IPCC Climate Change 2007 Report: Mitigation of Climate Change

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IPCC Climate Change 2007 Report: Mitigation of Climate Change


Since 1990, the Intergovernmental Panel on Climate Change (IPCC), an arm of the United Nations, has produced several reports on the state of scientific understanding of climate change. The latest of these, the Fourth Assessment Report (titled Climate Change 2007), was issued in 2007. The IPCC's reports are intended to provide an in-depth, authoritative view of the state of scientific knowledge on climate change to policymakers and other interested parties. The third part of the Fourth Assessment report, Climate Change 2007: Mitigation of Climate Change, describes a variety of possible means of combating global warming and the likely costs of those measures. It does not recommend or require specific responses from governments: the report serves as an advisory, not a directive.

Historical Background and Scientific Foundations

The Climate Change 2007: Mitigation of Climate Change report was issued in stages beginning in May 2007, as the third part of the IPCC's Fourth Assessment Report. The other three assessment reports were issued in 1990, 1995, and 2001. The Fourth Assessment Report pools the knowledge and experience of the world community of climate and weather scientists, with additional input from economists, engineers, and government officials. The other two parts of the Fourth Assessment, released earlier in 2007, are the Physical Science Basis report and the Impacts, Adaptation and Vulnerability report. The Physical Science Basis report is devoted to describing what climate changes are happening, what is causing them, and what changes are likely in the future. The Impacts report covers the affects that climate change is likely to have on the human and natural environments and what adaptations to it are possible.

The Mitigation report is more than 800 pages long. It offers both summary and in-depth views of the present state of scientific understanding of possibilities for mitigating climate change across the entire range of human activities that affect climate, from lifestyle choices to electricity generation and agriculture. The structure of the report is described in outline below, followed by a brief treatment of the report's main conclusions.

Structure of the Report

Because the Mitigation report is lengthy, its authors provide two summaries in a format consistent with those provided in the other two sections of the Fourth Assessment. The first of these is the “Summary for Policy-makers,” which highlights the scientific points most relevant for governmental and other persons involved in making decisions about public policy. The report does not only view government officials as “policymakers,” but takes a democratic view of decisionmaking. For example, Chapter 12 states: “Citizen groups have been major demanders of sustainable development and are critical actors in implementing sustainable development policy.” The second section is the “Technical Summary,” which takes a more detailed look at the science behind the statements made in the “Summary for Policymakers.”

The subsequent 13 chapters detail specific possibilities for mitigation. The report defines “mitigation” as “technological change and substitution that reduce resource inputs and emissions [of greenhouse gases] per unit of [economic] output.” This may involve producing fewer tons of greenhouse gases, or enhancing the effectiveness of sinks (processes that absorb greenhouse gases, primarily carbon dioxide, from the environment; a growing forest, for example, is a carbon sink). In the language of the report, “mitigation” does not refer to other changes in human behavior that would reduce greenhouse emissions, such as reducing the scale of industrial activity, reducing population sizes, or reducing affluence. It restricts itself to technological changes that

might be made in economic activities as currently conducted.

Chapter 1 describes the relevance of mitigation to the United Nations Framework Convention on Climate Change (UNFCCC), an international treaty drafted in Rio de Janeiro, Brazil, in 1992. The treaty, which has been signed by 190 countries, including the United States, is a non-binding promise to reduce greenhouse-gas emissions in order to stabilize world climate. Article 2 states that the treaty's ultimate objective is to achieve “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic (human-caused) interference with the climate system.” This stabilization should be achieved, the treaty says, “within a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.”

Chapter 1 notes that there is no single, scientific definition of the word “dangerous” in this context, where the possible harms of climate change (e.g., famine, species extinction) must be balanced against the possible harms of mitigation (e.g., poverty). Climate policy decisions, like other policy decisions, inevitably involve trade-offs between competing risks, groups, and benefits. However, Chapter 1 concludes, mitigation and sustainable economic activity are not necessarily in competition with each other. On the contrary, they probably are not. Failure to mitigate climate change will make poverty worse and undermine sustainable development, while mitigation can enhance sustainable development and reduce vulnerability to climate change.

Chapter 2 delves into what the report calls “framing issues.” These include the relationship between sustainable development and climate change, the nature of decision-making on long-term issues, definitions of risk and uncertainty, the balancing of costs and benefits, and the relationship of technology to mitigation. Chapter 3 discusses long-term issues related to mitigation, including the modeling of alternative possible futures. The future is inherently uncertain, so decisions about mitigation require the construction of a range of educated guesses about how today's decisions might affect tomorrow's climate.

Chapter 4 describes the status of the energy-production industry worldwide. (“Energy” here refers to energy delivered in the form of heat or electricity; energy used to move vehicles is treated separately, in Chapter 5.) It discusses in detail the potential of various energy technologies to reduce the emission of greenhouse gases. About two-thirds of mitigation potential is in the energy sector, where alternative sources, carbon capture and storage, and improved efficiency can all prevent greenhouse emissions.

Chapter 5 addresses transportation, including shipping, railroads, automobiles and aviation, while Chapter 6 describes the mitigation potential of residential and commercial buildings. Chapter 7 surveys industry as a whole; Chapter 8, agriculture; Chapter 9, forestry; and Chapter 10, waste management, including wastewater, landfills, sewage, garbage incineration, and recycling. Because no single sector—energy, transport, buildings, or other—exists in isolation, Chapter 11 is devoted to an overview of the connections between mitigation practices in all the sectors discussed in the preceding chapters. It discusses the implications of carbon pricing, schemes for transferring money from mitigation-derived savings in some sectors to expenses in other sectors, and related issues.

Chapter 12 describes the connections between sustainable development and mitigation. Like Chapter 2, it emphasizes the two-way relationship between sustainable development and mitigation: “policies pursuing sustainable development and climate change mitigation can be mutually reinforcing.” It also emphasizes that future global development pathways with lower greenhouse-gas emissions do not necessarily entail lower economic growth: in other words, fighting climate change need not make (or keep) people poor.

Finally, Chapter 13 discusses the relevance of government policies, treaties, and other international agreements. State and national policy tools for mitigation include standards and regulations, taxes and charges, tradable emissions permits (carbon trading), subsidies (e.g., to renewable energy sources or nuclear power), research and development programs, voluntary greenhouse-gas reduction agreements, and information policies that make environmental information openly available to the public. The main existing international agreement on climate change is the UNFCCC and its Kyoto Protocol.

Main Conclusions

The following conclusions are some of those highlighted in the “Summary for Policymakers” section of Climate Change 2007: Mitigation of Climate Change. The points in italics are paraphrased from the original report to increase clarity.

Greenhouse gas emissions have increased by 70% from 1970 to 2004, from 28.7 billion metric tons (gigatons, Gt) per year to 49 Gt per year, figured as equivalent tons of carbon dioxide. This has caused greenhouse gas concentrations in the atmosphere to increase greatly above pre-industrial (that is, roughly, pre–1760) levels. The largest amount of growth in emissions has come from the energy sector, which has increased its emissions by 145% since 1970. Transport emissions have increased 120%, industry 65%, land-use change and forestry 40%, agriculture 27%, and buildings 26%. Buildings, due to increased efficiency, have held constant in emissions since about 1990, but this does not count the electricity consumed in buildings.

Global energy intensity—the ratio of energy use to economic output—has decreased by a third since 1970, meaning that the efficiency with which energy is used to make goods and services has increased; however, global income growth (77%, for the average person) and population growth (69%, to about 6.7 billion) have more than wiped out the emissions gains from increased efficiency. Emissions of ozone-destroying chemicals have decreased from 1990 by about 20%, thanks to the Montreal Protocol, a treaty governing the use of such chemicals. Policies on climate change, energy security, and sustainable development have reduced greenhouse emissions in some sectors and countries, but not yet enough to offset global increases in emissions. Current per-person greenhouse emissions are highest for the United States and Canada, about 26 tons of CO2 equivalent per person per year (accounting for 19.4% of global greenhouse gas emissions), and lowest for south Asia, about 3 tons per year.


BIOFUEL: A fuel derived directly by human effort from living things, such as plants or bacteria. A biofuel can be burned or oxidized in a fuel cell to release useful energy.

CARBON PRICING: Assignment of a market price to carbon credits on some emissions trading market, such as the Chicago Climate Exchange or European Union Emissions Trading Scheme.

CARBON SINKS: Carbon reservoirs such as forests or oceans that take in and store more carbon (carbon sequestration) than they release. Carbon sinks can serve to partially offset greenhouse-gas emissions.

CARBON TRADING: Buying and selling of carbon credits, abstract instruments (like money) that each represent the right to emit 1 ton of carbon dioxide or an equivalent amount of other greenhouse gases. Carbon trading presently takes place under the European Union Emission Trading Scheme and the Chicago Climate Exchange.

DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosyn-thesis are no longer present and contributing to carbon storage.

IRON FERTILIZATION: Controversial speculative method for removing carbon from the atmosphere, in which adding powdered iron to ocean surface waters would cause single-celled aquatic plants (phytoplankton) to increase greatly in numbers, breaking down CO2 to obtain carbon for their tissues. The dead organisms would then, ideally, sink to deep waters or the ocean floor, sequestering their carbon content from the atmosphere and so mitigating climate change.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.

RENEWABLE ENERGY: Energy obtained from sources that are renewed at once, or fairly rapidly, by natural or managed processes that can be expected to continue indefinitely. Wind, sun, wood, crops, and waves can all be sources of renewable energy.

SUSTAINABILITY: The quality, in any human activity (farming, energy generation, or even the maintenance of a society as a whole), of being sustainable for an indefinite period without exhausting necessary resources or otherwise self-destructing.

If current climate-change mitigation policies and development policies remain in place, global greenhouse emissions will continue to increase for the next few decades. Today's policies, which, despite the Kyoto Protocol— which has not been ratified by the United States, and which does not specify constraints on the world's largest greenhouse-gas emitter as of 2007, China—are only weakly controlling emissions. The actual future of greenhouse-gas emissions depends on what policies and technologies are adopted now and in the next few decades, so no single prediction of future greenhouse emissions is possible.

Instead, the IPCC report describes a range of plausible pathways called the A1, A2, B1, and B2 storylines. In the A1 storyline, global population grows rapidly, peaks in the middle of the 21st century, and then decreases. The A1 storyline can be played out with heavy emphasis on fossil fuels (the A1F1 scenario); with emphasis on non-fossil-fuel energy sources, primarily nuclear power (A1T scenario); or a balance across energy sources (A1B scenario). In the A2 scenario family, population continuously increases and economic development is slower and more fragmented. In the B1 scenario family, population behaves as in A1 but with a less material-intensive economy and introduction of renewables and efficiency: B1 is the more or less eco-friendly scenario, oriented toward social equity and protection of the environment. The B2 scenario family combines the values of B1 with the more globalized patterns of A1.

None of these possible pathways assumes implementation of the UNFCCC's goal of greenhouse-gas stabilization, so the IPCC classes them all as “non-mitigation” scenarios. In this range of non-mitigation scenarios, greenhouse emissions in 2030 will have increased by some value between 9.7 Gt/year and 36.7 Gt/year CO2 equivalent, an increase of between 25% and 90%.

Many studies show that there is substantial economic (affordable) potential for mitigation of global greenhouse emissions over the next few decades. This potential, if realized, could hold greenhouse emissions steady or even decrease them below present-day levels. Mitigation opportunities with negative costs—measures that would actually save money, compared to business as usual— could reduce emissions by about 6 Gt/year CO2 equivalent by 2030.

The overall costs of mitigating greenhouse-gas emissions, with a goal of stabilizing levels between 445 and 710 parts per million CO2 equivalent (today's CO2 concentration is about 380 ppm), would require the expenditure of at most 3% of the world's economic output, or might even increase world economic output slightly. Costs for particular regions might be much higher or lower than the global average. Countries that depend heavily on oil exports—such as Russia, Venezuela, and Saudi Arabia—might be economically injured by a global shift away from fossil fuels.

Changes in lifestyle and behavior can help mitigation in all sectors. For example, driving less, buying less, recycling more, wasting less energy, living in smaller houses, turning down heating systems in unused spaces, and other individual behaviors can contribute significantly to the mitigation of climate change.

Mitigation will reduce air pollution; the reduced medical costs from reduced pollution will offset a substantial fraction of the costs of mitigation.

Mitigation gain in the transport sector may well be undone by increased transport activity (for example, increased automobile ownership in Asia). Consumers may prefer to buy larger vehicles even though smaller, more efficient vehicles would be less costly, less polluting, and equally safe. Shifting personal behaviors from individual automobile use to public transportation is also difficult.

Energy efficiency measures in new and old buildings can reduce carbon dioxide emissions considerably while saving money. About a third of possible emissions reductions in the building sector could yield a net profit. Such measures can also improve air quality indoors and out, enhance energy security, and make buildings more useful and pleasant.

Most mitigation potential in industry is in the energy-intensive industries such as metal production and cement production.

In agriculture, low-cost mitigation measures can increase absorption of carbon by soils, reduce greenhouse-gas emissions, and supply biofuels. Most of the mitigation potential of agriculture is in the storage of carbon by soils. Practices that enable soils to store carbon are also more sustainable (involve less soil loss to erosion, for example). Reducing emissions of the greenhouse gases—methane and nitrous oxide—is also possible in agriculture.


The National Academy of Sciences counsels: “Another way to prepare for climate change is to develop practical strategies that could be used to reduce economic and ecological systems' vulnerabilities to change. Some of these are ‘no-regrets’ strategies that will provide benefits whether a significant climate change ultimately occurs or not, potentially reducing vulnerability at little or no net cost. No-regrets measures could include low-cost steps to improve climate forecasting; to slow biodiversity loss; to improve water, land, and air quality; and to make our health care enterprise, financial markets, and transportation systems more resilient to major disruptions.”

SOURCE: Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.

In forestry, mitigation measures can reduce emissions and increase carbon absorption at low cost. Such measures would also assist sustainable development and adaptation to climate change. About 65% of mitigation potential is the tropics; about half could be realized by reducing deforestation.

Geo-engineering methods such as iron fertilization of the oceans are speculative and may have unforeseen negative side effects; reliable cost estimates for these exotic methods do not yet exist.

In the long term, mitigation efforts over the next 20 to 30 years will have a large impact on future chances to stabilize greenhouse gases in the atmosphere at a lower level. In other words, the more that is done soon, the less climate change will occur in the long run.

Impacts and Issues

All parts of the IPCC Fourth Assessment Report have been widely cited in the press and in the deliberations of United Nations bodies, citizen groups, and governments. However, the compilation of the Mitigation report, like the earlier two parts of the Fourth Assessment, did meet with some resistance from participating governments. (Participating governments have broad power to block the release of IPCC documents with which they disagree.) The final version of the report was only approved in early May 2007 after a marathon all-night session in which scientists, government officials, and economists from more than 100 countries debated its precise wording. China, for example, resisted the inclusion of language implying that fast-developing countries such as itself might also have to cut their greenhouse-gas emissions deeply. (In 2007, China became the largest producer of greenhouse gases.) The language was, however, included in the final report.

When the Mitigation report was released, some experts in the field were surprised by its hopefulness, given all the grim news contained in the first two parts of the Fourth Assessment Report. The journal Nature stated that the IPCC had “produced a surprisingly optimistic analysis of the possibility of mitigating climate change.” The report itself advises that significant mitigation can be achieved for low cost or at an actual profit, and that most mitigation can be achieved with currently available technologies. According to the IPCC, if policies are adopted that make carbon dioxide emissions cost $50 per metric ton, economics would naturally cause emissions in 2030 to be a fifth to a half lower than they would be otherwise (though still higher than today); this would slow world economic growth by only about a tenth of a percent.

Criticism of the report has been relatively slight. The outstanding issue is whether enough nations will take enough mitigation action in the near future to prevent the more dire consequences foretold by the IPCC in the earlier parts of its Fourth Assessment Report.

See Also IPCC Climate Change 2007 Report; IPCC Climate Change 2007 Report: Criticism; IPCC Climate Change 2007 Report: Impacts, Adaptation and Vulnerability; IPCC Climate Change 2007 Report: Physical Science Basis.



Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.


Hopkin, Michael. “Climate Panel Offers Grounds for Optimism.” Nature 447 (May 10, 2007): 120–121.

Revkin, Andrew C. “Climate Panel Reaches Consensus on the Need to Reduce Harmful Emissions.” The New York Times (May 4, 2007).

Web Sites

“Press Briefing on the Third Intergovernmental Panel on Climate Change Report on Climate Change [sic].” The White House, U.S. Government, May 4, 2007. <http://www.whitehouse.gov/news/releases/2007/05/20070504-2.html> (accessed September 12, 2007).

Larry Gilman