PNNL

Abstract

The Scenario Model Intercomparison Project (ScenarioMIP) defines and coordinates the primary future projections within the Coupled Model Intercomparison Project Phase 6 (CMIP6). This paper presents a range of its outcomes by synthesizing results from the participating global coupled Earth system models for concentration driven simulations. We limit the scope to the analysis of strictly geophysical outcomes on global averages and spatial patterns of change for surface air temperature and precipitation. We also compare CMIP6 projections to CMIP5 results for those scenarios that were designed to provide continuity across the CMIP phases, highlighting important differences in forcing composition as well as in results. The range of future temperature and precipitation changes by the end of the century encompassing the Tier 1 experiments and SSP1-1.9 spans both lower and higher values compared to the range of outcomes from CMIP5. This is due to both the wider range of radiative forcings that the new scenarios cover and to higher climate sensitivities in some of the new models compared to their CMIP5 predecessors. Spatial patterns of change for both variables averaged over models and scenarios have familiar features, and an analysis of their variations confirms model structural differences to be the dominant source of uncertainty. Models also differ with respect to the size and evolution of internal variability as measured by individual models’ ensemble spread, according to a set of initial condition ensemble simulations available under SSP3-7.0. Benefits of mitigation appear clearly when comparing scenarios developed under the same SSP, but to which different degrees of mitigation have been applied. It is also found that a mild overshoot in temperature of a few decades in mid-century, as represented in SSP5-3.4OS does not affect the end outcome in terms of temperature and precipitation changes by 2100, which return to the same level as those reached by SSP4-3.4. Central estimates based on the time at which the ensemble means of the different scenarios reach a given warming level show all scenarios reaching 1.5°C of warming compared to pre-industrial before the middle of the current decade, with the behavior of individual model trajectories spanning a window of 20 years from present. 2°C of warming is reached as early as the mid ‘30s under SSP5-8.5, but as late as the late ‘40s under SSP1-2.6 by the respective ensemble means. The highest warming level considered, 5°C, is reached only by the ensemble mean under SSP5-8.5 at the beginning of the last decade of the century.