How high is high enough? A multi-million-member ensemble analysis of future climate scenarios and their relevance

Abstract High end emissions and forcing scenarios used to drive climate models provide key insights into possible high-risk climate change impacts in the 21st century and beyond. Given rapid and ongoing societal changes (e.g., population growth, energy demand, technology, etc.), debates are increasing on the continued relevance of higher forcing scenarios, such as those designed to reach 8.5 W/m 2 by the end of the century (e.g., RCP8.5 and SSP5-8.5). Here, we determine an updated probability of exceeding 8.5 W/m 2 by 2100 by generating a multi-million-member ensemble from a newly developed database of probabilistic greenhouse gas (GHG) emissions scenarios, augmented here with improved characterization of aerosols and minor GHGs, coupled with the newest version of a reduced complexity climate model, with parameter distributions calibrated to match the IPCC 6th Assessment Report. The probability of exceeding 8.5 W/m 2 this century is estimated to be less than 1%: however, we discuss important rationales for the continued use of 8.5 W/m 2 scenarios, including for generating high temperature scenarios for use in calibrating damage functions, characterizing climate in the 22nd century (probability of exceeding 8.5 W/m 2 increases to ~7% by 2150), and assessing low-probability/high-impact futures.