Historically Hottest Summers Projected to be the Norm for more than half of the World's Population by 2035

Abstract:

Global mean temperatures are projected to increase by 3K and 4.9K above pre-industrial levels by 2100 in a moderate stabilization and a high-emission scenario (RCP4.5 and RCP8.5 from CMIP5). However, warming rates are regionally different. In this presentation, we focus on the regions defined by the IPCC SREX report. We investigate the year in the future in which historically hottest summers are projected to become the norm, i.e. to occur at least every other year. Using results from a detection and attribution analysis, we provide probabilistic estimates based on RCP4.5 and RCP8.5 simulations constrained by observations during 1950-2012. We also estimate the fraction of attributable risk (FAR), i.e. the probability for hot summers that is attributable to past emissions of anthropogenic greenhouse gases and aerosols. We find that the FAR is larger than 0.9 in many regions. We project that under RCP4.5, more than half of the world's population will experience the historically hottest summer of the past 63 years with a probability of 50% and 90% by 2035 and 2050, respectively. Under the higher emission scenario RCP8.5, historically hottest summers are projected to be more wide-spread. The Mediterranean region, Western and Eastern Asia, Northern Eurasia and the Sahara are among the first regions for which such hot summers might become the norm. Even under RCP4.5, more than 90% of summers are projected to be hotter than the historically hottest summers by 2025 and 2035 for Sahara and the Mediterranean regions, respectively.