Thirty-Five Year Record of Shortwave TOA radiative cloud forcing from SBUV Measurements

Abstract:

A thirty-five year record (1980-2014) of Shortwave TOA radiative cloud forcing is derived from UV Lambertian-Equivalent (LER) reflectivity data constructed using measured upwelling radiances from Nimbus-7 SBUV and seven NOAA SBUV/2 polar orbiting instruments. The approach is to scale the dimensionless UV LER data to match the CERES Shortwave Cloud radiative forcing when they are concurrent (2000-2014). The derived scaling factors are then used to produces a CERES-proxy of SW cloud forcing back to 1980. The good agreement between trends and anomalies of the CERES-proxy and CERES Shortwave Cloud forcing records during the overlapping data period supports using this new data set for extended climate studies.

During 1980-2000 period of globally warming surface temperatures, our record of shortwave cloud forcing shows a widespread global increase in forcing from reduced cloudiness. However, during the more recent pause in global warming 2000-2012, our record shows a reduction in SW cloud forcing from increased cloudiness.

Historical CMIP5 AMIP simulations from eight models do not capture the 1980-2000 reduction in cloudiness. Twenty-year segments of future RCP45 long-term runs with comparable surface temperature warming to that observed from 1980-2000, also do not simulate our observed reduction in cloud amount. This suggests that the climate sensitivity in the tested CMIP5 models is too low.