Abstract: The progress of the previous IPCC assessment reports indicate that human’s scientific cognition of climate system changes is constantly deepening, and global warming is unequivocal. Since the industrial age, human activities may have caused approximately 1 ℃. With the continuous improvement of climate system observations, the spatial coverage and time scale of observation data and reanalysis data have increased significantly. In terms of climate system models, the increase in resolution and the addition of more complex biogeochemical processes, making the model development has experienced progress from climate system model to earth system model. Compared with previous assessment reports, the sixth assessment report (AR6) will focus on problem-solving-oriented thinking, and strengthen the assessment content of water cycle changes, regional climate change and the detection and attribution of extreme weather and climate events.