Do climate models reproduce observed solar dimming and brightening over China and Japan?

Abstract

Previous research indicates that clear‐sky downward solar radiation measured at the surface over China significantly decreased by about −8.6 W m−2 per decade during 1961–1989 and insignificantly increased during 1990–1999. Furthermore, solar radiation over Japan remained relatively constant during 1971–1989 and significantly increased by +5.3 W m−2 per decade during 1990–1999. The present study compares observed trends with those from twentieth century simulations by 14 global climate models in the CMIP3/IPCC‐AR4. Since radiative forcing by aerosols is the primary contributor to long‐term variations in surface solar radiation, the simulations are expected to resemble the observed trends if the input aerosol histories are realistic. To minimize the confounding impact of different cloud realizations in the observations and models, the radiative effects of cloud cover anomalies are removed from the surface solar radiation anomalies via linear regression. Although all of the models exhibit significant dimming trends over China before 1990, the largest model trend is −3.4 W m−2 per decade, less than half the magnitude and significantly different from the observed trend. Models including black carbon aerosol produce stronger decreasing trends than those that do not. The models also fail to reproduce the trend during 1990–1999 over Japan, and the largest model trend is +2.3 W m−2 per decade, only about half of the observed trend. These results suggest that global climate models uniformly underestimate the increase in aerosol radiative forcing over China prior to 1990 and uniformly underestimate the decrease in aerosol radiative forcing over Japan after 1990.