Changes of heating and cooling degree days over China in response to global warming of 1.5 °C, 2 °C, 3 °C and 4 °C

Future changes of heating degree days (HDD) and cooling degree days (CDD) in the 21st century with and without considering population factor are investigated based on four sets of climate change simulations over East Asia using the regional climate model version 4.4 (RegCM4.4) driven by the global models of CSIRO-Mk3-6-0, EC-EARTH, HadGEM2-ES, and MPI-ESM-MR. Under global warming of 1.5 degrees C, 2 degrees C, 3 degrees C, and 4 degrees C, significant decrease of HDD can be found over China without considering population factor, with greater decrease over high elevation and high latitude regions, including the Tibetan Plateau, the northern part of Northeast China, and Northwest China; while population-weighted HDD increased in areas where population will increase in the future, such as Beijing, Tianjin, parts of southern Hebei, northern Shandong and Henan provinces. Similarly, the CDD projections with and without considering population factor are largely different. Specifically, without considering population, increase of CDD were observed over most parts of China except the Tibetan Plateau where the CDD remained zero because of the cold climate even under global warming; while considering population factor, the future CDD decreases in South China and increases in North China, the Sichuan Basin, and the southeastern coastal areas, which is directly related to the population changes. The different future changes of HDD and CDD when considering and disregarding the effects of population show that population distribution plays an important role in energy consumption, which should be considered in future research.