The Physical Climate at Global Warming Thresholds as Seen in the UK Earth System Model

A key goal of the 2015 Paris Climate Agreement is to keep global mean temperature change at 2 degrees C and if possible under 1.5 degrees C by the end of the century. To investigate the likelihood of achieving this target, we calculate the year of exceedance of a given global warming threshold (GWT) temperature across 32 CMIP6 models for Shared Socioeconomic Pathway (SSP) and radiative forcing combinations included in the Tier 1 ScenarioMIP simulations. Threshold exceedance year calculations reveal that a majority of CMIP6 models project warming beyond 2 degrees C by the end of the century under every scenario or pathway apart from the lowest emission scenarios considered, SSP1-1.9 and SSP1-2.6, which is largely a function of the ScenarioMIP experiment design. The U.K. Earth System Model (UKESM1) ScenarioMIP projections are analyzed in detail to assess the regional and seasonal variations in climate at different warming levels. The warming signal emerging by midcentury is identified as significant and distinct from internal climate variability in all scenarios considered and includes warming summers in the Mediterranean, drying in the Amazon, and heavier Indian monsoons. Arctic sea ice depletion results in prominent amplification of warming and tropical warming patterns emerge that are distinct from interannual variability. Climate changes projected for a 2 degrees C warmer world are in almost all cases exacerbated with further global warming (e.g.. to a 4 degrees C warmer world).