Location of air pollution has little influence on global radiation budget

Photo: S. Fiedler, MPI-M

Air pollution affects the global radiation budget. Where on Earth the pollution occurs, however, plays a minor role. Whether people pollute the air in Europe and the USA, or in East Asia, changes the global radiative forcing little. This is the result of a new study by Stephanie Fiedler, Bjorn Stevens and Thorsten Mauritsen at the Max Planck Institute for Meteorology (MPI-M) on the change of the global radiation budget by anthropogenic aerosol.

The researchers studied two aerosol distributions: today's observational data and values assumed for the 1970s. In both time periods, the global average air pollution was the same, but its distribution was fundamentally different. Previous studies with complex models showed contradictory results for the impact of the shift in air pollution from Europe and the USA to East Asia on the global radiative forcing. In order to better understand the problem, the MPI-M researchers repeatedly carried out climate experiments with the institute's atmosphere model.

The temporal sequences of weather systems in the model experiments, which cover decades, were varied, but the aerosol distributions were exactly repeated each year. "When we run the experiments long enough, our model shows a very small effect of the aerosol shift on the radiative forcing", says lead author Stephanie Fiedler. The researchers conclude from their simulations that contradictory model results on the magnitude of the global radiative forcing of anthropogenic aerosol can also arise solely through the variability of weather systems. Experiments in previous studies were probably too short to derive a reliable estimate of the effect of the spatial shift of air pollution on the global radiative forcing.

The magnitude of the radiative forcing of anthropogenic aerosol due to the interaction with clouds has long been unclear. Hence, complex aerosol models differently represent anthropogenic aerosol effects on clouds. Compared to the experiments with relatively weaker aerosol effects on clouds, the MPI-M scientists found a moderate influence of the aerosol shift on the global radiative forcing of 15 percent in experiments with a substantially stronger effect of the anthropogenic aerosol on clouds. This result suggests that models with a strong aerosol effect on clouds are more sensitive to aerosol shifts. Future studies will have to examine whether this occurs in aerosol models.

Background
Anthropogenic aerosol are man-made fine particles in the atmosphere. They influence the climate by their interaction with solar radiation and their influence on cloud droplets. The extent of such aerosol effects has been the reason for discussions for decades. On the one hand, complex models for representing the propagation and interaction of aerosol in the Earth system come to different results. On the other hand, it is difficult to separate possible aerosol effects from weather developments in observations. As a result, climate researchers have little confidence in current estimates of the radiative forcing of anthropogenic aerosol. The work of the MPI-M scientists, and the new method used in their study, improve the understanding of the uncertainties related to radiative forcing in climate models. Since their method will also be used in experiments for the next IPCC assessment report, there is new hope to better understand the effect of anthropogenic aerosol on the Earth's climate.

Original publication:
Fiedler, S., Stevens, B. and Mauritsen, T. (2017), On the sensitivity of anthropogenic aerosol forcing to model-internal variability and parameterizing a Twomey effect. J. Adv. Model. Earth Syst., 9, doi:10.1002/2017MS000932.

Contact:

Dr Stephanie Fiedler
Max Planck Institute for Meteorology
Phone: +49 40 41173 416
Email: stephanie.fiedler@we dont want spammpimet.mpg.de