This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations.
This project seeks to determine whether the current generation of 20th century climate simulations can reproduce the observed record of surface insolation, particularly the periods of solar "dimming" and "brightening". While past radiative forcing by greenhouse gases is reasonably well known, the record of aerosol forcing - the presumed agent of dimming and brightening - is not. The project is motivated by the fact that the earlier generation of models used for the Fourth Assessment report (AR4) did not successfully reproduce the observed dimming and brightening over Europe and East Asia, two regions where earlier research shows that insolation trends were driven by aerosols rather than changes in clouds. Given that the observed trends were driven by aerosols, the lack of agreement with models suggests that the aerosol histories used in the AR4 simuations were not accurate, hence research in the present project tests for improvements in aerosol-driven insolation variability between models used for AR4 and AR5. The work relies on a novel method for the removal of cloud cover effects in the computation of insolation trends, which is useful since the models do not have the same cloud and weather history as the observations.
The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. This project seeks in particular to identify and reduce uncertainty in aerosol radiative forcing, which will contribute to reducing uncertainty regarding the sensitivity of earth's climate to greenhouse gas increases.
One large uncertainty in our understanding of climate change is that we do not know very well how much pollution particles in the atmosphere have changed the amount of solar radiation absorbed by the climate system. The amount of pollution particles in the atmosphere has changed over time in different regions of the world in response to economic development, population growth, and clear air laws. Since widespread measurements of how the amount of pollution particles have changed over the past fifty years are not available, the amount of pollution particles must instead be indirectly inferred. Pollution particles in the atmosphere reflect and absorb solar radiation, thus reducing the amount of solar radiation reaching the surface of the Earth. Measurements of solar radiation at the surface are more available, and thus we use long-term measured changes in solar radiation as a proxy for long-term changes in the amount of pollution particles. We find that solar radiation has decreased over China and India due to increasing pollution over time. In Europe and Japan, solar radiation decreased prior to the mid 1980s and then increased, indicating that pollution increased prior to the mid 1980s and then decreased. Global climate models that use our best guess of the time history of pollution particles are unable to reproduce the magnitude and timing of observed regional multidecadal changes in solar radiation at the surface of the Earth. This suggests that global climate models may underestimate the amount or the radiative impact of pollution particles in the atmosphere. By using measurements of solar radiation as a constraint, we may be able to better understand how the amount of pollution particles has changed in the atmosphere over the past fifty years. This would enable us to better understand how Earth's climate will change in the next fifty years and what impacts that might have on human society.