Changes in IPCC Scenario Assessment Emulators Between SR1.5 and AR6 Unraveled

Nicholls, Z. ORCID: https://orcid.org/0000-0002-4767-2723, Meinshausen, M., Lewis, J., Smith, C. ORCID: https://orcid.org/0000-0003-0599-4633, Forster, P.M., Fuglestvedt, J.S., Rogelj, J. ORCID: https://orcid.org/0000-0003-2056-9061, Kikstra, J. ORCID: https://orcid.org/0000-0001-9405-1228, Riahi, K. ORCID: https://orcid.org/0000-0001-7193-3498, & Byers, E. ORCID: https://orcid.org/0000-0003-0349-5742 (2022). Changes in IPCC Scenario Assessment Emulators Between SR1.5 and AR6 Unraveled. Geophysical Research Letters 49 (20) 10.1029/2022GL099788.

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Abstract

The IPCC's scientific assessment of the timing of net-zero emissions and 2030 emission reduction targets consistent with limiting warming to 1.5°C or 2°C rests on large scenario databases. Updates to this assessment, such as between the IPCC's Special Report on Global Warming of 1.5°C (SR1.5) of warming and the Sixth Assessment Report (AR6), are the result of intertwined, sometimes opaque, factors. Here we isolate one factor: the Earth System Model emulators used to estimate the global warming implications of scenarios. We show that warming projections using AR6-calibrated emulators are consistent, to within around 0.1°C, with projections made by the emulators used in SR1.5. The consistency is due to two almost compensating changes: the increase in assessed historical warming between SR1.5 (based on AR5) and AR6, and a reduction in projected warming due to improved agreement between the emulators' response to emissions and the assessment to which it is calibrated.

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