The IPCC Sixth Assessment Report WGIII climate assessment of mitigation pathways: from emissions to global temperatures

Abstract

Abstract. While the IPCC’s physical science report usually assesses a handful of future scenarios, the IPCC Sixth Assessment Working Group III report (AR6 WGIII) on climate mitigation assesses hundreds to thousands of future emissions scenarios. A key task is to assess the global-mean temperature outcomes of these scenarios in a consistent manner, given the challenge that the emission scenarios from different integrated assessment models come with different sectoral and gas-to-gas coverage and cannot all be assessed consistently by complex Earth System Models. In this work, we describe the “climate assessment” workflow and its methods, including infilling of missing emissions and emissions harmonisation as applied to 1,202 mitigation scenarios in AR6 WGIII. We evaluate the global-mean temperature projections and effective radiative forcing characteristics (ERF) of climate emulators FaIRv1.6.2, MAGICCv7.5.3, and CICERO-SCM, discuss overshoot severity of the mitigation pathways using overshoot degree years, and look at an interpretation of compatibility with the Paris Agreement. We find that the lowest class of emission scenarios that limit global warming to “1.5 °C (with a probability of greater than 50 %) with no or limited overshoot” includes 90 scenarios for MAGICCv7.5.3, and 196 for FaIRv1.6.2. For the MAGICCv7.5.3 results, “limited overshoot” typically implies exceedance of median temperature projections of up to about 0.1 °C for up to a few decades, before returning to below 1.5 °C by or before the year 2100. For more than half of the scenarios of this category that comply with three criteria for being “Paris-compatible”, including net-zero or net-negative greenhouse gas (GHG) emissions, are projected to see median temperatures decline by about 0.3–0.4 °C after peaking at 1.5–1.6 °C in 2035–2055. We compare the methods applied in AR6 with the methods used for SR1.5 and discuss the implications. This article also introduces a ‘climate-assessment’ Python package which allows for fully reproducing the IPCC AR6 WGIII temperature assessment. This work can be the start of a community tool for assessing the temperature outcomes related to emissions pathways, and potential further work extending the workflow from emissions to global climate by downscaling climate characteristics to a regional level and calculating impacts.

Assessing the size and uncertainty of remaining carbon budgets

“…We therefore consider variations on this assumption, described in detail in Methods and plotted for a few scenarios in figure 3. We find that while in some scenarios different approaches will get very similar results, in other scenarios results may differ by over 0.1 • C. Some alternative approaches that can be considered are the non-CO 2 warming at the time of the model-reported net zero date (the date of net zero before emissions were harmonised to be consistent with recent emissions [37]); the maximum possible non-CO 2 warming at any point over the twenty-first century, and the non-CO 2 warming at maximum total temperature. The impact of changing between these measures is investigated in table 2.…”

“…For the non-CO 2 components of projections, we default to (and recommend using) the AR6 scenario database [33], but also investigate the use of the SR1.5 database [25] for comparison with previous IPCC RCBs. The emissions scenarios in both databases are vetted to ensure that key emissions species and socioeconomic variables are within reasonable ranges in the recent past and near future, then harmonised to match historic emissions precisely and infilled with any missing emissions [37].…”

Uncompensated claims to fair emission space risk putting Paris Agreement goals out of reach

“…However, a more complete assessment of the scenarios is necessary to compare the warming outcomes of the scenarios to criteria that have been suggested to operationalise the two textually linked elements of the LTTG ("hold warming well below 2°C" and "pursue efforts to limit warming to 1.5°C") . This requires assumptions to be made about the non-CO2 emission trajectories, and the use of an appropriately calibrated simple climate model to capture uncertainty in the response of the climate system to emissions (Brecha et al, 2022;Kikstra et al, 2022). In this paper, we first infer the methane (CH4) and nitrous oxide (N2O) emissions using a quantile-based infilling method (Lamboll et al, 2020), and then proceed to use the sequence of harmonisation, infilling, and climate assessment steps applied in AR6…”

“…WG III (Kikstra et al, 2022), with one key difference. We use the FaIR simple climate model (Smith et al, 2018) with the solar cycle forcing estimates removed from 2016, so that we only assess the anthropogenic warming contribution of these emission pathways (Rogelj, Schleussner and Hare, 2017).…”

“…We note that these results, especially those close to the thresholds, are somewhat sensitive to the choice of emulator (FaIR v1.6.2 in this case). Another simple climate model, MAGICC (Nicholls et al, 2020), which was applied for the scenario categorization in AR6 WGIII, exhibits faster nearterm warming, and slightly higher peak temperatures than FaIR, especially for scenarios consistent with a 1.5°C RCB (Kikstra et al, 2022). Hence, for some scenarios (e.g., GCAM 5.3 ecpc_minc1_maxc3 in Table 1), we would expect them to breach the thresholds identified by , if MAGICC was used for the climate assessment.…”