A long-term decline in downward surface solar radiation

Downward surface solar radiation (DSSR) is critical for the Earth system. It is well-known that DSSR over land has fluctuated on decadal timescales in the past. By utilizing a combination of station observations and the latest CMIP6 simulations, here we show that DSSR had a global consistent decline during 1959-2014, with comparable contributions from greenhouse gases (GHGs) and anthropogenic aerosols. The role of GHGs is even more important in the satellite period. The contribution from GHGs comes through rising temperature, which reduces the DSSR by increasing water vapor but is partly offset by reduced cloud. Future changes of DSSR are heavily dependent on climate change scenarios, which can be predicted well by global mean surface temperature (GMST) and aerosol concentrations. The sharp aerosol reduction and weak temperature rise in the SSP245/SSP126 scenarios will limit or stop the long-term decline of DSSR thus leading to a brighter future. Increased greenhouse gases reduced the solar radiation reaching the Earth surface as it was absorbed more by water vapor in the air. This is the same important as the aerosols in the historical period, which reduced the solar radiation by reflecting it back into the space.