Mid-summer surface air temperature and its internal variability over China at 1.5 °C and 2 °C global warming

Recently, extremely hot summers occurred frequently across China, and the mean mid-summer surface air temperature (SAT) continuously broke the records of the past decades, causing huge social and economic losses. As global warming accelerates, these extremely hot summers will undoubtedly occur more frequently. However, the issue of what will happen to the mid-summer SAT over China in the near future remains unclear. Therefore, we investigate the changes of mid-summer SAT and related internal variabilities over China at 1.5 degrees C and 2 degrees C global warming above preindustrial level by using the MPI-ESM Grand Ensemble simulations. The results indicate that compared to the present-day (1986-2005), national averaged mid-summer SAT will increase by 1.1 degrees C and 2.0 degrees C, in 1.5 degrees C and 2 degrees C warming scenarios respectively. This means that the mid-summer SAT is projected to increase by 0.9 degrees C due to an additional 0.5 degrees C global warming, which is higher than the annual value (0.8 degrees C) and almost two times the global warming rate. Regionally, in the two warming targets, the increase in mid-summer SAT will be more enhanced over the northwestern part of China. In addition, the extremely high monthly SAT would increase nationwide due to an additional 0.5 degrees C in global warming. Among all areas, the Qinghai and Xinjiang provinces would experience the strongest increase in extremely high monthly SAT. It is important to find that, from 1.5 degrees C to 2 degrees C global warming, changes of the internal variability of the mid-summer SAT differs across China. It would decrease over some parts of western Northwest China, North China, Northeast China and the Tibetan Plateau. However, it would significantly increase over Qinghai, Sichuan, and northern parts of Inner Mongolia. As a result, at 2 degrees C global warming, the increase of extreme SAT in Qinghai is caused by the synergistic effect of stronger warming rate and larger internal variability. Differently, the increase in Xinjiang province is mainly caused by the stronger local warming. Further analysis suggests that we can effectively reduce the intensity of extremely hot months over most regions of Northwest China by limiting global warming to 1.5 degrees C, rather than to 2 degrees C.