Policy ForumClimate Change

Recent Reductions in China's Greenhouse Gas Emissions

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Science  30 Nov 2001:
Vol. 294, Issue 5548, pp. 1835-1837
DOI: 10.1126/science.1065226

The prevailing wisdom about China's greenhouse gas [HN1] emissions is that they are increasing steadily, because of the large quantities of coal being used to fuel a fast-growing industrial economy, and most projections show China's greenhouse gas emissions continuing to grow in the coming decades [HN2] (1). However, we find that China's CO2 emissions declined by 7.3% between 1996 (the peak year) and 2000, and CH4 emissions declined by 2.2% between 1997 (the peak year) and 2000 (see the figure). This happened partly because China undertook a radical reform of its coal and energy industries [HN3] (2). In addition, China's economy suffered during the Asian economic crisis [HN4] of 1997 to 1998 to an extent not yet fully understood (3), and many factories curtailed production or shut down because of economic restructuring policies, resulting in a decline in coal production and consumption. Although China is not a party to the Kyoto Protocol [HN5], these recent developments have important implications for the formulation of climate change policies.

Trends in emissions

of (A) CO2 and (B) CH4 in China, 1990 to 2000.

Focusing only on CO2 emissions from fossil fuel combustion [HN6], against which we can compare other countries, we calculate that China's emissions dropped from 2950 Tg (teragrams of CO2, 1 Tg = 1 million tonnes) in 1996 to 2690 Tg in 2000, a reduction of 8.8%. This decrease, which China achieved while most other countries were increasing their emissions, represents about 1% of the global CO2 emissions from fossil fuel combustion in 2000 of 25,300 Tg (1). In the period 1995 to 1999, CO2 emissions from fossil fuel combustion in western Europe increased by 4.5%, in the United States by 6.3%, in Japan by 3.0%, and in India by 8.8% (4).

When energy data for the past 5 years began to emerge from China, it became clear that a transformation was in progress that had resulted in a reduction in energy use (2). This transformation had several aspects to it: the closing of small, inefficient industrial plants; improved efficiency of energy end-use; improved coal quality; the switching of many residential fuel users from coal to gas and electricity; technological progress in the energy-intensive sectors; and the opening up of coal and electricity markets. A slowdown in economic growth contributed to the decline in energy use (5). We have converted the changes in energy use and other activities into greenhouse gas emissions using guidelines from the Intergovernmental Panel on Climate Change (IPCC) [HN7] (6), as modified for China-specific conditions in the U.S.-sponsored China Climate Change Country Study [HN8] (7).

Coal combustion is the dominant contributor to CO2 emissions in China and is estimated to have grown from 22.1 exajoules (1 EJ = 1018 joules) in 1990 to a peak of 30.1 EJ in 1996 and then to have fallen to 25.1 EJ in 2000 (8). Oil and gas combustion both increased steadily during the decade, but their contributions to CO2 emissions are small. The use of biofuels (largely wood and agricultural residues for fuel) to provide cooking and heating services is widespread throughout rural China. However, its use has slowly declined during the decade, from 9.2 EJ in 1990 to about 7.6 EJ in 2000 (8), as kerosene, gas, and electricity have supplanted traditional fuels. Carbon dioxide is also released during the manufacture of cement [HN9], and China's use of cement for new construction grew from 210 million tons in 1990 to 600 million tons in 2000 (9). As a result of the reversal of past deforestation practices and recent promotion of afforestation activities [HN10], the net uptake of CO2 by forests in China has increased from 360 Tg in 1990 to 410 Tg in 2000 (10). Overall, we estimate that CO2 emissions grew from 2710 Tg in 1990 to a peak of 3470 Tg in 1996, thence dropping to 3220 Tg in 2000 (see the figure), a 7.3% reduction in the period 1996 to 2000 (11). This recent trend illustrates the potential for a fundamental change in the long-term pattern of emissions growth in China.

In the last few years, China's energy data have become more prone to error and uncertainty than they were in the early 1990s. The National Bureau of Statistics [HN11] has already revised 1999 coal output and consumption estimates (8). This has caused some skepticism about the reduction in energy use (12). Our analysis suggests that the reductions are real but not as great as previously believed. Further revisions to recent energy-use data are possible, which would necessitate a reanalysis of the data; however, we do not expect the trend to change.

Estimating CH4 emission [HN12] trends is a more difficult proposition, because it involves consideration of several nonenergy sources for which there are limited data available. Coal production is a large source of CH4, and it is known that coal production in China declined in recent years by an even larger amount than coal consumption, from 1370 million tons in 1996 to 1030 million tons in 2000 (8). This decline was due to overmining in the early part of the decade, which produced large stockpiles of coal that are now being drawn down (2). However, the benefit to CH4 emissions was not proportionately large, because much of the reduction in coal production was achieved at small, surface mines, owned by local governments and townships, which tend to emit less gas. Thus, over all mine types, we found a net increase in coal-bed emissions from 5.58 Tg CH4 in 1990 to 6.75 Tg in 1996, falling to 5.45 Tg in 2000 (13). Emissions from oil and gas extraction, processing, distribution, residential leakage, and combustion were also calculated, but these are small compared with coal emissions. Emissions from residential biofuel combustion were reduced from 2.76 Tg in 1990 to 2.28 Tg.

Agricultural emissions of methane grew in the middle of the decade but have leveled out recently. Emissions from rice cultivation declined slowly from 11.2 Tg in 1990 to 10.1 Tg in 2000, on the basis of annual trends in cultivated area (9). Emissions from livestock were calculated for three animal classes—large animals, sheep and goats, and pigs—following the method and emission rates used in the China Country Study (7), which includes both enteric fermentation and manure contributions. The numbers of animals have increased substantially during the decade (9), leading to an increase in CH4 emissions from 5.80 Tg in 1990 to 8.55 Tg in 2000.

Two final contributions to CH4 emissions were estimated, both of which are more uncertain: landfills and biomass burning. We estimate a large increase in landfill emissions from 2.43 Tg in 1990 to 4.35 Tg in 2000, due to changes in both the amount and composition of municipal garbage generated. Methane is also produced by the burning of biomass—whether of agricultural residues in the field after harvest, land clearing for production of new agricultural fields, deforestation, or simply wildfires in grassland and forests. Because this source is difficult to quantify and subject to interannual variability, we have adopted the estimate of Olivier et al. (14) for 1990 [HN13] and have included it at a constant annual value of 1.60 Tg across the decade. The combined estimate from all these source categories shows CH4 emissions in China rising from 30.7 Tg in 1990 to a peak of 34.1 Tg in 1997 and then falling to 33.3 Tg in 2000 (see the figure), a 2.2% reduction in the period 1997 to 2000.

What do we expect for the future? Emissions of CO2 (and probably CH4) in China are thought to have been roughly constant since 2000 (15). Some of the most painful reforms have already been made, and the economy is once again picking up speed—although accession to the World Trade Organization [HN14] is likely to bring a fresh wave of reforms to many sectors of the economy. A return to a slow increase in fossil fuel use has been projected (15), but on a much shallower trajectory and clearly with a large volume of avoided emissions with respect to previous expectations. There is some room for optimism, however, that further increases in greenhouse gas emissions in China might be averted for several years if energy efficiency improvements continue, markets continue to open up and lead to price reforms, persistent inefficiencies in the coal industry are removed, and natural gas continues to penetrate at a rapid rate. Much will depend on the vitality of the Chinese economy in the coming years.

We note that two other important species that influence radiative forcing [HN15], black carbon (BC) and sulfate, have been affected by these same trends in China. The importance of BC aerosol [HN16] has been stressed by Hansen et al. (16) and Jacobson (17); its contribution to positive radiative forcing (warming) in the modern era may be second only to CO2. Although we have insufficient information on changes in combustors and particulate controls to develop complete annual trends for BC, we do have two data points for 1995 and 2000. Our estimate is that BC emissions declined by 32%, from 1.34 Tg in 1995 (18) to 0.91 Tg in 2000 (19). The reduction in coal use was reinforced by a transition in urban areas from the use of raw coal to “smokeless” coal briquettes. Sulfur dioxide is converted in the atmosphere to sulfate aerosol [HN17], which has a negative radiative forcing (cooling). Emissions of SO2 declined from a peak of 26.2 Tg in 1996 (20) to an estimated 20.8 Tg in 2000 (19), a reduction of 21%.

A global-model calculation (17) of the effects of the 1995 to 2000 emission changes in China yields estimated changes in global mean temperatures over a 100-year period of +0.04 K for SO2, −0.026 K for BC, −0.003 K for CO2, and +0.001 K for CH4, an overall net change in global mean temperatures of +0.012 K (±0.02 K). In sum, the changes in emissions in China from 1995 to 2000 could slightly enhance global warming over a 100-year period, because of the dominant effect of the SO2 reductions. Whereas this calculation illustrates the need to address aerosol species in global-warming policies [HN18] (16), it is not intended to detract from the importance of the reductions in the conventional greenhouse gases CO2 and CH4 that China has achieved. China's experience suggests that there are actions that can be taken today in developing countries that would reduce their contribution to global greenhouse gas emissions far below “business-as-usual” projections.

Appendix

General Hypernotes

Dictionaries and Glossaries

A climate glossary is provided by NASA's Goddard Institute for Space Studies (GISS).

A glossary is provided by the Carbon Dioxide Information Analysis Center.

Glossaries for energy and greenhouse gases are provided by the Energy Information Administration of the U.S. Department of Energy (DOE).

The Encyclopedia of the Atmospheric Environment is provided by the Atmosphere, Climate and Environment Information Progamme of the Department of Environmental and Geographical Sciences, Manchester Metropolitan University, UK.

Web Collections, References, and Resource Lists

Yahoo! provides links to Internet resources on climate change.

The Open Directory Project provides links to Internet global change resources.

Global Warming Central, provided by the Energy Project of Pace University School of Law, provides links to Internet global warming resources.

The Carbon Dioxide Information Analysis Center (CDIAC) is DOE's primary global-change data and information analysis center. A table of current greenhouse gas concentrations is provided, as are a FAQ and a collection of global change Internet links.

The U.S. Global Change Research Information Office provides access to data and information on climate change research, adaptation/mitigation strategies and technologies, and global change-related educational resources on behalf of the Global Change Research Program.

The Global Change Electronic Edition Web site, maintained by the Pacific Institute for Studies in Development, Environment, and Security, provides a glossary and links to Internet resources with a section on climate change science.

The Tiempo Climate Cyberlibrary, maintained by the School of Environmental Sciences, University of East Anglia, Norwich, UK, is an electronic information service covering global warming, climate change, sea-level rise, and related issues.

The Internet Guide for China Studies is provided by the Institute of Chinese Studies, University of Heidelberg, Germany.

The China Data Center at the University of Michigan offers a collection of links to Internet resources. The University of Michigan Asia Library provides the China: New Resources Web site with Internet links.

Online Texts and Lecture Notes

The Warming of the Earth is a presentation offered by the Woods Hole Research Center.

The Global Warming Web site is provided by the U.S. Environmental Protection Agency (EPA). A glossary and links to Internet links are included.

The Center for International Earth Science Information Network provides data in scientific and policy fields related to global change studies.

The United Nations Framework Convention on Climate Change provides a Climate Change Information Kit. Included are introductions to the greenhouse effect and greenhouse gases.

The University of Michigan's Global Change Project makes available lecture notes for global change courses on physical processes, human impacts, and sustainability studies.

E. Takla, Department of Geological and Atmospheric Sciences, Iowa State University, provides lecture notes and Web links for a course on climate change. Lecture notes on the potential for global warming are included.

R. Myneni, Department of Geography, Boston University, provides lecture notes and Web resources for a course on global climate change and environmental impacts.

The School of the Environment, University of Leeds, UK, makes available lecture notes for a course on the scientific issues of climate change.

General Reports and Articles

The National Academy Press makes available on the Web a 2001 report Climate Change Science: An Analysis of Some Key Questions and a 2000 report Cooperation in the Energy Futures of China and the United States.

The Global Climate Change Briefing Book of the Congressional Research Service (CRS) is available from the National Library for the Environment, a service of the National Council for Science and the Environment (NCSE). An August 2001 CRS issue brief on global climate change by J. Justus and S. Fletcheris is available.

The Pew Center on Global Climate Change, an educational resource about the causes and potential consequences of climate change, makes available (in Adobe Acrobat format) a presentation by T. Wigle titled “The science of climate change: Global and U.S. perspectives.”

Sinosphere is an online journal of the Professional Association for China's Environment, an education and research organization whose primary purpose is to increase awareness of environmental problems in China.

International Energy Outlook 2001 is a report provided by the DOE's Energy Information Administration (EIA).

The GRID (Global and Regional Integrated Data) Centre, an office of the United Nations Environment Programme (UNEP) in Arendal, Norway, makes available the special reports on climate change of the Intergovernmental Panel on Climate Change (IPCC) (there is a U.S. mirror site). The IPCC makes available on the Web the three volumes of Climate Change 2001.

The vol. 4, no. 1, 1998, issue of Consequences had an article by J. Kasting titled “The carbon cycle, climate, and the long-term effects of fossil fuel burning.”

The 4 May 2001 issue of Science had an Enhanced Perspective by T. Crowley and R. Berner titled “CO2 and climate change.”

Numbered Hypernotes

1. Greenhouse gases and the greenhouse effect. An introduction to greenhouse gases is provided by the DOE's EIA. The World Data Centre for Greenhouse Gases, maintained by the Japan Meteorological Agency in cooperation with the World Meteorological Organization (WMO), provides an introduction to greenhouse gases. The Greenhouse Gas R&D Programme of the International Energy Agency (IEA) offers a presentation on greenhouse gases and climate change. GISS's Institute for Climate and Planets offers an presentation by H. Augenbraun, E. Matthews, and D. Sarma on the greenhouse effect, greenhouse gases, and global warming. I. Sokolik, Program in Atmospheric and Oceanic Sciences, University of Colorado, offers lecture notes on greenhouse gases and global warming for a course on air chemistry and pollution. G. Bothun, Department of Physics, University of Oregon, makes available lecture notes on the greenhouse effect for a course on the physics of energy and the environment. The 24 April 2001 issue of the Proceedings of the National Academy of Sciences had an article by B. Bolin and H. Kheshgi titled “On strategies for reducing greenhouse gas emissions.”

2. China's greenhouse gas emissions. The IPCC's Special Report on Emissions Scenarios provides projections for greenhouse gas emissions that extend to the end of the 21st century. The World Resources Institute offers a presentation on China and climate change. The University of Michigan's Global Change Project makes available 1999 lecture notes titled “Case study: Issues and options in greenhouse emissions control for China.” The Lycos Environment News Service offers a 15 June 2001 article by C. Lazaroff titled “China beats U.S. in greenhouse gas cuts” about a report by the Natural Resources Defense Council (NRDC) titled “China is aggressively reducing its carbon dioxide emissions”; the NRDC issued an October 2001 analysis that updates the earlier report. The 1999 Report on the State of the Environment in China, available from the China Environmental Protection Web site, includes a section on the atmospheric environment. DOE's EIA makes available a country analysis brief on China and environmental issues in China, as well as report titled “China: An energy sector overview” that includes information about carbon emissions. An August 1998 article by Z.-X. Zhang titled “Is China taking actions to limit its greenhouse gas emissions? Past evidence and future prospect” is made available by Resources for the Future (RFF); a 15 September 1998 feature about Zhang's article appeared on RFF's Weathervane Web site.

3. China's coal and energy industries. The IEA makes available the executive summary of a report titled “Coal in the energy supply of China.” The 12 July 2001 issue of Asia Times had an article titled “China: Coal fires still burning.” A 1997 student project by A. Littlefield on Chinese coal use and environmental impacts is made available by the Trade Environment Database of the School of International Service, American University, Washington, DC. The China Coal Information Network publishes the China Coal News. The China Energy Group at the Lawrence Berkeley National Laboratory makes available (in Adobe Acrobat format) an article by J. Sinton and D. Fridley titled “What goes up: Recent trends in China's energy consumption” that was published in the August 2000 issue of Energy Policy (2). DOE's Office of Industrial Technologies makes available a report on Chinese energy usage and energy efficiency. The China E-News Page, provided by the Advanced International Studies Unit of the Pacific Northwest National Laboratory, makes available (in Adobe Acrobat format) a June 2001 report by J. Logan titled “An update on recent energy and carbon dioxide trends in China.” The Environment, Science and Technology Section of the U.S. Embassy Beijing offers a August 2001 report on the controversy over China's reported falling energy use, as well as a summary of China's 2000 state of the environment report.

4. The Asian Crisis Home Page is maintained by Kar-yiu Wong, Department of Economics, University of Washington; links to articles and commentaries about China and the Asian crisis are included.

5. The text of the Kyoto Protocol is provided by Global Warming Central. The United Nations Framework Convention on Climate Change provides information about the convention and the Kyoto Protocol and related issues; a guide to the Kyoto Protocol is provided. Cambridge Scientific Abstracts offers a presentation on global warming and the Kyoto Protocol. A CRS report titled “Global climate change: The Kyoto Protocol” is available from the NCSE's National Library for the Environment.

6. CO2 emissions from fossil fuels. B. Shakhashiri, Department of Chemistry, University of Wisconsin, offers a presentation on carbon dioxide. EPA's Global Warming Web site offers a presentation on carbon dioxide emissions. Online Trends, a compendium of data on global change from the CDIAC, includes a section on global, regional, and national fossil fuel CO2 emissions. NASA's GISS offers a presentation titled “Tracking carbon dioxide emissions from fossil fuel burning.” The November 2000 issue of Physics Today had an article by A. Rosenfeld, T. Kaarsberg, and J. Romm titled “Technologies to reduce carbon dioxide emissions in the next decade.” Carbon Dioxide and Climate Change is the proceedings of a National Academy of Sciences colloquium (originally published in the 5 August 1997 issue of the Proceedings of the National Academy of Sciences).

7. The Intergovernmental Panel on Climate Change was established by WMO and UNEP to assess the scientific, technical, and socio-economic information relevant for the understanding of the risk of human-induced climate change. The IPCC National Greenhouse Gas Inventories Programme makes available a copy of the revised 1996 IPCC guidelines for national greenhouse gas inventories.

8. China Climate Change Country Study. The 1995 Interim Report on Climate Change Country Studies, issued by the U.S. Country Studies Program, includes a chapter titled “China: Studies addressing global climate change.”

9. CO2 emissions from cement manufacture. A fact sheet titled “Why cement-making produces carbon dioxide” is provided by the UNEP's Information Unit for Conventions.

10. Chinese afforestation efforts. The Chinese Academy of Forestry offers a presentation about forestry development in China with a section on afforestation programs. A section on forests and grasslands is included in the 1999 report on the state of the environment in China, available from the China Environmental Protection Web site. The Forests and Forestry Web site of the Food and Agriculture Organization (FAO) provides information about forests in China. The FAO's Asia-Pacific Forestry Sector Outlook Study includes a working paper by S. Kunshan et al. titled “China's country report on forestry.” The Environment, Science and Technology Section of the U.S. Embassy Beijing offers an August 2000 report titled “Trees vs. people? PRC natural forest protection,” as well as links to Chinese environmental Web Sites. The IPCC's Special Report on Land Use, Land-Use Change and Forestry includes a chapter on afforestation, reforestation, and deforestation activities.

11. China's National Bureau of Statistics makes available the China Statistical Yearbook 1999, as well as a statistical communique on the 2000 national economic and social development of China.

12. Methane emissions. B. Shakhashiri, Department of Chemistry, University of Wisconsin, offers a presentation on methane. Global Warming: Focus on the Future, a Web exhibit developed by the American Museum of Natural History and the Environmental Defense Fund, includes a section on methane. EPA's Global Warming Web site offers a presentation on methane emissions. The CDIAC's Online Trends Web site includes a section on methane emissions. GISS's Institute for Climate and Planets makes available a presentation by H. Augenbraun, E. Matthews, and D. Sarma on the global methane cycle. The IPCC's Special Report on Emissions Scenarios includes a section on methane emissions from fossil fuel and waste-disposal sources and sections on methane emissions from rice production and enteric fermentation. DOE's EIA offers an information page on U.S. methane emissions and their sources.

13. Olivier et al.'s 1990 data. The Emission Database for Global Atmospheric Research (EDGAR), maintained by J. Olivier, National Institute for Public Health and the Environment, Bilthoven, Netherlands, provides data for CH4 from anthropogenic sources in 1990, as well as a summary of the study (14).

14. WTO and China. The World Trade Organization (WTO) issued a 17 September 2001 press release and a 10 November 2001 press release about China's WTO membership. The United States-China Business Council provides information on the WTO and China. A CRS report on China and the WTO is provided by the National Library for the Environment. The Institute for International Economics makes available a topical page on China and the WTO; a 1999 policy brief by D. Rosen titled “China and the World Trade Organization: An economic balance sheet” is included. The Institute of International Economic Law at the Georgetown University Law Center offers information on China's WTO accession. CNN.com Asia provides a WTO special report; included is a 13 November 2001 article by W. Lam about China and the WTO titled “China's WTO membership — Now the hard part begins.”

15. Radiative forcing. Radiative forcing is defined in EPA's global warming glossary. The IPCC's Climate Change 2001: The Scientific Basis includes a chapter on radiative forcing of climate change. R. Myneni provides lecture notes on radiative forcing of climate change for a course on global climate change and environmental impacts. The School of the Environment, University of Leeds, UK, makes available lecture notes on radiative forcing, global warming potentials, and climate feedbacks for a course on the scientific issues of climate change. The 2001 report Climate Change Science: An Analysis of Some Key Questions includes a chapter on human-caused forcings. NASA's GISS makes available a presentation by J. Hansen titled “The forcing agents underlying climate change.”

16. Aerosols and black carbon aerosol. Aerosol is defined in the CDIAC glossary. The Encyclopedia of the Atmospheric Environment provides an introduction to aerosols. GISS's Institute for Climate and Planets offers a presentation by M. Reyes on aerosols in the atmosphere. The Atmospheric Chemistry Research Program of NOAA's Pacific Marine Environmental Laboratory provides a presentation on aerosol climate forcing; a collection of links to Internet resources is provided. The IPCC's Climate Change 2001: The Scientific Basis includes information about black carbon in the chapter on the direct and indirect effects of aerosols. The Global Emissions Inventory Activity Web site has entries for black carbon emissions from fossil fuel combustion and from biomass burning. The 8 February 2001 issue of Nature had an article by M. Jacobson tilted “Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols” (17) and a News and Views article by M. Andreae titled “The dark side of aerosols.” Stanford University issued a press release about Jacobson's research. The March 2001 issue of Geotimes had a news article by C. Reed about this research titled “Carbon's other warming role.” The Environment News Service had an 8 February 2001 article by C. Lazaroff about this research titled “Soot called major cause of global warming.”

17. Sulfate aerosol. Sulfate aerosol is defined in EPA's global warming glossary. The IPCC's Climate Change 2001: The Scientific Basis has a section on sulfates in the aerosol chapter. The Environmental News Network offers a 9 March 1999 article titled “Sulfate aerosols' role in climate change studied.” The 26 February 1999 issue of Science had an Enhanced Perspective by J. Kiehl titled “Solving the aerosol puzzle.”

18. Aerosols, global warming, and policy implications. The 29 August 2000 issue of the Proceedings of the National Academy of Sciences had an article by J. Hansen et al. titled “Global warming in the twenty-first century: An alternative scenario” (16) and a commentary by R. Cicerone titled “Human forcing of climate change: Easing up on the gas pedal.” A research brief by Hansen about this study is made available by GISS. http://science.nasa.gov/default.htm offers an article titled “Culprits of climate change” about Hansen et al.'s research. I. Sokolik offers lecture notes on global change due to anthropogenic aerosols for a course on air chemistry and pollution. J. Penner, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, makes available (in Adobe Acrobat Format) an encyclopedia article titled “Aerosols, effects on the climate” and a 1996 book chapter titled “The contribution of carbonaceous aerosols to climate change,” as well as other related publications. A presentation titled “Atmospheric aerosols and their effects on climate” is provided by the Global Hydrology and Climate Center. A chapter on climate forcing by aerosols is included in the 1996 report A Plan for a Research Program on Aerosol Radiative Forcing and Climate Change, available from the National Academy Press. The 10 November 2000 issue of Science had a Perspective by S. Smith, T. Wigley, and J. Edmonds titled “A new route toward limiting climate change?” The 12 May 2000 issue had a Perspective by S. Schwartz and P. Buseck about aerosols and climate titled “Absorbing Phenomena.”

19. D. G. Streets is in the Decision and Information Sciences Division, Argonne National Laboratory.

20. K. Jiang and X. Hu are at the Center for Energy, Environment and Climate Change, Energy Research Institute, Beijing.

21. J. E. Sinton. is in the Energy Analysis Department, Lawrence Berkeley National Laboratory.

22. X.-Q. Zhang and D. Xu are at the Forest Ecology and Environment Institute of the Chinese Academy of Forestry, Beijing.

23. M. Z. Jacobson is in the Department of Civil and Environmental Engineering, Stanford University.

24. J. E. Hansen is at the NASA Goddard Institute for Space Studies, New York.

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