Although emissions of CO2 are the largest anthropogenic contributor to the risks of climate change, other substances are important in the formulation of a cost-effective response. To provide improved facilities for addressing their role, we develop an approach for endogenizing control of these other greenhouse gases within a computable general equilibrium (CGE) model of the world economy. The calculation is consistent with underlying economic production theory. For parameterization it is able to draw on marginal abatement cost (MAC) functions for these gases based on detailed technological descriptions of control options. We apply the method to the gases identified in the Kyoto Protocol: methane (CH4), nitrous oxide (N2O), sulfur hexaflouride (SF6), the perflourocarbons (PFCs), and the hyrdoflourocarbons (HFCs). Complete and consistent estimates are provided of the costs of meeting greenhouse-gas reduction targets with a focus on "what" flexibility — i.e., the ability to abate the most cost-effective mix of gases in any period. We find that non-CO2 gases are a crucial component of a cost-effective policy. Because of their high Global Warming Potentials (GWPs) under current international agreements they would contribute a substantial share of early abatement.

© 2003 Kluwer Academic Publishers

The Kyoto Protocol is an international agreement aimed at limiting emissions of several greenhouse gases (GHGs; specifically: CO2, CH4, N2O, PFCs, HFCs, and SF6), and allows credit for approved sinks for CO2. It does not include consideration of several other trace atmospheric constituents that have important indirect effects on the radiative budget of the atmosphere. Here we show that inclusion of other GHGs and CO2 sinks greatly reduces the cost of achieving CO2 emissions reductions specified under the agreement. The Kyoto Protocol extrapolated to 2100 reduces predicted warming by only about 17%. The errors caused by simulating other GHGs with scaled amounts of CO2 on atmospheric composition, climate, and ecosystems are small. Larger errors come from failure to account for interactive and climatic effects of gases that affect atmospheric composition but are not included in the protocol (CO, NOx, and SOx). Over the period to 2100, the Global Warming Potential (GWP) indices based on a 100-year time horizon as specified in the protocol appear to be an adequate representation of trace gas climatic effects. The principal reason for the success of this simplified GWP approach in our calculations is that the mix of gas emissions resulting from a carbon-only rather than a multi-gas control strategy does not change by a large amount.

In the effort to understand and address global climate change, most analysis has focused on rapidly rising emissions of carbon dioxide (CO₂) and options for reducing them. Indeed, carbon dioxide, a byproduct of fossil fuel combustion, is the principal greenhouse gas contributing to global warming. However, other greenhouse gases including methane, nitrous oxide, and a number of industrial-process gases also are important contributors to climate change. From both an environmental and an economic standpoint, effective climate strategies should address both carbon dioxide and these other greenhouse gases.

Non-CO₂ gases account for 17 percent of total greenhouse gas emissions in the United States and a much larger percentage in developing countries such as India and Brazil. In addition, a host of local and regional air pollutant emissions interact in the atmosphere’s complex chemistry to produce either additional warming or cooling effects. Understanding how these gases interact—and how to craft policies that address a range of environmental impacts—is vital to addressing both local and global environmental concerns.

In this report, authors John Reilly, Henry Jacoby, and Ronald Prinn of M.I.T. unravel some of the complexities associated with analyzing the impacts of these multiple gases and opportunities for reducing them. Emissions originate from a wide range of sectors and practices. Accurate calculation of emissions and emission reductions is easier for some sources than for others. For policy purposes, various greenhouse gases are compared on the basis of “global warming potentials,” which are based on the atmospheric lifetime of each gas and its ability to trap heat. However, these do not yet accurately capture the climatic effects of all the substances contributing to climate change and so must be used with some caution. While scientists have recognized the various roles of non-CO₂ gases and other substances that contribute to climate change for some time, only recently have the various pieces of the puzzle been fit together to provide a more complete picture of the critical role these gases can play in a cost-effective strategy to address climate change.

Using M.I.T.’s general equilibrium model, the authors demonstrate that including all greenhouse gases in a moderate emissions reduction strategy not only increases the overall amount of emissions reductions, but also reduces the overall cost of mitigation: a win-win strategy. In fact, due to the high potency of the non-CO₂ gases and the current lack of economic incentives, this analysis concludes that control of these gases is especially important and cost-effective in the near term. The policy implications are clear: any attempt to curb warming should include efforts to reduce both CO₂ and non-CO₂ greenhouse gases.

Under the Kyoto Protocol, reductions in emissions of several radiative gases can be credited against a carbon equivalent emissions cap. We investigate the economic implications of including other greenhouse gases and sinks in the climate change control policy using our revised and updated version of the Emissions Prediction and Policy Analysis (EPPA) model. In addition we amended our methane abatement curves based on different interpretations of estimates that substantial abatement of methane can be obtained at no cost. The inclusion of other greenhouse gases and CO2 sinks reduces the costs of achieving CO2 emissions reductions specified under the agreement.