiting temperature overshoot to less than 0.1°C. Cumulative CO2


emissions until the end of


the century for any available below 1.5°C by 2100 scenario are at most 380 GtCO2


for the below 1.8°C scenario are 900 GtCO2 from 2018 to the time of reaching net zero.


66 percent probability of the IPCC Special Report. Maximum cumulative CO2


equates to a 66 percent


Using the IPCC Special Report assessment, this 900 GtCO2


time of reaching net zero are 1,300 GtCO2 from 2018 to 2100 are 1,200 GtCO2


probability of limiting warming to about 1.8°C. This also corresponds to about a 50 percent probability of limiting warming to 1.7°C. For the below 2°C scenario, maximum cumulative CO2


emissions from 2018 to the and . Using the


with at least 66 percent probability by 2100, though there is a slightly lower probability at peak warming during the century. This suggest that the probability of limiting warming to 1.9°C is about 50 percent.


IPCC Special Report assessment, this 1,200 GtCO2


equates to keeping warming below 2°C


is less than the remaining carbon budget of 420 GtCO2


, which


for limiting warming to 1.5°C with emissions


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