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complement prior studies that highlight theimportance of short- and medium-lived pollutants ( 14

– 17 ). The top 10 pollutant-generating activities contributing to net RF (positive RF minusnegative RF) in year 20 are shown in the bottom chart, page 526), which takes into account the emission of multiple pollutants from each source activity ( 18 ). The seven sources that appear only on the left side (purple bars) would be overlooked by mitigation strategies focusing exclusively on long-lived pollutants.The distinctly different sources of nearterm and long-term RF lend themselves to the aforementioned two-pronged mitigation approach. This decoupling is convenient for policy design and implementation; whereasthe importance of long-term climate stabilization is clear, the perceived urgency of near-term mitigation will evolve with ourknowledge of the climate system. Additionally, optimal near-term mitigation strategies will refl ect decadal oscillations ( 19 ), seasonal and regional variations ( 20

, 21 ), and evolving knowledge of aerosol-climate effects ( 22

, 23 ) and methane-atmosphere interactions ( 22 )—considerations unique to the near term. Thus, short- and medium-lived sources (black carbon, tropospheric ozone, and methane) must be regulated separately and dynamically. The long-term mitigation treaty should focus exclusively on steady reduction of long-lived pollutants. A separate treaty for short- and medium-lived sources shouldinclude standards that evolve based on periodic recommendations of an independent international scientifi c panel. The framework of “best available control technology” (strict) and “lowest achievable emissions rate” (stricter) from the U.S. Clean Air Act ( 24 ) can be used as a model.Such a two-pronged institutional framework would reflect the evolving scientific understanding of near-term climate change,the scientifi c certainty around long-term climate change, and the opportunity to separately adjust the pace of near-term and longterm mitigation efforts.

References and Notes
1. D. Archer et al

., Annu. Rev. Earth Planet. Sci. 37 , 117 (2009).2. The e-folding time (required to decrease to 37% of original airborne amount) is on the order of days to weeks for short-lived pollutants (e.g., black and organic carbon, tropospheric ozone, and sulfur dioxide), a decade for medium-lived (e.g., methane and some halocarbons), and a century for long-lived (e.g., nitrous oxide, some halocarbons). CO 2 takes roughly a century to reach 37%, then decays more slowly over millennia. 3. C. P. McMullen, J. Jabbour, Eds., Climate Change Science Compendium 2009 (U.N. Environment Programme, Nairobi, EarthPrint, 2009); www.unep.org/compendium2009/.
4. S. Solomon et al

., Climate Change 2007: The Physical 

# T he accounting now used for assessing compliance with carbon limits in theKyoto Protocol and in climate legislation contains a far-reaching but fi xable fl aw that will severely undermine greenhouse gas reduction goals ( 1 ). It does not count CO 2 

## emitted from tailpipes and smokestacks when bioenergy is being used, but it also does not count changes in emissions from land use when biomass for energy is harvested or grown. This accounting erroneously treats all bioenergy as carbon neutral regardless of the source of the biomass, which may cause large differences in net emissions. For example, the clearing of long-established forests to burn wood or to grow energy crops is counted as a 100% reduction in energy emissions despite causing large releases of carbon. Several recent studies estimate that this error, applied globally, would create strong incentives to clear land as carbon caps tighten. One study ( 2 ) estimated that a global CO 2 

## target of 450 ppm under this accounting would cause bioenergy crops to expand todisplace virtually all the world’s natural forests and savannahs by 2065, releasing up to 37 gigatons (Gt) of CO 2 

## per year (compa

# Fixing a Critical Climate Accounting Error **CLIMATE CHANGE**



## Timothy D. Searchinger, 1 

## * Steven P. Hamburg, 2 

## * Jerry Melillo, 3 

## William Chameides, 4 

## Petr Havlik, 5 

## Daniel M. Kammen, 6 

## Gene E. Likens, 7 

## Ruben N. Lubowski, 2 

## Michael Obersteiner, 5 

## Michael Oppenheimer, 1 

## G. Philip Robertson, 8 

## William H. Schlesinger, 7 

## G. David Tilman 9



## Rules for applying the Kyoto Protocol and national cap-and-trade laws contain a major, but fi xable, carbon accounting fl aw in assessing bioenergy.

1 

### Princeton University, Princeton, NJ 08544, USA. 2 

### Environmental Defense Fund, Boston, MA 02108, and Washington, DC 20009, USA. 3 

### Marine Biological Laboratory, Woods Hole, MA 02543, USA. 4 

### Duke University, Durham, NC 27708, USA. 5 

### International Institute for Applied Systems Analysis, Laxenburg 2361, Austria. 6 

### University of California at Berkeley, Berkeley, CA 94720, USA. 7 

### Cary Institute of Ecosystem Studies, Millbrook, NY 12545, USA. 8 

### Michigan State University, Hickory Corners, MI 49060, USA. 9 

### University of Minnesota, St. Paul, MN 55108, USA.

*Authors for correspondence. E-mail: shamburg@edf.org (S.P.H.); tsearchi@princeton.edu (T.D.S.). Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the IPCC (Cambridge Univ. Press, New York, 2007).
5. S. Rahmstorf et al

., Science 316 , 709 (2007). 6. J. B. Smith et al

., Proc. Natl. Acad. Sci. U.S.A. 106 , 4133 (2009). 7. A. Sokolov et al

., J. Clim. 22 , 5175 (2009). 8. T. Stocker, Quat. Sci. Rev. 19 , 301 (2000).
9. T. M. Lenton et al

., Proc. Natl. Acad. Sci. U.S.A. 105

, 1786 (2008). 10. J. Hansen et al

., Open Atmos. Sci. J. 2 , 217 (2008). 11. RF is a property of the climate at a point in time. Increases in RF create planetary energy imbalance, with more incoming solar radiation than outgoing infrared radiation and a warming effect on the system. 12. T. F. Stocker, A. Schmittner, Nature 388 , 862 (1997). 13. R. B. Alley et al

., Science 299 , 2005 (2003). 14. J. Hansen et al

., Philos. Trans. R. Soc. London Ser. A 365

, 1925 (2007). 15. P. K. Quinn et al

., Atmos. Chem. Phys. 8 , 1723 (2008). 16. M. Z. Jacobson, J. Geophys. Res. Atmos. 107 , 4410 (2002). 17. F. C. Moore, M. C. MacCracken, Intl. J. Strategic Change Mgmt. 1 , 42 (2009). 18. D. Koch, T. C. Bond, D. Streets, N. Unger, Geophys. Res.
Lett. 34 , L05821 (2007). 19. K. Trenberth et al. , in ( 4 ), pp. 235–336. 20. D. Koch, T. Bond, D. Streets, N. Unger, G. van der Werf, J. Geophys. Res. 112 , D02205 (2007). 21. A. Stohl, J. Geophys. Res. 111 , D11306 (2006).
22. P. Forster et al. , in ( 4 ), pp. 129–234. 23. V. Ramanathan, G. Carmichael, Nat. Geosci. 1 , 221 (2008). 24. U.S. Clean Air Act, www.epa.gov/oar/caa/. 25. The same analysis applied to the IPCC’s SRES marker scenarios (A1, A2, B1, and B2) ( 26 ) produces results that fall largely within the bounds of these two scenarios (fi g. S1). 26. N. Nakicenovic, R. Swart, Eds., Special Report on Emissions Scenarios (IPCC, Cambridge Univ. Press, Cambridge, 2000). 27. Data for year 2000 RF are based on ( 14 ), emissions are from ( 28 ), decay rates are based on the lifetimes on p. 212 in ( 22 ) and historical CO
2 decay is calculated according to p. 824 in ( 29 ). Growth rates are from ( 28 ) and ( 30 ). Zero growth of emissions assumed for BC, OC, SO 2 , and halocarbons Each year’s RF for short-lived pollutants (BC, OC,
O 3 , SO 2 ) is due only to emissions in that year; thus, the RFdoes not accumulate from one year to the next. The contributions of black carbon and ozone are conservative, as they do not refl ect recent near-double estimates of black carbon’s RF (

23 ) nor recent estimates of ozone’s indirect land sink effect (

31 ). 28. EDGAR 3.2 (www.mnp.nl/edgar/model/). 29. G. Meehl et al. , in ( 4 ), pp. 747–845. 30. Climate Analysis Indicators Tool v6.0 (http://cait.wri.org). 31. S. Sitch, P. M. Cox, W. J. Collins, C. Huntingford, Nature
448 , 791 (2007).
32. T. C. Bond et al

., Global Biogeochem. Cycles 21 , GB2018 (2007). 33. The author thanks J. Harte for providing encouragement and critique.



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rable to total human CO 2 

## emissions today). Another study predicts that, based solely on economic considerations, bioenergy could displace 59% of the world’s natural forest cover and release an additional 9 Gt of CO 2

## per year to achieve a 50% “cut” in greenhouse gases by 2050 ( 3 ). The reason: When bioenergy from any biomass is counted as carbon neutral, economics favor large-scale land conversion for bioenergy regardless of the actual net emissions ( 4 ). The potential of bioenergy to reduce greenhouse gas emissions inherently dependson the source of the biomass and its net landuse effects. Replacing fossil fuels with bioenergy does not by itself reduce carbon emissions, because the CO 2 

## released by tailpipes and smokestacks is roughly the same per unit of energy regardless of the source ( 1

, 5 ). Emissions from producing and/or refi ning biofuels also typically exceed those for petroleum ( 1

, 6 ). Bioenergy therefore reduces greenhouse emissions only if the growth and harvesting of the biomass for energy captures carbon above and beyond what would be sequestered anyway and thereby offsets emissions from energy use. This additional carbon may result from land management changes that increase plant uptake or from the use of biomass that would otherwise decompose rapidly. Assessing such carbon gains requires the same accounting principles used to assign credits for other land-based carbon offsets.For example, if unproductive land supports fast-growing grasses for bioenergy, or if forestry improvements increase tree growth rates, the additional carbon absorbed offsets emissions when burned for energy. Energy use of manure or crop and timberresidues may also capture “additional” carbon. However, harvesting existing forests for electricity adds net carbon to the air. That remains true even if limited harvest rates leave the carbon stocks of regrowing forests unchanged, because those stocks would otherwise increase and contribute to the terrestrial carbon sink ( 1 ). If bioenergycrops displace forest or grassland, the carbon released from soils and vegetation, plus lost future sequestration, generates carbon debt, which counts against the carbon the crops absorb ( 7

, 8 )

. The Intergovernmental Panel on ClimateChange (IPCC) has long realized that bioenergy’s greenhouse effects vary by sourceof biomass and land-use effects. It also recognizes that when forests or other plants are harvested for bioenergy, the resulting carbon release must be counted either as land-use emissions or energy emissions but not both. To avoid double-counting, the IPCC assigns the CO 2 

## to the land-use accounts and exempts bioenergy emissions from energy accounts ( 5 ). Yet it warns, because “fossil fuel substitution is already ‘rewarded’” by this exemption, “to avoid underreporting . . . any changes in biomass stocks on lands . . . resulting from the production of biofuels would need to be included in the accounts” ( 9 ). This symmetrical approach works for the reporting under the United Nations Framework Convention on Climate Change (UNFCCC) because virtually all countries report emissions from both land and energy use. For example, if forests are cleared in Southeast Asia to produce palm biodiesel burned in Europe, Europe can exclude the tailpipe emissions as Asia reports the large net carbon release as land-use emissions.However, exempting emissions from bioenergy use is improper for greenhouse gas regulations if land-use emissions are not included.The Kyoto Protocol caps the energy emissions of developed countries. But the protocol applies no limits to land use or any otheremissions from developing countries, and special crediting rules for “forest management” allow developed countries to cancel out their own land-use emissions as well ( 1

, 10 ). Thus, maintaining the exemption for CO 2 

## emitted by bioenergy use under the protocol ( 11 ) wrongly treats bioenergy from all biomass sources as carbon neutral, even if the source involves clearing forests for electricity in Europe or converting them to biodiesel crops in Asia . This accounting error has carried over into the European Union’s cap-and-trade law and the climate bill passed by the U.S. House of Representatives ( 1

, 12

, 13 ). Both regulate emissions from energy but not land use and then erroneously exempt CO 2 

## emitted frombioenergy use. In theory, the accounting system would work if caps covered all land-use emissions and sinks. However, this approach is both technically and politically challenging as it is extremely hard to measure all land-useemissions or to distinguish human and natural causes of many emissions (e.g., fi res). The straightforward solution is to fi x the accounting of bioenergy. That means tracingthe actual fl ows of carbon and counting emissions from tailpipes and smokestacks whether from fossil energy or bioenergy. Instead of an assumption that all biomass offsets energy emissions, biomass should receive credit to the extent that its use results in additional carbon from enhanced plant growth or from the use of residues or biowastes. Under any creditingsystem, credits must refl ect net changes in carbon stocks, emissions of non-CO 2 

## greenhouse gases, and leakage emissions resulting from changes in land-use activities to replace crops or timber diverted to bioenergy ( 1 ). Separately, Europe and the United Stateshave established legal requirements for minimum use of biofuels, which assess greenhouse gas consequences based on life-cycle analyses that refl ect some land-use effects ( 1

, 14 ). Such assessments vary widely incomprehensiveness, but none considers biofuels free from land-based emissions. Yet the carbon cap accounting ignores land-use emissions altogether, creating its own large, perverse incentives. Bioenergy can provide much energy and help meet greenhouse caps, but correct accounting must provide the right incentives.

References and Notes 1. Additional references supporting the themes of this Policy Forum can be found in the supporting online material.
2. M. Wise et al

., Science 324 , 1183 (2009). 3. J. M. Melillo et al

., Unintended Environmental Consequences of a Global Biofuel Program (MIT Joint Program Report Series, Massachusetts Institute of Technology, Cambridge, MA, 2009). 4. International Energy Agency, Energy Technology Perspectives: In Support of the G8 Plan of Action: Scenarios and Strategies to 2050 [Organization for Economic Cooperation and Development (OECD)/IEA, Paris, 2008]. 5. IPCC, 2006 IPCC Guidelines for National Greenhouse Gas
Inventories , prepared by the National Greenhouse GasInventories Programme [Institute for Global Environmental Strategies (IGES), Tokyo, Japan, 2007]. 6. E. Manichetti, M. Otto, in Biofuels: Environmental Consequences and Interactions with Changing Land Use: Proceedings of the Scientifi c Committee on Problems of the Environment , R. W. Howarth, and S. Bringezu, Eds. (Cornell Univ. Press, Ithaca, NY, 2009), pp. 81–109. 7. T. Searchinger et al

., Science 319 , 1238 (2008). 8. J. Fargione, J. Hill, D. Tilman, S. Polasky, P. Hawthorne,
Science 319 , 1235 (2008).
9. R. Watson et al ., Eds., Land Use, Land-Use Change, and Forestry

(IPCC, Cambridge Univ. Press, Cambridge, 2000). 10. UNFCCC, Report of the Conference of the Parties on Its Seventh Session: Action taken by the COP (FCCC/CP/20001/13/Add.1, UNFCCC, Geneva, 2002), Addendum, part 2. 11. UNFCCC, Updated UNFCCC reporting guidelines onannual inventories following incorporation of the provisions of decision 14/CP.11 [FCCC/Subsidiary Body for Scientifi c and Technological Advice (SBSTA)/2006/9, Geneva, 2006], p. 23. 12. European Commission, Directive 2003/87/EC of the European Parliament and of the Council of 13 October
2003 , Offi cial Journal of the European Union L 275,
25.10.2003. 13. The American Clean Energy and Security Act of 2009, H.R. 2454, 111th Cong., 1st Sess. (as passed by U.S. House of Representatives July 2009). 14. T. D. Searchinger, in Biofuels: Environmental Consequences and Interactions with Changing Land Use: Proceedings of the Scientifi c Committee on Problems of the Environment, R. W. Howarth and S. Bringezu, Eds. (Cornell Univ. Press, Ithaca, NY, 2009), pp. 37–52. 15. The authors express thanks for the support of the German Marshall Fund of the United States.



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