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

Abstract

Abstract. While the Intergovernmental Panel on Climate Change (IPCC) physical science reports usually assess a handful of future scenarios, the Working Group III contribution on climate mitigation to the IPCC's Sixth Assessment Report (AR6 WGIII) assesses hundreds to thousands of future emissions scenarios. A key task in WGIII is to assess the global mean temperature outcomes of these scenarios in a consistent manner, given the challenge that the emissions scenarios from different integrated assessment models (IAMs) 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 1202 mitigation scenarios in AR6 WGIII. We evaluate the global mean temperature projections and effective radiative forcing (ERF) characteristics of climate emulators FaIRv1.6.2 and MAGICCv7.5.3 and use the CICERO simple climate model (CICERO-SCM) for sensitivity analysis. We discuss the implied overshoot severity of the mitigation pathways using overshoot degree years and look at emissions and temperature characteristics of scenarios compatible with one possible interpretation of the Paris Agreement. We find that the lowest class of emissions scenarios that limit global warming to “1.5 ∘C (with a probability of greater than 50 %) with no or limited overshoot” includes 97 scenarios for MAGICCv7.5.3 and 203 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 in this category that comply with three criteria for being “Paris-compatible”, including net-zero or net-negative greenhouse gas (GHG) emissions, 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 their implications. This article also introduces a “climate-assessment” Python package which allows for fully reproducing the IPCC AR6 WGIII temperature assessment. This work provides a community tool for assessing the temperature outcomes of emissions pathways and provides a basis for further work such as extending the workflow to include downscaling of climate characteristics to a regional level and calculating impacts.

A food system transformation can enhance global health, environmental conditions and social inclusion

“…We ran MAGICC with a probabilistic setup following the IPCC's latest WG1 report 102 (see Cross-Chapter Box 7.1 in Chapter 7 of AR6 WG1). For emissions not included in REMIND-MAgPIE (e.g., Montreal Protocol species), we followed methods from the latest WG3 report 103,104 . As input to MAGICC, we combined AFOLU emissions from MAgPIE (CO 2 , CH 4 , N 2 O) with non-AFOLU emissions (e.g., energy, transport, industry, waste) from prior REMIND scenarios (see REMIND section), ensuring coherence between bioenergy demand and energy transformation levels across the modeled scenarios.…”

Climate uncertainty impacts on optimal mitigation pathways and social cost of carbon

“…Results from the offline historical FaIR runs are saved out for 2023 and used as initial conditions for DICE. The temperatures of the three ocean layers in 2023 are re-baselined such that the uppermost layer (a proxy for global mean near-surface air temperature) is defined to be 0.85 • C above pre-industrial over the 1995-2014 mean, this being the best estimate assessed warming in the IPCC AR6 WG1 [31] and following the treatment of scenario assessment in IPCC AR6 WG3 [3,27,41]. The other two ocean layers are adjusted by the same amount that was required to fix the uppermost layer at 0.85 • C, maintaining relative differences.…”

Potential impacts of rapidly changing european use of fossil fuels on global warming

“…FaIR is an open-source, simple and computationally fast climate emulator of fullcomplexity ESMs. FaIR is one of the models used in the Intergovernmental Panel on Climate Change (IPCC) 6th Assessment Report (AR6) Working Group WGI and WGIII reports (IPCC 2021, IPCC 2022, Kikstra et al 2022. The model consists of a simplified representation of the global carbon cycle coupled with a climate response model with two ocean layers (Millar et al 2017, Smith et al 2018.…”