Biomass sinks and biomass energy:key issues in using biomass to protect

the global climateSivan Kartha

Stockholm Environment Institute and Tellus Institute, 11 Arlington St., Boston, Massachusetts, United States 02116

1. IntroductionThere are two features of biomassthat link it inextricably to the chal-lenge of keeping carbon out of the at-mosphere and preventing climatechange. The first is that biomass, inthe form of plants and the soils inwhich they grow, stores a hugeamount of carbon. The second is thatbiomass can substitute for fossil fuel.It is not clear how these two featureswill be exploited in the global effortto address climate change, but it isclear that a tremendous amount ofcontroversy surrounds biomass. Infact, at the Hague in November 2000,a primary reason for the breakdownof negotiations at the meeting of sig-natories to the Framework Conven-tion on Climate Change was theargument over the role of biomass.This article aims to explain the maincontentious issues surrounding thepotential role of biomass in address-ing the climate challenge.

2. Two roles for biomass: sinksand energy

Biomass serves as a so-called ‘‘carbonsink’’, absorbing carbon from the at-mosphere and storing it in the formof living plants, dead plant matter,and decomposed plant matter in soils.Vast amounts of carbon are continu-ally being exchanged between thisbiospheric sink and the atmosphere.Through photosynthesis, plants ab-sorb carbon from the atmosphere tomake plant tissues (carbohydrates),and through respiration, plants re-lease carbon as they metabolizestored carbohydrates. Dead plant mat-ter also releases carbon dioxide to theatmosphere, as soil bacteria decom-pose it. Approximately 120 billiontonnes (i.e., gigatonnes) of carbon(GtC) is absorbed annually through

photosynthesis, and a comparableamount is released through respira-tion and decomposition. This naturalflux of carbon between the biosphereand the atmosphere is finely bal-anced, and has been so for millennia.

In the past century, however, wehave perturbed this equilibriumenough to disrupt the climate system[IPCC, 2001]. Human activities, suchas the clearing of land for agricultureand logging, currently release an ad-ditional 2 GtC to the atmosphere eachyear. In addition, roughly 6 GtC is re-leased to the atmosphere annuallyfrom burning fossil fuels. Since thedawn of the industrial age, approxi-mately 175 GtC of excess carbon hasaccumulated in the atmosphere due tohuman activity -- enough to raise theconcentration of carbon dioxide from285 to 370 parts per million (ppm).(See Table 1.)

The amount of carbon held in thebiosphere is vast -- more than threetimes as much as is in the atmos-phere. A relatively small change inthe amount of carbon in the biospherewould cause a significant change inthe greenhouse balance of the atmos-phere. Indeed, the release of merely7 % of the biospheric stock of carbonis enough to double the 175 billiontonnes (GtC) that human activitieshave so far added to the atmosphere.And releasing merely 0.3 % of thebiospheric stock would double the 8GtC emitted annually due to humanactivity. The biosphere’s critical placein the global carbon cycle has in-spired many to consider the possibil-ity of enlisting biospheric sinks in ourefforts to avert climate change. Pos-sible strategies include reducing de-forestation, restoring degraded lands,improving land management prac-tices, and shifting to land uses that

increase the amount of carbon storedon the land.

There is a second set of strategiesfor enlisting the biosphere to helpavert climate change. These are basedon the fact that the biosphere containsnot only a vast amount of carbon, butalso a vast amount of energy.Biomass energy can be used to dis-place fossil energy. This will result inlower carbon emissions, providingthat biomass is grown and harvestedthrough a ‘‘carbon-neutral’’ cycle,wherein the carbon released when thebiomass is consumed is offset by anequal amount of carbon absorbedwhen each successive crop ofbiomass is produced. The use ofbioenergy might not be preciselycarbon-neutral in totality, insofar asfossil fuels are used to grow, manage,fertilize, harvest, and process thebiomass fuels. But, unlike fossil fuelcycles, well-designed biomass fuelcycles can be close to carbon-neutral.Bioenergy is likely to provide a cru-cial non-fossil energy source for apost-fossil fuel world.

3. Biomass sinks

From the standpoint of the green-house effect, both options -- biomasssinks and biomass energy -- areequivalent. It doesn’t matter whetheryou keep a tonne of carbon dioxideout of the atmosphere by preventingthe clearing of some tropical forest inBrazil, or by substituting some coalwith some biomass in a United Statespower plant. The two tonnes wouldhave equally potent greenhouse im-pacts, and the atmosphere is mixedquickly enough that the location ofthe source is irrelevant. What, then,are the chief differences between thetwo biomass-based routes to reducingcarbon emissions? To start, it is useful

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to clarify the main features of sink-based approaches to mitigation, in-cluding the key contentious issuesthat arise.3.1. Biomass sinks -- reasons forenthusiasmThere are several compelling reasonsfor the enthusiastic pursuit ofbiospheric sinks. First, as Table 1shows, human impacts on the bio-sphere result in approximately 2GtC/yr of carbon emissions -- roughly25 % of total human emissions ofcarbon. In the long run, the climatecannot be stabilized unless totalglobal emissions (fossil emissionsplus land-use emissions) fall belowthis level, so, even if emissions fromfossil fuels were completely halted,emissions from biospheric stocksmust ultimately be reduced as welland, ideally, reversed.

Broadly, there are four meansthrough which this can be done.1. We can reduce the rate at which

we destroy forests and other largereservoirs of carbon.

2. We can restore degraded ecosys-tems that have lost plant matterand soil carbon.

3. We can adopt new land-manage-ment practices that enable land tohold more carbon.

4. We can shift to entirely new landuses that store more carbon thancurrent uses.

Table 2 illustrates the rough magni-tude of emission benefits to be gainedfrom a worldwide program to preventdeforestation (Group 1), restore land(Group 2), improve land management(Group 3), and change land uses(Group 4) [IPCC, 2000]. This verycoarse estimate suggests a benefit of1.9 GtC/yr by the year 2010 wouldbe achievable if deforestation rateswere halved relative to today’s levelsand further efforts were deployedacross a considerable portion of otherland categories. For reference, theKyoto Protocol committed the indus-trialized countries to a collectiveemission ceiling that is approximately0.5 GtC/yr below their projected levelof emissions in the year 2010. Indus-trialized countries could in theorymeet their commitments relyingsolely on a fraction of the sink-basedactivities listed in the table, with noneed to resort to reducing fossil fuelconsumption. Admittedly, this wouldentail an ambitious policy agendawith a level of success that dramati-cally exceeds efforts to date, yet it il-lustrates the magnitude of theopportunities available through sink-

based mitigation activities.It is difficult to estimate the cost

of such measures, but many advo-cates of sink-based activities haveclaimed they will be inexpensive. Inits efforts to address the climate prob-lem, the global community has in-vested considerable effort insearching for the cheapest options forkeeping CO2 out of the atmosphere,often expressing the total cost in $/tof carbon. Based on an inventory ofsink-based projects implemented todate, the cost of carbon reductions istypically in the range $ 1-15/t carbon[IPCC, 2000]. This is highly attrac-tive to investors interested in carbonreductions. The above inventory ofsink-based projects represents $ 160million invested -- before the KyotoProtocol has entered into force. TheWorld Bank has projected that the an-nual commerce in sink-based projectscould reach $ 150 billion per year in2020 [Lohmann, 1999].

The emission of carbon frombiospheric sinks is usually a sign ofdeeper problems, so steps to preserveor enhance those sinks can yield tre-mendous ancillary benefits. Indeed,carbon is kept out of the atmosphere,but arguably the primary gain wouldderive from stopping deforestation,restoring wastelands, preventing soilerosion, preserving wetlands, protect-ing watersheds, and making agricul-ture more sustainable. One mightargue that climate change is a less im-mediate problem than declining bio-diversity, deteriorating freshwaterresources, or agricultural pollution.Directing climate change efforts to-ward sinks could provide a financialincentive to resolve crises for whichresources are now sorely lacking.

Sink-based options might be theonly emission mitigation activitiesthat direct resources toward theworld’s rural poor. Activities aimed atreducing fossil fuel emissions willflow toward those who use (generallylarge quantities of) fossil fuels. Theestimated two billion people whohave neither electricity nor refinedcooking fuels will remain irrlevant tothe billions of dollars invested insuch activities. In contrast, resourcesaimed at preserving biospheric car-bon might naturally flow toward the

Table 1. Terrestrial biomass contains huge amounts of carbon, hence considerable potentialfor climate change mitigation

Carbon stocks and fluxes

Fluxes

Annual natural exchange between biosphere and atmosphere (balance between photosynthesis and respiration/decomposition)

∼120 GtC/yr

Annual excess releases from biosphere(due to human land-use activities)

+ 2 GtC/yr

Annual releases from fossil fuels(due to human energy activities)

+ 6 GtC/yr

Annual gross emissions to atmosphere due to human activity + 8 GtC/yr

Stocks

Biospheric carbon 2500 GtC

of which : soils 2000 GtC

vegetation 500 GtC

Preindustrial atmosphere (285 ppm CO2) 600 GtC

Today’s atmosphere (370 ppm CO2) 775 GtC

Net addition to atmosphere to date due to human activity + 175 GtC

Sources: Figures taken from IPCC [2000] and IPCC [2001].

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rural poor. Subsistence farmers mighteagerly collaborate in agroforestryprograms, indigenous communitiesmight welcome efforts that help themprotect their native forests, and land-less poor could be the prime benefi-ciaries of the restoration of thedegraded lands on which their liveli-hoods rely.3.2. Biomass sinks -- reasons forcautionFor the above reasons, the enthusiasmfor sink-based projects is great,among investors as well as some en-vironment and development NGOs.However, serious problems also arise,which have led to a strong resistanceto their unrestricted deploymentwithin the international climate re-gime.

First, although sink-related emis-sions are important, fossil fuel-relatedemissions are by far inflicting moreharm on the climate. They accountfor three-quarters of global emissionsand are increasing more rapidly. Toresolve the climate problem, it is

indisputably necessary for fossil fuelemissions to be cut radically. Thelonger this is postponed, the longerthe delay in developing and deploy-ing low-carbon technologies, and thegreater the investments sunk in fossilfuel-intensive capital and infrastruc-ture. Long-lived capital, such as ve-hicles, buildings, and factories, is along-term commitment to emit, whichcannot be broken without consider-able cost. Seemingly inexpensivesink-based options might prove to bea false economy in the longer term.

Second, crediting sink-based miti-gation is problematic. The science ofbiospheric carbon is extremely com-plex, introducing considerable uncer-tainty into quantifying the carbonbenefits of any given activity. Meas-uring the carbon contained on a par-cel of land is a fairly impreciseexercise. The proportional change incarbon induced by some sink-basedactivities (especially those related tosoil carbon) can be very small, andthe natural variability -- both spatial

and temporal -- can be much greater.Accurate measurements can thereforebe difficult and costly. It might there-fore be prudent to postpone the for-mal crediting of sink-based activitiesuntil the science and monitoring tech-nology is more advanced [WBGU,1998].

Third, the benefits of sink-basedactivities are difficult to credit notonly because of technical uncertain-ties, but because of social uncertain-ties as well. A particular parcel offorest can certainly be protected fromdeforestation, but unless the underly-ing cause is addressed, it is likely thatthe offending logging or land-clear-ing will simply ‘‘leak’’ to a nearbyparcel of forest outside of the formalboundaries of the project. Sink-basedactivities that aim to protect ecosys-tems, therefore, cannot be effectiveunless they are focused primarily onthe demand -- rather than supply --side of the equation. Arguably, pro-jects aimed at recycling wood prod-ucts and reducing (land-intensive)

Table 2. Illustrative potential for reducing emissions using biospheric reservoirs

Total global area(Mha)

Percentagetargeted in 2010

Annual change incarbon stock

(tC/ha/yr)

Estimated changein carbon in 2010

(MtC/yr)

1. Halting deforestation

Temperate forests current annual rate: 2 50% 60 60

Tropical forests current annual rate: 14 50% 120 840

2. Restoring degraded ecosystems

Wetland restoration 230 5% 0.4 4

Restoring severely degraded land 280 5% 0.3 3

3. Improving land management

Forest management 4,050 10% 0.4 170

Cropland management 1,300 30% 0.3 125

Grazing land management 3,400 10% 0.7 240

Agroforestry 400 20% 0.3 26

Rice paddies 150 50% 0.1 7

Urban land management 100 5% 0.3 2

4. Changing land uses

Agroforestry on unproductive lands 630 20% 3.1 390

Cropland conversion from grassland 1,500 3% 0.8 38

Emission reduction potential 1,900

Source: Adapted from IPCC [2000]. Section 1 adapted from Table 3, Sections 2-4 adapted from Table 4.

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meat consumption would more effec-tively reduce deforestation than pro-jects aimed at fencing and guardingplots of land.

Fourth, the net result of a sink-based project is the enhancement ofa biospheric stock of carbon -- eitherby augmenting an existing stock orpreventing its release. This is an im-permanent result that can be undoneat any time, through human activitiesor natural events. A forest, though itmay be protected for some indefinitemeantime, remains a potential targetfor logging or land-clearing and re-mains vulnerable to being consumedby fire, ravaged by pests, or devas-tated by storms. Most dangerously, awarming climate could itself degradeforests and other ecosystems, releas-ing carbon that further accelerateswarming in a positive feedback of un-predictable magnitude [IPCC, 2000].This reflects the fact that a vast flowof carbon rapidly cycles between thebiospheric and atmospheric reservoirsvia processes over which humankindhas little control. In contrast, fossilcarbon rests in a secure undergroundreservoir that is not vulnerable to un-intentional releases to the atmos-phere. This distinction is critical. Itunderlies the potential hazard in ra-tionalizing the continued release offossil carbon by ‘‘offsetting’’ it withcarbon sequestered in a much morelabile biospheric reservoir.

Finally, sink-based activities holdout the promise of channeling re-sources towards activities that delivera host of environmental and socialbenefits. This is, perhaps, the mostcompelling reason for undertaking

such activities. However, these bene-fits do not automatically materializeas carbon is stored on the land. Itmust appear as the result of well-de-signed activities that explicitly allo-cate resources towardenvironmentally and socially sustain-able results, with carbon storage as acomponent but not the driving ele-ment in the effort. If an internationalclimate agreement such as the KyotoProtocol confers an economic valueon carbon, then investors’ sink-basedactivities will respond to that incen-tive with projects that maximize thestorage of carbon. It is not difficultto imagine sink-based activities thatvery effectively sequester carbon,without delivering environmental orsocial benefits, and perhaps deliver-ing detrimental impacts. For instance,some intensive monoculture planta-tions have replaced native grasslandsthat were more biodiverse; and someforests have been ‘‘protected’’ by dis-placing native communities; andsome degraded common lands havebeen restored by excluding communi-ties whose livelihoods had relied onthat land [Lohmann, 1999; FordFoundation, 1998].

4. Biomass energy

Bioenergy-based options for reducingcarbon emissions address most, butnot all, of these concerns. Each of thefive concerns cited above is discussedhere. First, bioenergy reduces fossilcarbon emissions, and offers a set oftechnological options that are likelyto prove critical to the long-term tran-sition away from a fossil economy.(See, for example, the other papers in

this issue and in Energy for Sustain-able Development Vol 4 No 3, SpecialIssue on Modernized Biomass En-ergy.) It cannot be assumed that thetimely development and deploymentof the needed technologies will occurspontaneously. The over-reliance onbiomass-based sinks to satisfy obliga-tions under the Kyoto Protocol woulddilute the incentive to develop suchalternative technologies.

Table 3 gives an approximate com-parison of the carbon dioxide emis-sions from selected fossil fuel andbiomass electricity generation tech-nologies. The emissions from eachfuel/technology combination dependon the efficiency of the generationtechnology and the carbon content ofthe fuel. In the case of biomass, it isassumed that the biomass feedstockis grown sustainably, leading to nolong-term reduction in carbon on theland, only the cyclical variations asthe land passes through periodicstages of growth and harvest. (For ex-ample, it is assumed that biomass fuelis not obtained by clearing virginland.) It is also assumed that the pro-duction, harvesting and delivery ofbiomass consume fossil fuels thatamount to approximately 1/12 of theenergy of the delivered biomass[Turhollow and Perlack, 1991]. Thecarbon benefit of a given bioenergytechnology will depend on the fossiltechnology it displaces. For example,approximately 85% reduction in car-bon emissions would result from dis-placing diesel oil with biogas in adiesel generator. A comparable bene-fit would result from displacing natu-ral gas with gasified biomass in a gasturbine. In the long term, low-carbontechnological options such as thesewill need to provide the basis for theglobal energy system.

Second, compared with sink-basedactivities, mitigation projects basedon bioenergy are less subject to meas-urement uncertainty. These problemsdo exist, but they are less severe. Onecan straightforwardly measure the en-ergy produced at a bioenergy facility.The greater challenge is to determinewhat fossil fuel-based energy hasbeen displaced. This so-called ‘‘base-line’’ is not amenable to direct meas-urement, but reflects a counterfactual

Table 3. Approximate full fuel cycle carbon dioxide emissions from sample biomass andfossil technologies

Fuel and technology Generation efficiency

Grams ofCO2 per

kWh

Small diesel generator 20 % 1320

Coal steam cycle power plant 33 % 1000

Natural gas combined cycle power plant 45 % 410

Biogas digester with diesel generator (with 15% diesel pilot fuel) 18 % 220

Biomass steam cycle power plant 22 % 100

Biomass gasifier/gas turbine power plant 35 % 60

Source: adapted from Kartha and Larson [2000]

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situation that must be conjectured onthe basis of projections and observa-tions of the surrounding energy sectorcontext. This is likely to be a vexingexercise for both bioenergy projectsand sink-based projects. While quan-tifying the benefits of individualbioenergy projects will be difficult,there is no doubt that dramatic carbonbenefits would result from a transi-tion to an energy strategy that reliesmore on bioenergy and less on fossilenergy.

Third, the ‘‘leakage’’ of emissionsoutside of the bioenergy projectboundary is largely irrelevant. Abioenergy project satisfies a specificdemand for energy services withbiomass fuels instead of fossil fuels.This avoids carbon emissions fromfossil fuels that would have otherwiseoccurred, and fundamentally differsfrom simply shifting demand to an-other location -- which provides nocarbon benefits.

Leakage might occur, however, ifthe decreased demand for a fossil fueltriggers a decrease in its market price,causing demand to increase else-where. A careful calculation of carbonbenefits would attempt to account forthis effect. Under conditions of scar-city (such as a power sector withchronic under-capacity) a bioenergyfacility might not be displacing anyfossil facilities -- existing or planned-- but merely supplementing them. Insuch cases, it is necessary to takeleakage into account.

Fourth, emission reductions earnedthrough bioenergy activities do notsuffer from the problem of imperma-nence. Once a biomass fuel has dis-placed a fossil fuel over some periodof time in a specific application, ithas irreversibly satisfied some spe-cific energy demand. Certainly, thebiomass fuel might at some futuretime be given up in favor of the fossilfuel, but this would not undo thecarbon benefits that had been pre-viously gained. This differs from thestorage of carbon in biomass sinks,

the gains of which are undone oncethe activity is reversed.

Finally, it is comparably difficult toensure the environmental and socialbenefits of bioenergy activities andbiomass sink activities. Both areland-intensive undertakings thatstrongly affect the ecosystems andlivelihoods. Over time, bioenergy op-tions are more land-efficient thanbiomass sink options [IPCC, 2000].The carbon-absorbing capacity of thelatter is eventually saturated overtime, requiring that progressivelymore land be allocated to storingcarbon if a constant storage rate is tobe sustained. The options listed inTable 2, for example, saturate over aperiod of 20-40 years [IPCC, 2000].Even bioenergy strategies, however,can exert pressure on valuable landresources, especially if they requirehighly productive land in order to beeconomically attractive [Azar andLarson, 2000].

5. Conclusions and recommendations

If activities based on either bioenergyor biomass sinks are to play impor-tant roles in addressing the climatechange problem, they will need to beconducted in ways that not only re-duce carbon emissions, but are envi-ronmentally and socially sustainable.Parties to the Framework Conventionon Climate Change and the KyotoProtocol can adopt -- although theyhave not done so yet -- specific pro-visions to ensure that the climateagreements motivate only biomass-related projects that are sustainable.Such provisions can ensure that ade-quate effort is directed at fosteringthe timely transition away from fos-sil-intensive technologies, in light ofthe long-term need for technology de-velopment and the replacement oflong-lived capital stock. They canrecognize only those carbon reduc-tions that are rigorously verifiable,permanent (or scrupulously risk-man-aged), and without unaccounted leak-age. They can require that biomass

activities are consistent with other en-vironmental agreements, such as theConvention on Biodiversity, and prin-ciples of environmental integrity. Andthey can insist on an effective impactassessment process that incorporatesparticipatory approaches and posi-tively affects local livelihoods.

Instituting such precautions canhelp resolve some of the controversysurrounding biomass, and allow it tofulfill its promise as part of the solu-tion to climate change and, more gen-erally, sustainable development.The author can be contacted at:Phone: (+)1-617-266-8090; Fax:(+)1-617-266-8303E-mail: skartha@tellus.org

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