New estimates of the future effect of air pollution on Earth´s radiation budget

Almost all IPCC scenarios for 2015 to 2100 show a reduction in the air pollution by anthropogenic aerosols for the future, but their effects on the radiation budget are different. Dr Stephanie Fiedler, scientist in the department "The Atmosphere in the Earth System" at the Max Planck Institute for Meteorology (MPI-M), and her co-authors come to this conclusion in a new study published in Geoscientific Model Development.

The changes in air pollution until 2100 are based on the new future scenarios that the Intergovernmental Panel on Climate Change (IPCC) will use for their new assessment report on climate change. For the first time, these scenarios were interpreted with the help of climate simulations that use the parameterization of anthropogenic aerosols developed at MPI-M. In order to understand the impact of the new scenarios on the Earth's radiation budget, the authors performed several climate simulations with MPI-M´s atmospheric model ECHAM6.3 and the new scenarios for anthropogenic aerosols. In the experiments, the temporal development of weather systems was slightly modified, but the aerosol distributions were annually repeated. "When we run the simulations long enough, we can determine the effect of anthropogenic aerosols on the global radiation budget of the model with a good precision", explains Dr Stephanie Fiedler. Based on the model analysis, the scientists were able to estimate the uncertainty in anthropogenic aerosol forcing due to the latest assumptions for the societal, economic and technological developments. By the end of the 21st century, this uncertainty is 30-108 % of the anthropogenic aerosol forcing for the year 2005.

The magnitude of the radiative forcing of anthropogenic aerosols due to their interaction with clouds is still unclear. Therefore, complex climate models also calculate different impacts of aerosol changes on clouds. The implication of such uncertainty for the results of the new model estimate of the aerosol forcing at the end of the 21st century was further investigated. The authors performed experiments with a particularly strong effect of anthropogenic aerosols on clouds and found an almost twice as strong change in the future radiation budget as in their previous estimates. This result implies that the anthropogenic aerosols would more strongly counteract the warming effect of greenhouse gases than indicated in the previous calculations by the authors. Therefore, the magnitude of the relative cooling effect by anthropogenic aerosols depends on the sensitivity of clouds to anthropogenic changes of aerosols, which is currently not well understood. Future studies have to address the relevance of such aerosol-cloud effects for the global climate.

Background: Anthropogenic aerosol are man-made fine particles in the atmosphere. They influence the climate through their effect on solar radiation and cloud droplets. The strength of these aerosol effects has been discussed for decades. On the one hand, complex models for representing the transport and interaction of aerosols 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. Climate researchers therefore have little confidence in current estimates of the radiative forcing of anthropogenic aerosol. The new method used in the study improves the understanding of the uncertainties related to radiative forcing in climate models. Since it will also be used in experiments for the next IPCC assessment report, there is new hope to better understand the effect of anthropogenic aerosols on the Earth's climate.

Original research article:

Fiedler, S., Stevens, B., Gidden, M., Smith, S. J., Riahi, K., and van Vuuren, D. (2019) First forcing estimates from the future CMIP6 scenarios of anthropogenic aerosol optical properties and an associated Twomey effect, Geosci. Model Dev., 12, 989-1007, doi:10.5194/gmd-12-989-2019.

Dr Stephanie Fiedler
Max Planck Institute for Meteorology
Phone: +49 (0) 40 41173 416
Email: stephanie.fiedler@we dont want