Editing IPCC:AR6/WGIII/Chapter-3 (section)

=== Box 3.3 | The Likelihood of High-end Emissions Scenarios ===

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At the time the Representative Concentration Pathways (RCPs) were published, they included three scenarios that could represent emission developments in the absence of climate policy: RCP4.5, RCP6 and RCP8.5, described as, respectively, low, medium and high-end scenarios in the absence of strong climate policy ( [[#van%20Vuuren--2011|van Vuuren et al. 2011]] ). RCP8.5 was described as representative of the top 5% scenarios in the literature. The SSPs-based set of scenarios covered the RCP forcing levels, adding a new low scenario (at 1.9 W m –2 ). [[#Hausfather--2020|Hausfather and Peters (2020)]] pointed out that since 2011, the rapid development of renewable energy technologies and emerging climate policy have made it considerably less likely that emissions could end up as high as RCP8.5. Still, emission trends in developing countries track RCP8.5 [[#Pedersen--2020|Pedersen et al. (2020)]] , and high land-use emissions could imply that emissions would continue to do so in the future, even at the global scale (Schwalm et al. 2020). Other factors resulting in high emissions include higher population or economic growth as included in the SSPs ( [[#3.3.1|Section 3.3.1]] ) or rapid development of new energy services. Climate projections of RCP8.5 can also result from strong feedbacks of climate change on (natural) emission sources and high climate sensitivity (AR6 WGI Chapter 7), and therefore their median climate impacts might also materialise while following a lower emission path (e.g., Hausfather and Betts 2020). The discussion also relates to a more fundamental discussion on assigning likelihoods to scenarios, which is extremely difficult given the deep uncertainty and direct relationship with human choice. However, it would help to appreciate certain projections (e.g., Ho et al. 2019). All in all, this means that high-end scenarios have become considerably less likely since AR5 but cannot be ruled out. It is important to realise that RCP8.5 and SSP5-8.5 do not represent a typical ‘business-as-usual’ projection but are only useful as high-end, high-risk scenarios. Reference emission scenarios (without additional climate policy) typically end up in the C5–C7 categories included in this assessment.

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'''Box 3.4, Figure 1 | Cumulative CO 2 emissions from AR6 scenario categories (coloured dots), adjusted for distinct 0.''' '''1°C warming levels (black bars) in comparison to the WGI remaining carbon budgets (grey bars).''' The cumulative carbon emissions for the AR6 scenarios are shown for the median peak warming '''(a)''' , the 33rd-percentile peak warming '''(b)''' and the upper 67th-percentile peak warming '''(c)''' calculated with the WGI-calibrated emulator MAGICC7 (IPCC AR6 WGI, Cross-Chapter Box 7.1). The adjustment to the nearest 0.1°C intervals is made using AR6 WGI TCRE (at the relevant percentile, e.g., the 67th-percentile TCRE is used to adjust the 67th-percentile peak warming), with the 5–95% range of adjusted scenarios provided by the black bar. The AR6 WGI remaining carbon budget is shown, including the WGI estimate of at least a ±220 GtCO 2 uncertainty due to non-CO 2 emissions variations across scenarios (grey bars). For median peak warming (panel a) projections below 2°C relative to 1850–1900, the AR6 WGIII assessment of cumulative carbon emissions tends to be slightly smaller than the remaining carbon budgets provided by WGI but well within the uncertainties. Note that only a few scenarios in WGIII limit warming to below 1.5°C with a 50% chance, thus statistics for that specific threshold have low confidence.

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