Determining the impact of air pollution on climate

Air pollutants like aerosol particles play an important role in the Earth's climate, because the interfere with the radiative transfer through the atmosphere.

In the Climate Change Unit we are interested how past as well as future changes in air pollution impact on climate. To study the effect air pollutants have on the Earth's climate we apply the state-of-the-art ECHAM5-HAM model. ECHAM5 is a comprehensive general circulation model of the atmosphere developed at the Max Planck Institute for Meteorology, Germany. ECHAM5 has been extended by a complex microphysical aerosol model (HAM), which allows the assessment of aerosol impacts on climate (Stier et al, 2005). With this ECHAM5-HAM gives the framework to study the combined effect of changes in greenhouse concentrations and aerosols. Such an integrated approach is important, as changes in greenhouse gas concentrations have a different effect on climate compared to changes in aerosols and it is expected that these different climate effects add in a non-linear way.

ECHAM5-HAM considers the major aerosol compounds in the atmosphere: sulfate, black carbon, organic carbon, sea salt and mineral dust. Aerosols arise from natural sources, such as dust emissions, sulfate emissions from volcanoes or sea spray. They also have an anthropogenic source, in particular the combustion of fossil- and bio-fuels. Aerosol and aerosol - precursor emission data are compiled by various databases, such as e.g. EDGAR. ECHAM5-HAM includes a sulfur chemistry scheme simulating the oxidation of SO2 into sulfate. Is uses prescribed offline oxidant concentrations as simulated within chemistry transport models, e.g. TM5.

We apply ECHAM5-HAM for long-term equilibrium climate simulations with various air pollutant scenarios, to investigate the trade-offs and co-benefits of air pollution mitigation policies in the means of climate change. In an equilibrium simulation ECHAM5-HAM uses aerosol emissions and greenhouse gas concentrations of a specific year and keeps them constant during a model run that spans several decades until the model is in equilibrium, the last decades are then used to derive statistics of the climate. These simulations offer the advantage of capturing climate variations on annual and decadal timescales, and therefore they are statistically the most robust method to investigate the response of the climate system.