Let's not get stuck on CO2 - other gas emissions may be easier to reduce

Most discussions of climate change have focused on the human contribution to increasing atmospheric concentrations of CO2 and on strategies to limit its emissions from fossil fuel use. Among long-lived greenhouse gases (GHGs) from human activity, CO2 is the largest contributor to climate change and its relative role is expected to increase in the future.

An emphasis on CO2 is justified but the near-exclusive attention to this single contributor to global warming has directed attention away from other GHGs, where some of the most cost-effective abatement options exist. The non-CO2 GHGs emitted directly by human activities include methane (CH4), nitrous oxide (N2O), and a group of industrial gases including perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF6). Together with the already banned chlorofluorocarbons (CFCs), the climate significance of these gasses over the past century is roughly equivalent to that of CO2.

In next half-century, it is also the case that feasible reductions in methane and other non-CO2 gases can make a contribution to slowing global warming as large as, or larger than, similarly feasible reductions in CO2. To effectively limit climate change in a cost-effective manner requires climate policies that deal with CO2 and non-CO2 gases alike.

There are several reasons why attention has been focused on CO2 even though the full list of GHGs is targeted for control under international climate agreements. Emissions of CO2 from fossil sources are readily estimated from market data on fuel use and the analysis of abatement options for fossil emissions benefits from decades of research on energy markets, energy efficiency, and alternative energy supply technologies - work that was spurred by concerns about the security of supply and prices of fossil fuels. The analytical capability developed to study energy markets was then readily applied to the climate issue.

Now that the capability to measure and assess the non-CO2 GHGs has improved, it is clear that their control is also an essential part of a cost-effective climate policy.

In addition to the main non-CO2 GHGs identified above, there are other emissions from human activities that are not included in existing climate policy agreements but that nonetheless retard or enhance the greenhouse effect.

Tropospheric ozone (O3) is a natural greenhouse constituent of the atmosphere. Emissions of carbon monoxide (CO), nitrogen oxides (NOX), aerosols, non-methane volatile organic compounds (NMVOCs), and ammonia (NH3) all affect the chemistry of tropospheric ozone and methane. Black carbon or soot, though not well understood, is thought to contribute to warming as well. Other human emissions have the opposite of a greenhouse effect. Sulfur dioxide (SO2) and nitrogen oxides (NOX), mainly from fossil fuel combustion, are converted by chemical processes in the atmosphere into cooling aerosols. These various 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. Thus, policies that reduce CO2 also may affect emissions of SO2, NOX, and CO, as well as the non-CO2 greenhouse gases.

Designing a cost-effective approach for control of these multiple substances requires some way of accounting for the independent effects of each on climate. The current method for doing so is a set of indices or weights known as global warming potentials (GWPs). These have been developed for the main GHGs but not for SO2 and other local and regional air pollutants. By design, the GWP for CO2 is 1.0 and the values for other GHGs are expressed in relation to it. These indices attempt to capture the main differences among the gases in terms of their instantaneous ability to trap heat and their varying lifetimes in the atmosphere. By this measure, for example, methane is, ton-for-ton, more than 20 times as potent as CO2, while N2O is about 300 times as potent, and the industrial gases are thousands of times as potent when taking into account the atmospheric effects of these gases over the next 100 years.

The relative value of controlling non-CO2 gases, as expressed by these GWPs, 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, particularly in the early years. Given the high carbon-equivalent values of the non-CO2 gases, even a small carbon-equivalent price on these gases would create a huge incentive to reduce emissions. 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, whereas price signals exist via energy costs to curb CO2 emissions from fossil fuels.

If, for example, the total GHG emissions reduction required to meet a target were in the order of 10 or 15 per cent, as would be the case if total GHG emissions in the United States were held at year 2000 levels through 2010, nearly all of the cost-effective reductions could come from the non-CO2 greenhouse gases. Compared to a particular reduction achieved by CO2 cuts alone, inclusion of the non-CO2 abatement options could reduce the carbon-equivalent price of such a policy by two-thirds.

This large contribution of the non-CO2 gases, and their potential effect on lowering the cost of a climate policy, is surprising because it is disproportionate to their roughly 20 per cent contribution to total U.S. GHG emissions. In developing countries like India and Brazil, non-CO2 gases currently account for well over one-half of GHG emissions. Any cost-effective effort to engage developing countries in climate mitigation will, therefore, need to give even greater attention to the non-CO2 gases.

Control of these gases is only part of an effective response to the climate threat. Even if they were largely controlled, we would still be left with substantial CO2 emissions from energy use and land-use change. Over the longer term, and as larger cuts in GHGs are required, the control of CO2 will increase in its importance as an essential component of climate policy.

There remain a number of uncertainties in calculating the climatic effects of non-CO2 gases. One of these is the accuracy of global warming potentials. Analysis has shown that the GWPs currently in use significantly underestimate the role of methane. This error is due in part to omitted interactions, such as the role of methane in tropospheric ozone formation. Any correction of this bias would amplify the importance of the non-CO2 greenhouse gases.

The GWPs also fail to adequately portray the timing of the climate effects of abatement efforts. Because of its relatively short lifetime in the atmosphere, abatement efforts directed at methane have benefits in slowing climate change over the next few decades, whereas the benefits of CO2 abatement are spread over a century or longer.

To the extent one is concerned about slowing climate change over the next 50 years methane and HFCs - that last a decade or so - have an importance that is obscured when 100-year GWPs are used to compare the contributions of the various gases. Economic formulations of the GWP indices have been proposed that would address these concerns, but calculations are bedeviled by 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. The main issue with these substances is that, even though their climatic effects are important, a more immediate concern is that they also cause local and regional air pollution affecting human health, crop productivity, and ecosystems. Moreover, their climatic effects are mainly regional, or even local, which creates difficulties in designing a single index to represent their effects across the globe. It is essential to consider these substances as part of climate policy, but more research and analysis is needed to quantitatively establish their climate influence and to design policies that take account of their local and regional pollution effects.

Putting aside the local and regional air pollutants, the quantitative importance of the other non-CO2 greenhouse gases has now been relatively well established. One of the major remaining concerns is accurate measurement and monitoring so compliance can be assured, whatever set of policies are in place. This has less to do with the type of greenhouse gas than with the nature of its source. 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. Methane released from large landfills can be easily measured, and is in the United States, but it is impractical to measure the methane emitted from each head of livestock, or the N2O from every farmer's field. A different regulatory approach may be needed for different sources.

Scientists have long recognized the roles of non-CO2 greenhouse gases and other substances that contribute to climate change. Only in the past few years have that the pieces of this complex puzzle come together to demonstrate how critical the control of these gases is for a cost-effective strategy to slow climate change. Control of non-CO2 greenhouse gases is a critical component of a cost-effective climate policy, and particularly in the near term these reductions can complement early efforts to control carbon dioxide.