Global Short-Lived Climate Forcers/Regional Air Quality Regulations
Critical interactions can occur between policies aimed at reducing greenhouse gas emissions and those targeted on reducing emissions of criteria pollutants like sulfur oxides, nitrogen oxides, mercury, volatile organic compounds and particulate matter. These interactions have to do with the level of the emissions or concentration targets set, retirement of old equipment, development of new control technologies and development of new substitute technologies. A number of groups have now demonstrated that controlling end of pipe emissions in the short run and GHG emissions a bit later can, under certain conditions, lead to much higher costs than the case where emission free technologies are substituted for fossil fuel based technologies before the end of pipe control decisions need to be made. There are now enough models in the U.S., Europe and possibly Asia to implement a productive and informative model comparison exercise in this area. Such a study would be difficult to implement with global scale models at present and, therefore, might be gainfully conducted in parallel with the next study which does require global scale models to be executed. A key issue in analyses of the cost, economic and energy system impacts of achieving global climate targets like limiting the global mean temperature relative to pre-industrial levels or limiting the total radiative forcing of the atmosphere, is accounting for the impacts of non-Kyoto gas emissions (generally short lived particles of sulfur or carbon and tropospheric ozone pre-cursors) and control on our ability to achieve those targets. There are large scientific uncertainties about the direct and especially the indirect (generally through effects on cloud formation) of these substances. In addition, the location of the emissions and size of the particles can have a large impact on the concentrations observed through meteorological processes (e.g., wind speeds and directions, sunlight, and inversion layer height). Finally, the emissions themselves are determined by industrial and individual technology choices, air quality standards and control technology development all of which are highly uncertain.
There are a number of models that include consideration of these substances Scenarios could include various assumptions about emissions drivers, both joint and individual species controls and a wide range of forcing assumptions for each species. Given the possible difference in emissions profiles between biofuels and conventional fossil fuels, this study could also be usefully co-ordinated with the bio-energy study discussed next.