John M. Reilly

Multi-Gas Contributors to Global Climate Change


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
February 2003

John M. Reilly, Henry D. Jacoby, and Ronald G. Prinn
Massachusetts Institute of Technology

Press Release

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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.

Executive Summary


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.

Henry D. Jacoby
John M. Reilly
Ronald G. Prinn

Agriculture & Global Climate Change: A Review of Impacts to U.S. Agricultural Resources

Agriculture & Global Climate Change: A Review of Impacts to U.S. Agricultural Resources

Prepared for the Pew Center on Global Climate Change
February 1999

Richard M. Adams, Oregon State University
Brian H. Hurd, Stratus Consulting Inc.
John Reilly, Massachusetts Institute of Technology

Press Release

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Eileen Claussen, Executive Director, Pew Center on Global Climate Change

In order to intelligently respond to climate change, we must first understand the likely consequences on our environment and health. This report, the first in a series of environmental impact reports, will explore anticipated effects of climate change on U.S. agriculture. Other reports in this series will assess what is known about the impact of climate change on weather and include analyses of its impact on water resources, coastal areas, human health, ecosystems, and forests. In evaluating the current state of scientific knowledge regarding the anticipated effects of climate change on U.S. agriculture, this report yields several key observations:

Climate change will result in agricultural shifts and changes across the United States. Given the requisite time and resources to adapt, the United States is likely to continue to be able to feed itself; however, there will clearly be regional winners and losers.

If the rate of greenhouse gas emissions exceeds projected levels or if unanticipated or more frequent extreme events accompany this change, the outlook for the United States would likely worsen. The projections in this report, for example, are based on a doubling of carbon dioxide (CO2) in the atmosphere which could understate the severity of climate change impacts over the long-term.

Some countries will experience more negative effects on agriculture associated with climate change. The situation will be particularly acute in developing nations that do not have the same resources as the United States to respond to the agricultural changes projected.

This report broadly outlines projected effects on U.S. agricultural regions. The complexity of the climate system itself and its relationship to agricultural resources make it difficult to project specific effects on individual states or communities. More research is needed to better understand this complex system and to incorporate relevant factors into future climate models and assessments. The report does, however, provide an objective foundation upon which to build and clearly demonstrates the impact climate change will have, both direct and indirect, on U.S. agricultural systems.

In addition to reporting on the environmental impacts of climate change, the Pew Center undertakes analyses on domestic and international policy matters and economics. The Center was established in 1998 by the Pew Charitable Trusts to bring a new, cooperative approach and critical scientific, economic and technological expertise to the global climate change debate.

A number of major corporations have taken a bold and historic step in joining the Center's Business Environmental Leadership Council. In doing so, they have accepted "the views of most scientists that enough is known about the science and environmental impacts of climate change for us to take actions to address its consequences." Understanding the potential environmental impacts of climate change, as this report illustrates, is an important step toward promoting informed action.

Executive Summary

This paper analyzes the current state of knowledge about the effects of climate change on U.S. food production and agricultural resources. The paper also considers regional changes in agricultural production, including distributional impacts.

The linkages between agriculture and climate are pronounced, often complex, and not always well understood. Temperature increases can have both positive and negative effects on crop yields, with the difference depending in part on location and on the magnitude of the increase. Crop yields in the northern United States and Canada may increase, but yields in the already warm, low-latitude regions of the southern United States are likely to decline. Evidence also suggests positive crop yield effects for mild to moderate temperature increases such as 2°C to 3°C (3.6°F to 5.4°F). However, once average global temperatures rise beyond about 4°C (7.2°F), yields begin to fall. Increases in precipitation level, timing, and variability may benefit semi-arid and other water-short areas by increasing soil moisture, but could aggravate problems in regions with excess water. Although most climate models predict precipitation increases, some regions will experience decreased precipitation, which could exacerbate water shortages and droughts. Higher carbon dioxide (CO2) levels in controlled experiments increase crop growth and decrease water use. However, these experiments often have demonstrated a more positive response than observed under actual field conditions.

Agricultural systems are most sensitive to extreme climatic events such as floods, wind storms, and droughts, and to seasonal variability such as periods of frost, cold temperatures, and changing rainfall patterns. Climate change could alter the frequency and magnitude of extreme events and could change seasonal patterns in both favorable and unfavorable ways, depending on regional conditions. Increases in rainfall intensity pose a threat to agriculture and the environment because heavy rainfall is primarily responsible for soil erosion, leaching of agricultural chemicals, and run off that carries livestock waste and nutrients into water bodies. Currently available climate forecasts cannot resolve how extreme events and variability will change; however, both are potential risks to agriculture. The rate of change is also uncertain. Adjustment costs are likely to be higher with greater rates of change.

Agricultural systems are managed. Farmers have a number of adaptation options open to them, such as changing planting and harvest dates, rotating crops, selecting crops and crop varieties for cultivation, consuming water for irrigation, using fertilizers, and choosing tillage practices. These adaptation strategies can lessen potential yield losses from climate change and improve yields in regions where climate change has beneficial effects. At the market level, price and other changes can signal further opportunities to adapt as farmers make decisions about land use and which crops to grow. Thus, patterns of food production respond not only to biophysical changes in crop and livestock productivity brought about by climate change or technological change, but also to changes in agricultural management practices, crop and livestock prices, the cost and availability of inputs, and government policies. In the longer term, adaptations include the development and use of new crop varieties that offer advantages under changed climates, or investments in new irrigation infrastructure as insurance against potentially less reliable rainfall. The extent to which opportunities for adaptation are realized depends upon a variety of factors such as information flow, access to capital, and the flexibility of government programs and policies.

Climate change can also have a number of negative indirect effects on agro-environmental systems effects that have been largely ignored in climate change assessments. These indirect effects include changes in the incidence and distribution of pests and pathogens, increased rates of soil erosion and degradation, and increased tropospheric ozone levels from rising temperatures. Regional shifts in crop production and expansion of irrigated acreage may stress environmental and natural resources, including water quantity and quality, wetlands, soil, fish, and wildlife.

The focus of this paper is on the impacts of climate change on agriculture. However, agriculture is also a potential source of greenhouse gas (GHG) emissions, and it can play an important role in mitigating these emissions. Methane from rice paddies and livestock, nitrous oxide (N2O) from cultivated soils and feedlots, and CO2 from the cultivation of virgin agricultural lands and intensive production methods contribute to global warming. Changes in management can reduce emissions from these sources. Agriculture can reduce atmospheric CO2 through tree-planting and similar programs that sequester significant amounts of carbon and through increased planting of biofuel crops that could replace fossil fuels.

The following describes the current understanding regarding the potential impacts of climate change on U.S. agriculture:

CROPS AND LIVESTOCK ARE SENSITIVE TO CLIMATE CHANGES IN BOTH POSITIVE AND NEGATIVE WAYS. Understanding the direct biophysical and economic responses to these changes is complicated and requires more research. In addition, indirect effects - such as changes in pests and water quality and changes in extreme climate events - are not well understood.


THE IMPACT OF CLIMATE CHANGE ON U.S. AGRICULTURE IS MIXED. Climate change is not expected to threaten the ability of the United States to produce enough food to feed itself through the next century; however, regional patterns of production are likely to change. Regions such as the Northern Great Plains and Great Lakes may have increased productivity while the Southern Plains, Delta states, and possibly the Southeast and portions of the Corn Belt could see agricultural productivity fall. However, the form and pattern of change are uncertain because changes in regional climate cannot be predicted with a high degree of confidence.

CONSIDERATION OF ADAPTATION AND HUMAN RESPONSE IS CRITICAL TO THE ACCURATE AND CREDIBLE ASSESSMENT OF CLIMATE CHANGE IMPACTS.However, because of the long time horizons involved in climate change assessments and uncertainties concerning the rate at which climate will change, it is difficult to predict accurately what adaptations people will make. This is particularly challenging since adaptations are influenced by many factors, including government policy, prices, technology research and development, and agricultural extension services.

BETTER CLIMATE CHANGE FORECASTS ARE KEY TO IMPROVE DASSESSMENTS OF THE IMPACTS OF CLIMATE CHANGE. In the meantime, farmers and the agricultural community must consider strategies that are economically and environmentally viable in the face of uncertainty about the course of climate change.

AGRICULTURE IS A SECTOR THAT CAN ADAPT, BUT THERE ARE SOME FACTORS NOT INCLUDED IN ASSESSMENTS THAT COULD CHANGE THIS CONCLUSION.Changes in the incidence and severity of agricultural pests, diseases, soil erosion, and tropospheric ozone levels, as well as changes in extreme events such as droughts and floods, are largely unmeasured or uncertain and have not been incorporated into estimates of impacts. These omitted effects could result in a very different assessment of the true impacts of climate change on agriculture. If the rate or magnitude of climate change is much greater than anticipated, adaptation could be more difficult and impacts could be greater than currently expected.
Overall, the consensus of economic assessments is that global climate change of the magnitudes currently being discussed by the Intergovernmental Panel on Climate Change (IPCC) and other organizations (i.e., +0.8°C to +4.5°C or +1.4°F to +8.1°F) could result in some lowering of global production but will have only a small overall effect on U.S. agriculture and its ability to provide sufficient food and fiber to both domestic and global customers over the next 100 years. However, distributional effects within the United States can be significant because consumers, producers, and local economies will gain in some regions and lose in others.

Warming beyond that reflected in current studies (i.e., associated with a continued rise in CO2 beyond the doubling that has been commonly investigated) is expected to impose greater costs, decreasing agricultural production in most areas of the United States and substantially limiting global production. This reinforces the need to determine the magnitude and rate of warming that may accompany the CO2 and greenhouse gas build-up currently underway in the atmosphere.

About the Authors

Richard M. Adams
Oregon State University, Corvallis, OR

Richard M. Adams received his Ph.D. in Agricultural Economics from the University of California, Davis, in 1975. He is currently a professor of Agricultural and Resource Economics at Oregon State University, a position he has held since 1983. His research interests include the economic analysis of resource and environmental issues, with emphasis on the consequences of environmental change. Professor Adams has served on numerous governmental advisory and research committees dealing with environmental issues. He has published over 160 journal articles, book chapters and research reports, including 20 on the effects of climate change on agriculture and agricultural resources. He has served on the editorial boards of five journals and w as editor of the American Journal of Agricultural Economics from 1992 to 1994.

Brian H. Hurd
Stratus Consulting Inc., Boulder, CO

Brian H. Hurd is a Senior Associate in the climate change group at Stratus Consulting, a Boulder-based environment and energy research firm. He received his Ph.D. in agricultural economics from the University of California, Davis in 1992, where he analyzed technology changes in production agriculture. His passion for interdisciplinary research and for contributing to public decision-making regarding natural resources has led to his current focus on climate change. He has developed regional and national models of water resource impacts, analyzed land use changes in forestry and agriculture, and investigated adaptation and mitigation strategies, while serving a variety of public- and private-sector clients such as U.S. EPA, U.S. Department of Energy, National Science Foundation, National Institute for Global Environmental Change, and the Electric Power Research Institute.

John Reilly
Massachusetts Institute of Technology, Cambridge, MA

Dr. Reilly is the Associate Director for Research in the Joint Program on the Science and Policy of Global Change at the Massachusetts Institute of Technology. He spent 12 years with the Economic Research Service of USDA, most recently as the Acting Director and Deputy Director for Research of the Resource Economics Division. He has been a scientist with Battelle's Pacific Northwest National Laboratory and with the Institute for Energy Analysis, Oak Ridge Associated Universities. He received his Ph.D. in economics from the University of Pennsylvania in 1983 and holds a BS in economics and political science from the University of Wisconsin. He has conducted research on the economics of climate change for 19 years. He was a principal author for the Intergovernmental Panel on Climate Change's Second Assessment Report and has served on many Federal government and international committees on climate change and agricultural research.



Brian Hurd
John M. Reilly
Richard M. Adams
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