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important role in reducing current emissions of CO2, CH, and N2O and in enhancing carbon sinks. A number of measures could conserve and sequester substantial amounts of carbon (approximately 60-90 Gt C in the forestry sector alone) over the next 50 years. In the forestry sector, costs for conserving and sequestering carbon in biomass and soil are estimated to range widely but can be competitive with other mitigation options. Factors affecting costs include opportunity costs of land; initial costs of planting and establishment; costs of nurseries; the cost of annual maintenance and monitoring; and transaction costs. Direct and indirect benefits will vary with national circumstances and could offset the costs. Other practices in the agriculture sector could reduce emissions of other greenhouse gases such as CH, and N2O. Land use and management measures include:

Sustaining existing forest cover

The net amount of carbon per unit area conserved or sequestered in living biomass under a particular forest management practice and present climate is relatively well understood. The most important uncertainties associated with estimating a global value are (i) the amount of land suitable and available for forestation, regeneration, and/or restoration programs; (ii) the rate at which tropical deforestation can actually be reduced; (iii) the long-term use (security) of these lands; and (iv) the continued suitability of some practices for particular locations given the possibility of changes in temperature, water availability, and so forth under climate change.

Promoting agroforestry

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emissions of greenhouse gases or sequestering carbon. Current analysis suggests the following:

Competing Uses of Land, Water, and Other Natural Resources. A growing population and expanding economy will increase the demand for land and other natural resources needed to provide, inter alia, food, fiber, forest products, and recreation services. Climate change will interact with the resulting intensified patterns of resource use. Land and other resources could also be required for mitigation of greenhouse gas emissions. Agricultural productivity improvements throughout the world and especially in developing countries would increase availability of land for production of biomass energy. Geoengineering Options. Some geoengineering approaches to counterbalance greenhouse gas-induced climate change have been suggested (e.g., putting solar radiation reflectors in space or injecting sulfate aerosols into the atmosphere to mimic the cooling influence of volcanic eruptions). Such approaches generally are likely to be ineffective, expensive to sustain, and/or to have serious environmental and other effects that are in many cases poorly understood.

Policy Instruments

Mitigation depends on reducing barriers to the diffusion and transfer of technology, mobilizing financial resources, supporting capacity building in developing countries, and other approaches to assist in the implementation of behavioral changes and technological opportunities in all regions of the globe. The optimum mix of policies will vary from country to country, depending upon political structure and societal receptiveness. The leadership of national governments in applying these policies will contribute to responding to adverse consequences of climate change. Governments can choose policies that facilitate the penetration of less greenhouse gas-intensive technologies and modified consumption patterns. Indeed, many countries have extensive experience with a variety of policies that can accelerate the adoption of such technologies. This experience comes from efforts over the past 20 to 30 years to achieve improved energy efficiency, reduce the environmental impacts of agricultural policies, and meet conservation and environmental goals unrelated to climate change. Policies to reduce net greenhouse gas emissions appear more easily implemented when they are designed to address other concerns that impede sustainable development (e.g., air pollution and soil erosion). A number of policies, some of which may need regional or international agreement, can facilitate the penetration of less greenhouse gas-intensive technologies and modified consumption patterns, including:

Reducing or removing other subsidies (e.g., agricultural and transport subsidies) that increase greenhouse gas emissions Tradable emissions permits

Voluntary programs and negotiated agreements with industry Utility demand-side management programs

Regulatory programs, including minimum energy efficiency standards (e.g., for appliances and fuel economy) Stimulating RD&D to make new technologies available Market pull and demonstration programs that stimulate the development and application of advanced technologies Renewable energy incentives during market build-up Incentives such as provisions for accelerated depreciation and reduced costs for consumers

Education and training; information and advisory measures Options that also support other economic and environmental goals.

Accelerated development of technologies that will reduce greenhouse gas emissions and enhance greenhouse gas sinksas well as understanding the barriers that inhibit their diffusion into the marketplace-requires intensified research and development by governments and the private sector.

Authors/Reviewers

Robert T. Watson, USA; M.C. Zinyowera. Zimbabwe, Richard H. Moss, USA, Roberto Acosta Moreno, Cuba, Sharad P. Adhikary, Nepal. Michael Adler, USA: Shardul Agrawala. India: Adrian Guillermo Aguilar, Mexico: Saiyed Al-Khouli, Saudi Arabia, Barbara AllenDiaz, USA. Mitsuru Ando, Japan, Rigoberto Andressen, Venezuela; 8.W. Ang. Singapore. Nigel Arnell, UK, Anne Arquit-Niederberger, Switzerland. Arthur 1. Askew, Switzerland. Walter Baethgen, Uruguay, Bryson Bates. Australia, Martin Beniston, Switzerland. Max Beran, UK, Sien Bergström, Sweden, Luitzen Bijlsma, The Netherlands; Rouna Bierbaum, USA; Michel Boko, Republic of Benin, Bert Bolin, Sweden, Suzanne Bolton, USA; Evelyne Bravo, Venezuela: Sandra Brown, USA; James P. Bruce, Canada, Peter Bullock, UK, Melvin G.R. Cannell, UK. Osvaldo F. Canziani, Argentina: Rodolfo Carcavallo, Argenona: William Chandler, USA; Fred C. Cheghe, Kenya, Chunzhen Liu, China, Carlos Clemente Cem, Brazil; Bryan W. Christmas, New Zealand, Vernon Cole, USA, Richard M. Cowling, South Africa, Wolfgang P. Cramer, Germany; Rex Victor Cruz, Philippines, Ogunlade Davidson, Sierra Leone, Ehrlich Desa, India, Deying Xu, China, Sandra Diaz, Argentna. Andrew F. Dlugolecki, Scotland, David Dokken, USA, James A. Edmonds, USA, Lin Erda, China, John Everett, USA; Zhou Fenggi, China, Andreas Fischlin, Switzerland; Bernard Fisher, UK; B. Blair Fitzharris, New Zealand; Douglas G. Fox, USA. Jaafar Friaa, Tunisia, Margaret Friedel, Australia, Alexander Rauja Gacuhi, Kenya, W. Galinski, Poland, Habiba Gitay, Australia; Peter M. Groffman, USA. Arnulf Grubler, Austria, Howard Gruenspechi, USA. Steven P. Hamburg, USA; M.T. (Timm) Hoffman, South Africa, Jarle Inge Holten, Norway, Hisashi Ishitani, Japan: Venugopalan Inekkot, Germany, Michael Jefferson, UK, David S. Jenkinson, UK. Thomas B. Johansson, Sweden, Zdzisław Kaczmarek, Poland, Takao Kashuwagi, Japan; Robert M. Kirk, New Zealand, Miko Kirschbaum, Australia; Paul Komor, USA; Andrei Krovnin, Russian Federation; Richard J.T. Klein, The Netherlands; S.M. Kulshrestha, India; Herbert Lang, Switzerland; Henry Le Houerou, France, Susan E. Lee, UK; Rik Leemans, The Netherlands, Mark D. Levine, USA, Daniel Lluch-Belda, Mexico, Michael MacCracken, USA: John J. Magnuson, USA; Gabriel M. Mailu, Kenya, Joseph Mworia Maitima, Kenya. Gregg Marland. USA. Kathy Maskell, UK. Roger F. McLean, Australia. Anthony J. McMichael, UK. Laurie Michaelis, France, Ed Miles, USA. William Moomaw, USA. Roberto Moreira, Brazil, Nebojsa Nakicenovic, Austria. Neville Nicholls, Australia, Robert J. Nicholls, UK, Shuzo Nishioka, Japan, Ian Noble, Australia; Leonard A. Nurse. Barbados: Rispa Odongo, Kenya, Ryousuke Ohashi, Japan. Ezekiel Okernwa, Kenya, Mats Oquist Sweden. Flo Ormond, USA, Martin L. Parry, UK, Martha Perdomo, Venezuela, Michel Petit France, Rick Piltz, USA, Warren T. Piver, USA. P.S. Ramakrishnan, India; N.H. Ravindranath, India, John Reilly, USA, John C. Rodda, Switzerland, Hans-Holger Rogner, Canada; Jayant Sathaye, USA. Dieter R. Sauerbeck, Germany, Hans-Wilhelm Scharpenseel. Germany, Robert J. Scholes, South Africa, Michael Scott, USA; Subodh K. Sharma, India, David Shriner, USA; S K. Sinha, India. Jim F Skea, UK, Allen M. Solomon, USA; Eugene Z. Sukhiv, USA, Oedon Starosolszky, Hungary, Su Jilan, China, Avelino Suarez, Cuba, Bo Svensson, Sweden, Hidekazu Takakura, Japan. Kevin Russell Tate, New Zealand, Melissa Taylor, USA; Lucien Tessier, France. Dennus Tirpak, USA; Tran Viet Lien, Vietnam, JeanPaul Troadec, France, Hiroshi Tsukamoto, Japan, Itsuya Tsuzaka, Japan, Laura Van Wie. USA. Pier Vellinga, The Netherlands, Richard Warrick, New Zealand, Marthas Weber, Switzerland, Stanley Jeffress Williams, USA, Ted Williams, USA, Youyu Xie, China, Patrick Young, USA. Peter P Zhou, Botswana, Lewis H. Ziska. The Philippines