Historical global land surface air apparent temperature and its future changes based on CMIP6 projections.

The warming magnitudes under different shared socioeconomic pathways (SSPs) and the spatial distribution of global land surface air apparent temperature (APT) since the early of the 20 century were systematically analyzed, based on the comparisons among in-situ observations, extended reanalysis, and the CMIP6 model output. The warming of APT by the mid and late 21st century was then projected, as well as under the 1.5 °C and 2.0 °C threshold for global warming. The study reveals: 1) the CMIP6 multi-model ensemble mean (MME) agrees well with the observations in terms of the climatological mean and temporal variations for the global land surface air temperature (SAT) and the calculated APT over the past 100 years. 2) Although the spatial gradient distribution of SAT and APT is quite similar under SSP2-4.5 and SSP5-8.5, the warming trend of global surface APT over land is significantly larger than that of SAT. Population living in low latitudes will be more vulnerable to the enhanced warming of APT. 3) Under the global warming thresholds of 1.5 °C and 2.0 °C, the global mean APT estimated under SSP2-4.5 and SSP5-8.5 is identical, which are 1.9 °C and 2.7 °C, respectively. The projected APT will increase by 3.9 °C under SSP2-4.5 and 6.7 °C under SSP5-8.5 at the end of the 21st century relative to the pre-industrial. This study highlights that the probability and intensity of extreme warm events for land SAT and APT around the globe under SSP5-8.5 will be remarkably higher than SSP2-4.5 in the 21st century, implying the urgent demand of regulating greenhouse gas emissions toward reducing thermal discomfort in the future.