2016 Volume 94 Issue 5 Pages 393-414
We reviewed the long-term trends and inter-annual variations in the surface shortwave irradiance in China and Japan. Pyranometer observations revealed decreases followed by increases in the shortwave irradiance in China and Japan between the 1960s and 2000s, while obvious long-term trends were not evident in the satellite observations after 1983. In China, surface shortwave irradiance decreased from 1961 until around 1990, but then began to increase. In Japan, on the contrary, the decreasing trend stopped in the 1960s, with little inter-annual variation during the 1970s and 1980s, and an increase began around 1990. The causes of the differences between the shortwave irradiance trends in China and Japan were ascribed to an increase in light-absorbing aerosols in China that began in the 1960s and a decrease in absorbing aerosols in Japan that began in 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 aerosols could have direct effects 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 phases of the minimum periods in the two regions slightly differed. An increase in anthropogenic aerosol was responsible for the variation in the shortwave irradiance through the direct radiative effect of aerosol in the polluted area, while indirect radiative effects, i.e., changes in cloud cover due to an increase in cloud condensation nuclei, dominated in pristine areas. The effects 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.