Article ID: 2016-024
We reviewed the long-term trends and inter-annual variations in surface shortwave irradiance in China and Japan. Pyranometer observations indicated a decrease followed by an increase in shortwave irradiance in China and Japan during the period from the 1960s to 2000s, while obvious long-term trends were not found from satellite observations after 1983. In China, surface shortwave irradiance decreased from 1961 to around 1990, but then began to increase. In Japan, on the other hand, the decreasing trend stopped in the 1960s, with little inter-annual variation during the 1970s and 1980s, and an increase that began around 1990. The causes of the difference in shortwave irradiance trends between China and Japan were ascribed to an increase in light-absorbing aerosols in China since the 1960s and a decrease in absorbing aerosols in Japan since the late 1970s. Absorbing aerosols decrease both direct and diffuse radiation, while non-absorbing aerosols decrease direct radiation but increase diffuse radiation. Although these aerosol influences are generally found under clear-sky conditions, absorbing aerosol could have a direct effect even under cloudy-sky conditions. The trends of surface shortwave irradiance in China and Japan are in line with the so-called global dimming and brightening dimming processes, although the phase of the minimum period differed slightly between the two regions. An increase in anthropogenic aerosol was responsible for the variation in shortwave irradiance through the direct radiative effect of aerosol in the polluted area, while an indirect radiative effect, i.e., changes in cloud cover due to an increase in cloud condensation nuclei, dominated in pristine areas. The effect of other factors, such as variations in water vapor and natural aerosol levels, appear to be small compared to the effects of cloud and anthropogenic aerosols.