Urban air pollution and climate are closely connected due to shared generating processes (e.g., combustion) for emissions of the driving gases and aerosols. They are also connected because the atmospheric lifecycles of common air pollutants such as CO, NOx and VOCs, and of the climatically important methane gas (CH4) and sulfate aerosols, both involve the fast photochemistry of the hydroxyl free radical (OH). Thus policies designed to address air pollution may impact climate and vice versa. We present calculations using a model coupling economics, atmospheric chemistry, climate and ecosystems to illustrate some effects of air pollution policy alone on global warming. We consider caps on emissions of NOx, CO, volatile organic carbon, and SOx both individually and combined in two ways. These caps can lower ozone causing less warming, lower sulfate aerosols yielding more warming, lower OH and thus increase CH4 giving more warming, and finally, allow more carbon uptake by ecosystems leading to less warming. Overall, these effects significantly offset each other suggesting that air pollution policy has a relatively small net effect on the global mean surface temperature and sea level rise. However, our study does not account for the effects of air pollution policies on overall demand for fossil fuels and on the choice of fuels (coal, oil, gas), nor have we considered the effects of caps on black carbon or organic carbon aerosols on climate. These effects, if included, could lead to more substantial impacts of capping pollutant emissions on global temperature and sea level than concluded here. Caps on aerosols in general could also yield impacts on other important aspects of climate beyond those addressed here, such as the regional patterns of cloudiness and precipitation.

© 2007 Cambridge University Press

The experience of other environmental problems suggests that policies yielding uniform marginal costs across sectors, as most analyses assume, are not likely to be realized in practice. Some sectors will be favored over others, yielding different levels of control. Using the MIT Emissions Prediction and Policy Analysis Model, the national cost of such differentiation across sectors is shown to be very high. Moreover, because of interactions and feedbacks in the economy, measures that differentiate in this way may not even aid the sectors they are intended to protect.

This technical note documents the inventory of non-CO2 greenhouse gas (GHG) and traditional air pollutant emissions for the MIT Emissions Prediction and Policy Analysis model version five (EPPA 5). The non-CO2 GHG species considered include methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Traditional air pollutants include carbon monoxide (CO), sulfur dioxide (SO2), nitrous oxides (NOx), ammonia (NH3), black carbon (BC), organic carbon (OC), and non-methane volatile organic compounds (NMVOCs). In considering non-CO2 GHG data sets, we evaluate bottom-up inventories from the Emissions Database for Global Atmospheric Research version 4.1 (EDGAR v4.1), the “U.S. Environmental Protection Agency Global Non-CO2 Anthropogenic Emissions: 1990-2020” report (EPA 2006), and a recent inventory from the Global Trade Analysis Project (GTAP v7). For traditional air pollutants we consider EDGAR v4.1 and EDGAR-HTAP v1. Since EPPA 5 is also used in connection with the MIT Integrated Global System Model (IGSM) to study environmental effects, good agreement with measured GHG concentrations is crucial and we compare bottom-up and top-down estimates to gauge for consistency. We conclude that the EDGAR v4.1 inventory is best suited for benchmarking non-CO2 GHGs in EPPA 5 due to good disaggregation between economic sectors and species, and because it provides the closest fit with top-down estimates.

In the absence of significant greenhouse gas (GHG) mitigation, many analysts project that atmospheric concentrations of species identified for control in the Kyoto protocol could exceed 1000 ppm (carbon-dioxide-equivalent) by 2100 from the current levels of about 435 ppm. This could lead to global average temperature increases of between 2.5° and 6° C by the end of the century. There are risks of even greater warming given that underlying uncertainties in emissions projections and climate response are substantial. Stabilization of GHG concentrations that would have a reasonable chance of meeting temperature targets identified in international negotiations would require significant reductions in GHG emissions below “business-as-usual” levels, and indeed from present emissions levels. Nearly universal participation of countries is required, and the needed investments in efficiency and alternative energy sources would entail significant costs. Resolving how these additional costs might be shared among countries is critical to facilitating a wide participation of large-emitting countries in a climate stabilization policy. The 2°C target is very ambitious given current atmospheric concentrations and inertia in the energy and climate system. The Copenhagen pledges for 2020 still keep the 2°C target within a reach, but very aggressive actions would be needed immediately after that.