Solar Radiation and Cloud Change in China Calculated in a 20th Century Simulation with a Climate Model

Significant long-term trends in various climate elements in China have been observed in recent decades. The elements include surface temperature, precipitation, evaporation, surface solar radiation, cloud cover, etc. It is of particular interest that coexistence of decreasing trends both in solar radiation and cloud cover have been observed over most of China, since a decreasing trend of solar radiation would be expected to be accompanied by an increasing trend of cloud cover that reduces the radiation from the sun. Here, we investigate the cause of this confusing fact with the aid of a 20th century simulation performed with the MIROC climate model. Simulated trend in the surface solar radiation shows the decreasing trend over entire China, although the trend averaged over China is about -1.0 W/m2 which is smaller than observed trend. While the simulated trend in cloud cover averaged over China is indiscernible as opposed to the observation, a small but significant decreasing trend similar to the observation could be simulated in the southern part of China. In the southern part of China, our model could capture the coexistence of decreasing trends both in solar radiation and cloud cover, which could also be simulated an experiment forced only anthropogenic climate forcings (i.e. increases in greenhouse gases and aerosols). As for the decrease in the surface solar radiation simulated in the model, the dominant cause is the increase in aerosols. Simplified analysis revealed the relative importance of direct and indirect effect of aerosols on the decrease in the surface solar radiation in the model; both effects have comparable contributions in some regions. The increase in greenhouse gasses has some effect on the decrease in cloud cover in the southern part of China in the model.