Multi-Gas Contributors to Global Climate Change: Climate Impacts and Mitigation Costs of Non-CO2 Gases
Prepared for the Pew Center on Global Climate Change
John M. Reilly, Henry D. Jacoby, and Ronald G. Prinn
Massachusetts Institute of Technology
Press Release 
Eileen Claussen, President, Pew Center on Global Climate Change
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.
The Center and the authors are grateful to James Hansen, Keith Paustian, Ev Ehrlich, Francisco Delachesnaye, and Dina Kruger for their helpful comments on previous drafts of this report. The authors also acknowledge support, through the M.I.T. Joint Program on the Science and Policy of Global Climate Change, and the research assistance provided by Marcus Sarofim.
Most discussions of the climate change issue have focused almost entirely on the human contribution to increasing atmospheric concentrations of carbon dioxide (CO2) and on strategies to limit its emissions from fossil fuel use. Among the various long-lived greenhouse gases (GHGs) emitted by human activities, CO2 is so far the largest contributor to climate change, and, if anything, its relative role is expected to increase in the future. An emphasis on CO2 is therefore justified, but the near-exclusive attention to this single contributor to global warming has had the unintended consequence of directing attention away from the other GHGs, where some of the most cost-effective abatement options exist. The non-CO2 GHGs emitted directly by human activities include methane (CH4) and nitrous oxide (N2O), and a group of industrial gases including perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF6). When taken together with the already banned chlorofluorocarbons (CFCs), their climate significance over the past century is roughly equivalent to that of CO2. Looking to likely emissions over the next half-century, it is also the case that feasible reductions in emissions of methane and other non-CO2 gases can make a contribution to slowing global warming that is as large as or even larger than similar reductions in CO2 emissions. To effectively limit climate change, and to do so in a cost-effective manner, thus requires that climate policies deal with CO2 and non-CO2 gases alike.
There are several reasons why attention has been focused so heavily on CO2 even though the full list of GHGs has been targeted for control under international climate agreements. Emissions of CO2 from fossil sources can be readily estimated from market data on fuel use, whereas the other gases present measurement difficulties. Also, 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 via energy costs exist to curb CO2 emissions from fossil fuels.
If, for example, the total GHG emissions reduction required to meet a target were on the order of 10 or 15 percent, 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 would come from the non-CO2 greenhouse gases. Compared to a particular reduction achieved by CO2 cuts alone, inclusion of the non-CO2 abatement options available 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 particularly surprising because it is disproportionate to their roughly 20 percent 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.
Of course, these gases are 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, and one is the accuracy of global warming potentials. Analysis has shown that the GWPs currently in use significantly underestimate the role of methane, and any correction of this bias would amplify the importance of the non-CO2 greenhouse gases. This error is due in part to omitted interactions, such as the role of methane in tropospheric ozone formation. 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 that take effect over the next few decades, whereas the benefits of CO2 abatement are spread out over a century or more. To the extent one is concerned about slowing climate change over the next 50 years, therefore, the control of methane and HFCs—the gases that last a decade or so—has 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 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. The main issue with these substances is that, even though their climatic effects are important, a more immediate concern is that they cause local and regional air pollution that affects human health, crop productivity, and ecosystems. Moreover, their climatic effects are mainly regional, or even local, and this feature creates difficulties for the use of a single index to represent their effects across the globe. In the end, 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 in including them 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. In fact, the ability to monitor and measure 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 directly measure the methane emitted from each head of livestock, or the N2O from every farmer’s field. The difficulty of monitoring and measuring emissions implies that a different regulatory approach may be desirable for different sources, at least initially.
Scientists have long recognized the various roles of non-CO2 greenhouse gases and other substances that contribute to climate change. It is only in the past few years, however, that the various pieces of this complex puzzle have been fit together to provide a more complete picture of just how critical the control of these gases can be in 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.