Conclusions

Multi-Gas Contributors to Global Climate Change: Climate Impacts and Mitigation Costs of Non-CO2 Gases

Conclusions

Of the various GHGs emitted by human activities, CO₂ is the largest contributor to climate change. With its relative role expected to increase in the future, a continuing emphasis on reducing CO₂ emissions is therefore justified. However, to effectively limit climate change, and to do so in a cost-effective manner, climate policies must also take into account the importance of non-CO₂ greenhouse gases. Improvements in our ability to measure and assess the non-CO₂ gases in recent years have made it clear that their control is an essential part of a cost-effective climate policy. Efforts to engage developing countries in climate mitigation will need to give even greater attention to the non-CO₂ greenhouse gases since these gases typically account for a higher percentage of their overall emissions. Non-CO₂ gases currently account for well over one-half of the GHG emissions in Brazil and India, for example, as compared to 20 percent in the United States.

The non-CO₂ greenhouse gases that human activities emit directly include methane (CH₄), nitrous oxide (N₂O), and a group of industrial gases that include perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF₆). In addition to these, several other substances play a role in retarding or enhancing the greenhouse effect, but are not included in existing climate policy agreements. These include carbon monoxide (CO), nitrogen oxides (NO_x), sulfur oxides (SOX), aerosols, non-methane volatile organic compounds (NMVOCs), and ammonia. These gases and aerosols are related to one another by their common generation in industry and agriculture as well as by their interaction in the chemistry of urban areas, the lower atmosphere, and the stratosphere.

Modeling studies indicate that a cost-effective abatement strategy would focus heavily on the non-CO₂ gases in the early years. The relative value of controlling non-CO₂ gases, as indicated by the indices or weights known as global warming potentials, is one key reason that inclusion of the non-CO₂ gases in policies to address climate change can be so effective in lowering implementation costs, especially in the short term. Another reason is that, historically, economic instruments (i.e., prices, taxes, and fees) have not been used to discourage or reduce emissions of non-CO₂ gases, even though price signals via energy costs exist to curb fossil fuel emissions. Given the high carbon-equivalent values of the non-CO₂ gases, even a small carbon-equivalent price on these gases could create a huge incentive to reduce emissions. For smaller percent reductions, such as a case where total GHG emissions in the United States are held at year 2000 levels through 2010, nearly all of the cost-effective cuts would come from the non-CO₂ gases. Including the abatement options available for these gases would reduce the carbon-equivalent price of the policy by two-thirds from that needed if the same level of abatement were achieved only through reductions in CO₂ emissions from fossil fuels.

Designing an effective approach to controlling these multiple substances requires that a number of issues be resolved. A major issue is the challenge of finding a more accurate way of accounting for the independent effects of each gas on climate and of comparing their relative reductions. The GWP indices are currently used for doing so, but analysis has shown that the current GWPs significantly underestimate the role of methane. This is due in part to omitted interactions such as the role of methane in tropospheric ozone formation. GWPs also fail to adequately portray the timing of the climate effects of abatement efforts. Economic formulations of the GWP indices have been proposed that would address these concerns, but calculations using these economic-based formulae are bedeviled by a variety of deeper uncertainties, such as how to monetize the damages associated with climate change.

A still more difficult issue is whether and how to compare efforts to control other substances that affect the radiative balance of the atmosphere, such as tropospheric ozone precursors, black carbon, and cooling aerosols. These pollutants have immediate effects on human health, crop productivity, and ecosystems. Further, their climatic effects are mainly regional or even local, which creates difficulties in using a single index to represent the effects of emissions across the globe. It is essential to consider these substances as part of a climate policy, but more research and analysis is needed to quantitatively establish their climate influence and to design policies that take into account their local and regional pollution effects.

Another major concern in including non-CO₂ gases in a control regime is whether their emissions can be measured and monitored accurately so that, whatever set of policies are in place, compliance can be assured. The ability to monitor and measure has less to do with the type of GHG than with the nature of the GHG source—i.e., it is far easier to measure and monitor emissions from large point sources, such as electric power plants, than from widely dispersed non-point sources, such as automobile and truck tailpipes or farmers’ fields. Resolving this difficulty may require different regulatory approaches for different sources, at least initially.

There remain large uncertainties in scientific estimates of the relative climate-changing properties of the various greenhouse gases and aerosols. But these uncertainties do not change the basic conclusion that control of non-CO₂ greenhouse gases is a critical component of a cost-effective climate policy, particularly in the near term, and a key complement to carbon dioxide control efforts.