Climate assessment of emissions scenarios for use in WG3 of the IPCC’s Sixth Assessment Report

<p>Consistent and comparable climate assessments of scenarios are critical within the context of IPCC assessment reports. Given the number of scenarios assessed by WG3, the assessment “pipeline” must be almost completely automated. Here, we present the application of a new assessment pipeline which combines state-of-the-art components into a single workflow in order to derive climate outcomes for integrated assessment model (IAM) scenarios assessed by WG3 of the IPCC. A consistent analysis ensures that WG3’s conclusions about the socioeconomic transformations required to maintain a safe climate are based on the best understanding of our planetary boundaries from WG1. For example, if WG1 determines that climate sensitivity is higher than previously considered, then WG3 could incorporate this insight by e.g. considering much smaller remaining carbon budgets for any given temperature target.</p><p> </p><p>The scenario-climate assessment pipeline is comprised of three primary components. First, a consistent harmonization algorithm which maintains critical model characteristics between harmonized and unharmonized scenarios [1] is employed to harmonize emissions trajectories to a common and consistent historical dataset as used in CMIP6 [2]. Next, a scenario’s reported emissions trajectories are analyzed as to the completeness of its species and sectoral coverage. A consistent set of 14 emissions species are expected, aligning with published work within ScenarioMIP and CMIP6 (see ref [2], Table 2). Should any component of this full set of emissions trajectories be absent for a given scenario, an algorithm (e.g., generalised quantile walk [3]) is employed in order to “back-fill” missing species at the native model regional resolution. Finally, full emissions scenarios are analyzed by an Earth System Model emulator, e.g., MAGICC [4].</p><p> </p><p>In this presentation, we explore differences in climate assessments and estimated remaining carbon budgets across various components of the pipeline for available scenarios in the literature. We consider the impact of alternative choices, especially those made in prior assessments by the IPCC (AR5, SR15), including, for example, the historical emissions database used, the effect of harmonization and back-filling, as well as the version and setup of MAGICC used. </p><p> </p><p>References</p><p> </p><p>[1] Gidden, M.J., Fujimori, S., van den Berg, M., Klein, D., Smith, S.J., van Vuuren, D.P. and Riahi, K., 2018. A methodology and implementation of automated emissions harmonization for use in Integrated Assessment Models. Environmental Modelling & Software, 105, pp.187-200.</p><p> </p><p>[2] Gidden, M. J., Riahi, K., Smith, S. J., Fujimori, S., Luderer, G., Kriegler, E., van Vuuren, D. P., van den Berg, M., Feng, L., Klein, D., Calvin, K., Doelman, J. C., Frank, S., Fricko, O., Harmsen, M., Hasegawa, T., Havlik, P., Hilaire, J., Hoesly, R., Horing, J., Popp, A., Stehfest, E., and Takahashi, K.: Global emissions pathways under different socioeconomic scenarios for use in CMIP6: a dataset of harmonized emissions trajectories through the end of the century, Geosci. Model Dev., 12, 1443-1475, https://doi.org/10.5194/gmd-12-1443-2019, 2019.</p><p> </p><p>[3] Teske, S. et al., Achieving the Paris Climate Agreement Goals. Springer, 2019.</p><p> </p><p>[4] Meinshausen, M., Raper, S.C. and Wigley, T.M., 2011. Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6–Part 1: Model description and calibration. Atmospheric Chemistry and Physics, 11(4), pp.1417-1456.</p>