WP 6: Regional and global climate effects

Overview and background

Atmospheric aerosols and greenhouse gases are well known to have environmental and climatic effects at the global and the regional scale (IPCC, 2001). Although for policy purposes air quality and climate change are often considered separately, they are inextricably linked, since the same emitted species are responsible for both. Urban activities related to the development of megacities emit large amounts of pollutants and aerosols (Molina and Molina 2004; Lawrence et al., 2006). These can be expected to substantially alter climate in the surrounding environment and possibly in remote regions via long range transport. This effect will be ever more important in the future, as megacities in the world will grow due to the population flux from rural to urban areas. In addition, climate change can affect urban air quality by changing regional climate patterns. The emissions of air quality pollutants being studied in WPs 4 and 5 will nearly all have effects on climate. The main objective of this WP is to quantify the climate effects of megacity emissions on both regional and global scales. Emissions of primary aerosols (such as back or organic carbon) will have a direct climate impact. Emissions of reactive gases (such as NOx, SO2, volatile organic compounds) will form ozone or secondary aerosols and will affect the lifetime of methane. The spatial extent of the climate impacts of megacity emissions depends largely on the lifetime of the species in the atmosphere, and for short-lived species their impacts will depend particularly on the location and regional characteristics of the megacities from which they is emitted. Short-lived species (such as sulphate aerosols) will have their greatest impact on the regional scale close to the sources, whereas long-lived species (such as ozone) will impact larger scales. Carbon dioxide and methane, although not considered air quality pollutants, will have global climate impacts.

Methodology and advancement beyond the state-of-the-art

The tasks in this WP are designed to explore these differences. It is critical to assess the effects of megacity emissions on climate and possible feedbacks between these effects and the emissions, both in present day and future conditions. This goal can be achieved through the use of combined air quality and climate models or through the use of fully coupled chemistry-climate modelling systems. In the latter case simplified chemistry/aerosol modules need to be used for computational requirements. These two strategies are complimentary in that the un-coupled one allows the use of more detailed atmospheric composition schemes, while the coupled one allows us to capture feedback effects. Therefore, in this WP both strategies will be used and will be inter-compared to examine the respective advantages and limitations. Regional and global modelling components are essential in the present WP. By using regional and global climate and air quality models in comprehensive studies of coupled and uncoupled mode in this WP, we will quantify the patterns of surface temperature changes and other important meteorological variables. The focus of the regional component will be on megacities in Europe and surrounding regions (such as north Africa). In addition, at least one non-European region where megacities can have important climatic effects will also be investigated. Candidate regions for this purpose are East Asia and central America, where large megacities are expected to develop and global warming is expected to have large impacts.

This WP will take as a basis the fields of constituents calculated and analysed in WP5 and use these within global and regional climate models to quantify the climate impacts. Additional studies will use the same emission fields as in WP5, but simulate the constituent fields in interactive mode with the climate and atmospheric composition evolving together. The constituent concentrations will have been evaluated in WP5, but additional evaluation will take place in this WP to compare the radiative forcings and aerosol optical properties against measurements. On both scales (global and regional) the WP will focus on the climate effects from both direct aerosol/gas-radiation interaction and indirect aerosol/gas-cloud interactions. WP6 will also provide meteorological driving fields for future climate conditions to WP5. Finally, the climate impacts of well-mixed greenhouse gases (e.g., CO2) emitted from megacities will be quantified using simple analytical formulae (Ramaswamy et al., 2001).

FP7 EC MEGAPOLI, 2008-2011