Impact of Cloud Changes on the Radiation Budget Under Global Warming

Clouds play a significant role in our climate system by influencing radiative balances. Global clouds, by changing the amount and optical property, alter the amount of the net radiation by trapping outgoing longwave radiation or reflecting the incoming solar radiation. Therefore, under global warming, the future cloud change and its impact on the radiation budget is a question at issue. This study examined the trend in cloud radiative effect in current models and satellite observations. We targeted Total Cloud Fraction (TCF) and net radiation using the Coupled Model Intercomparison Project-6 (CMIP6) for 1950-2100 over the globe. We utilized historical data (1950-2014) and the shared socio-economic pathway (SSP) (2015-2100) data. This 1950-2100 period spans a period from the rapid growth point in global surface temperature relative to 1850-1900 according to the IPCC AR6 to the future climate provided by the SSP scenarios. The modeled trends of TCF and net radiation are calculated based on linear regression analysis. As a result, net radiation increases by 0.16 W/m²/decade with −0.14 %/decade changing TCF over the globe. In more detail, TCF changes an average of −0.15±0.8 %/decade and −0.20±1.6 %/decade in low and middle latitudes, whereas TCF increases by 0.08±2.5 %/decade in high latitudes for both hemispheres. On the other hand, the net radiation increases an average of 0.1±0.9 W/m²/decade, 0.22±1.06 W/m²/decade, and 0.22±1.38 W/m²/decade in low, middle, and high latitudes for both hemispheres. Therefore, the decrease in TCF may have allowed more solar radiation into the earth, contributing to surface warming in mid and low latitudes. The clear cloud radiative effects will be investigated by the difference between cloudy and clear-sky radiation trends. In high latitudes where surface albedo is high, the increase in TCF does not necessarily mean a decrease in net radiation although increased clouds reflect more incident solar radiation. This is because the reduction of sea ice albedo has a larger effect on the net radiation than the cloud increase. These model results were validated by the Clouds and the Earth’s Radiant Energy System (CERES) satellite data for 2001-2021. The correlation coefficients of TCF between CMIP6 and CERES are an average of −0.05, 0.32, and 0.2 in low, middle, and high latitudes for both hemispheres. Net radiation shows the correlation coefficients as an average of −0.18, 0.41, and 0.26 in low, middle, and high latitudes for both hemispheres. We will show cloud trends for each level (low, middle, and high) using CloudSat and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data during 2007-2015 in order to investigate the contribution of vertical clouds to each zonal mean net radiation trend. This study would contribute to the enhancement of cloud parameterization in climate models.