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complement prior studies that highlight the 

importance of short- and medium-lived pollutants ( 14– 17). 

The top 10 pollutant-generating activities 

contributing to net RF (positive RF minus 

negative 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; whereas 

the importance of long-term climate stabilization is clear, the perceived urgency of 

near-term mitigation will evolve with our 

knowledge 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 should 

include 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). CO2 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 

 The accounting now used for assessing 

compliance with carbon limits in the 

Kyoto 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 

CO2 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 

CO2 target of 450 ppm under this accounting 

would cause bioenergy crops to expand to 

displace virtually all the world’s natural forests and savannahs by 2065, releasing up to 

37 gigatons (Gt) of CO2 per year (compaFixing 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.

1Princeton University, Princeton, NJ 08544, USA. 2Environmental Defense Fund, Boston, MA 02108, and Washington, DC 20009, USA. 3Marine Biological Laboratory, Woods 

Hole, MA 02543, USA. 4Duke University, Durham, NC 

27708, USA. 5International Institute for Applied Systems 

Analysis, Laxenburg 2361, Austria. 6University of California at Berkeley, Berkeley, CA 94720, USA. 7Cary Institute of 

Ecosystem Studies, Millbrook, NY 12545, USA. 8Michigan 

State University, Hickory Corners, MI 49060, USA. 9University 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 CO2 decay is calculated according to 

p. 824 in ( 29). Growth rates are from ( 28) and ( 30). Zero 

growth of emissions assumed for BC, OC, SO2, and halocarbons Each year’s RF for short-lived pollutants (BC, OC, 

O3, SO2) is due only to emissions in that year; thus, the RF 

does 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.

Supporting Online Material

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528 23 OCTOBER 2009 VOL 326 SCIENCE www.sciencemag.org 

POLICYFORUM

rable to total human CO2 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 CO2

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 depends 

on the source of the biomass and its net landuse effects. Replacing fossil fuels with bioenergy does not by itself reduce carbon 

emissions, because the CO2 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 timber 

residues 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 bioenergy 

crops 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 Climate 

Change (IPCC) has long realized that bioenergy’s greenhouse effects vary by source 

of 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 CO2 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 other 

emissions 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 CO2 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 CO2 emitted from 

bioenergy 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-use 

emissions 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 tracing 

the 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 crediting 

system, credits must refl ect net changes in carbon stocks, emissions of non-CO2

 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 States 

have 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 in 

comprehensiveness, 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 Gas 

Inventories 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 on 

annual 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.

Supporting Online Material

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10.1126/science.1178797

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