Background Arctic temperatures have increased at almost twice the global average rate over the past 100 years. Warming in the Arctic has been accompanied by an earlier onset of spring melt, a longer melt season and changes in the mass balance of the Greenland ice sheet. The lengthening of the melt season changes the Earth’s albedo, a positive feedback effect which leads to further warming. Reducing the atmospheric burden of carbon dioxide (CO2) is the only meaningful way to mitigate climate forcing in the Arctic. However, reducing the concentration of short-lived climate forcing agents, such as black carbon (BC), might be used to slow the process, allowing time for other measures to take effect. BC is produced by the incomplete burning of fossil fuels and is the major constituent of soot; it absorbs solar radiation and when deposited on snow and ice darkens their surfaces and accelerates melting. Recent research has estimated that BC is the second most important pollutant, after CO2, forcing climate change. The advantage of reducing emissions of short-lived agents like BC is that the effect on the radiative balance of the atmosphere is noticed much quicker than with long-lived greenhouse gases. Objectives The MACEB project’s objective was to demonstrate, by using the best available tools, an innovative approach to mitigating warming of the Arctic climate by reducing black carbon (BC) emissions at mid latitudes. The project sought to identify knowledge gaps and uncertainties in how BC emissions can be linked to radiative forcing in Arctic areas, by using existing modelling and measurement tools. MACEB aimed to assess the impact of air quality and climate-relevant legislation on BC emissions and their transport to the Arctic, and to evaluate an extensive set of mitigation measures targeting BC emissions that could contribute to existing European legislation relating to climate change and air quality. Results The MACEB project carried out a black carbon (BC) inventory at several observations sites in Finland. Emission sources of black carbon are mostly anthropogenic, with the only natural sources being vegetation and forest fires. In general, residential combustion and the transportation sector accounted for around 80-90% of black carbon emissions in Finland. Coordinating beneficiary the Finnish Meteorological Institute (FMI) collaborated with three associated beneficiaries - the Finnish Environment Institute (SYKE), the International Institute for Applied Systems Analysis (IIASA) in Austria, and the Aerosol Research Group from the University of Helsinki - to assess possible measures to reduce BC emissions, thereby helping to mitigate Arctic warming. The black carbon inventory results were combined within an innovative integrated modelling system. This integrated model was run with Finnish and European-wide emissions data, with 2005 used as a base year for emissions and projections generated for 2020 and 2030. The modelling system was developed to assess black carbon emissions from different areas and source categories, and was linked with black carbon concentrations in surface air and snow over Arctic areas and their corresponding radiative forcing (warming). The final operational toolbox enables a much greater level of interaction between the research community and policy makers in terms of assessing the effects of black carbon. Monthly average concentrations maps for black carbon in surface air and snow in the Arctic were created, along with simulated concentrations for current and future climate emission scenarios. Simulated black carbon concentrations were converted into corresponding monthly-mean concentration maps over Arctic areas. Further, the project calculated the black carbon radiative forcing over the spring-summer period. Forcing was determined for both current and future climate predictions, and compared with forcing by greenhouse gases. Black carbon emissions are not subject to direct climate change-related legislation, but are affected indirectly by air pollutant emission legislation such as that controlling particulate matter (PM) emission limits for vehicles. The relationship between EU air pollution legislation and climate policies, and black carbon emissions, was analysed by enumerating a large number of existing and relevant directives. A number of regulatory framework issues could benefit from information generated by the project, including the Directive on the sulphur content in liquid fuels, MARPOL implementation regarding sulphur content of marine fuels, and the Fuel Quality directive on the quality of petrol and diesel fuels. The project produced a priority list for further mitigation actions to reduce black carbon emissions in the European Union and an estimate of their cost with respect to climate impacts in the Arctic. MACEB revealed that in the current legislation and regulatory framework there are a number of technically-feasible measures that could be implemented to effectively regulate black carbon emissions. These utilise already existing technology at low marginal cost. For instance, the setting of emission limits as product standards for new residential combustion equipment and for road vehicles. The project also concluded that large black carbon emission reductions could be reached through non-technical measures, especially informing and educating people of the adverse impacts of black carbon on human health. The project website was designed to disseminate information on black carbon emissions and mitigation measures to a wide audience (e.g. the scientific community, local authorities, and the general public) globally. The toolbox developed by the LIFE MACEB project in Finland is readily transferable for use in other countries, for example, to analyse different mitigation strategies or to identify the relative black carbon emission contributions of different areas (e.g. USA, China). Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).