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2012 | OriginalPaper | Buchkapitel

Global Consequences of the Bioenergy Greenhouse Gas Accounting Error

verfasst von : Tim Searchinger

Erschienen in: Energy, Transport, & the Environment

Verlag: Springer London

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Abstract

Like the global financial crisis, which resulted in part from misguided accounting of mortgages, global policies to expand transportation biofuels and bioelectricity reflect an accounting error. Although the carbon accounting in these policies assumes that plant growth offsets all carbon released by burning biofuels, only “additional” plant growth can provide an offset. Because they double count biomass and land already used by people or sequestering carbon, many policy proposals aim for bioenergy to supply 20% or more of the world’s energy by 2050. That would require almost doubling the present global harvest of plants for all uses, which would likely lead to extensive deforestation and increase greenhouse gases. Fixing the accounting would focus policies on the more limited potential for truly low carbon biofuels.

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Vorheriges Kapitel Deciding What Transport is for: Connectivity and the Economy

A ton of carbon biomass released by processing and burning biomass only displaces a fraction of a ton replaced by processing and burning fossil fuels for a variety of reasons, including greater energy in processing (even if some of that energy comes from the biomass itself), a higher carbon to energy ratio for biomass than fossil fuels, the higher water content of biomass, and in many situations, a lower generation efficiency.

In reality, even when bioenergy results from harvesting mature forests, it is in reality sacrificing an alternative use of ongoing plant growth although that growth may no longer result in additional net sequestration. A mature forest that not no longer adds carbon storage each year actually continues to absorb carbon from the air through plant growth in great amounts. Its net primary productivity—its degree of annual carbon absorption—may be even higher than that of growing forests. It no longer accumulates carbon or accumulates carbon slowly only because the rate of consumption of its carbon by heterotrophic organisms—particularly microorganisms—breaks down as much biomass as the forest annually accumulates. In this case, the annual carbon uptake is in reality serving to replenish the stored carbon that would otherwise decline. For this reason, the use of plant growth for bioenergy comes at the cost of using that plant growth to replenish existing stores of carbon in the existing forest.

Net ecosystem productivity is the absorption of carbon after accounting for decomposition of plants by animals and microorganisms.

This point is confused by some analysts. Policymakers are likely to require greater emissions reductions in the future than at present, and the more reductions are required, the higher the cost, which means the market value of emissions reductions are likely to increase in the future if any policy phases in reductions. But the market value in such a case reflects the policy not the value of the reductions. Such a policy in fact probably would reflect the judgment that costs of emissions reductions will decline over time and so should be phased in rather than required immediately. Such a policy would not mean that emissions reductions in the future are more valuable than achieving those same levels of reductions today.

In this study, most of the bioenergy potential existed in shrublands in Asia and temperate zones. By visual inspection, this analysis is likely to be capturing many re-growing forests, and the study did not estimate carbon losses from the foregone sequestration of such lands.

The biofuel mix estimate for 2020 was produced by a consulting firm for the British Renewable Fuels Agency, but mostly mirrors the mix of world biofuels in place in 2007 scaled up to reflect government biofuel targets for 2020.

Plevin [42] contains an excellent discussion of these estimates of switchgrass yields.

These emissions have been documented from actual plants by the Partnership for Policy Integrity http://www.pfpi.net/carbon-emissions and are the figures used in Walker [55].

Table 17A provides changes in carbon stocks at different ages for live timber, standing dead timber, understory, dead down wood and forest litter. In the analysis here, the dead down wood and forest litter increase postharvest in an amount equal to the loss of standing dead timber, which implicitly assumes that all above-ground vegetation is removed without any amount left for residues. The calculations shown here assume that.

The math is best illustrated by an example. Start with 100 tons of carbon in live trees in the forest. Harvest them and remove 68 tons to burn, leaving 32 tons of roots and residues to decompose. (That assumes 12% of the carbon is left as unharvested tree tops and branches for long-term soil fertility and wildlife, and that 20% of live tree carbon is unharvested roots [28]. The burning and decomposition of the wood generate 100 tons of carbon emissions, and that saves 25 tons of carbon if the biomass replaces gas, and 34 tons if it replaces coal. That results in an initial increase in emissions of 75 and 66 tons of carbon, respectively, to burn biomass. Before bioelectricity emissions can equal emissions from fossil fuel use, net regrowth must sequester precisely these levels of carbon, which is in turn equal to 75 and 66% of the carbon in the harvested trees.

For example, the Buckholz paper [8] carefully analyzed the availability of biomass through increased forest uptake in the northeastern U.S., but all that biomass would be part of the terrestrial carbon sink and therefore cannot reduce emissions. (The authors were apparently in disagreement about proper bioenergy accounting and noted the counter argument in some caveats although the whole analysis was based on the assumption that forest growth is carbon free.) Schlamadinger and Marland [44] developed one of the first models to look at bioenergy harvests in conjunction with timber harvests and use of timber in long-lived wood products using idealized forest growth figures. A careful review suggests that the bioenergy portion of the harvest appears not to pay off from a greenhouse gas perspective for many decades, but the incremental consequences of the bioenergy are not directly presented.

Because of higher prices, some timber products will not be replaced, generating greenhouse gas benefits from reduced consumption but also generating some other greenhouse gas emissions from replacement products. Some forest products will be replaced by harvesting other forests, triggering those emissions and different re-growth consequences. Ultimately, land managers may plant some additional plantation forests either on grasslands or forests, which sequesters more carbon directly but typically triggers land use change (or more reduced consumption) to replace the food products. The main problem with this kind of analysis is that it is extremely difficult and inherently uncertain, with world forest models and data even less developed and trustworthy than world agricultural models.

The Abt paper [1] examined the regional effects of harvesting Southern U.S. forests for electricity and estimated that much of the bioenergy demand would displace other uses of such harvests and also trigger additional forest plantings. Even so, the paper was not particularly optimistic about bioenergy, but this kind of regional analysis ignores the adverse land use changes elsewhere necessary to replace the lost timber products and food.

DIRECTIVE 2009/28/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 23 April 2009 on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC (http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:140:0016:0062:en:PDF.

RGGI has model rules set forth at http://www.rggi.org/docs/Model%20Rule%20Revised%2012.31.08_OnlinePDF.pdf. Emissions of carbon dioxide are not counted if they result from the use of “eligible biomass,” which is defined as follows:

Eligible biomass. Eligible biomass includes biofuels harvested woody and herbaceous fuel sources that are available on a renewable or recurring basis (excluding old-growth timber), including dedicated energy crops and trees, agricultural food and feed crop residues, aquatic plants, unadulterated wood and wood residues, animal wastes, other clean organic wastes not mixed with other solid wastes, biogas and other neat liquid biofuels derived from such fuel sources. Biofuels harvested will be determined by the regulatory agency.

Perhaps the best analysis of the final EPA rule was by Richard Plevin of Berkeley, and this paper relies on his analysis [42].

The U.S. Energy Information Agency estimates world energy use will grow by 50% just between 2007 and 2035 in their baseline scenario. U.S. Energy Information Agency, International Energy Outlook 2010, World Economic Demand and Energy Outlook (DOE/EIA-0484(2010), Washington) http://www.eia.gov/oiaf/ieo/world.html.

It is impossible to set this number too precisely because it depends on the degree of forest manipulation and grassland use that should be considered human use.

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Titel: Global Consequences of the Bioenergy Greenhouse Gas Accounting Error
verfasst von: Tim Searchinger
Verlag: Springer London
Buch: Energy, Transport, & the Environment
Print ISBN: 978-1-4471-2716-1

Electronic ISBN: 978-1-4471-2717-8

Copyright-Jahr: 2012
DOI: https://doi.org/10.1007/978-1-4471-2717-8_36

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