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Science
| 05/02/18

As US touts ‘carbon neutral’ wood pellets, scientists say it’s not so simple

TCE

Burning wood for energy is often thought of as carbon neutral. The European Union treats woody biomass this way and last week, the head of the US Environmental Protection Agency unveiled new regulations declaring the same thing. But forestry researchers suggest it’s not that simple.

Under international reporting requirements, emissions from combustion of wood are reported in the land sector, and are therefore not reported in the energy sector.

From the perspective of the energy sector, these emissions are treated as ‘carbon neutral’, but this concept may over-simplify the way politicians account for the release and drawdown of greenhouse gas (GHG) emissions. This was the warning that forest researchers made in a new paper in which they reviewed the latest raft of papers exploring emissions over the course of the full life cycle of production and combustion of bioenergy.

The emissions from the combustion of wood are in principle offset when in the future trees draw down carbon dioxide from the air as they grow. But the researchers, including two members of the Forest Carbon Management Project, supported by the Pacific Institute for Climate Solutions, say that this rough sketch of carbon flux in the forest fails to take into account a great many other factors including the timing of when emissions and subsequent removals from the atmosphere occur.

As a result, governments may be overselling the climate benefits of wood pellets.

Instead of a single study, which may show one result that is later contradicted by another paper, the researchers performed a literature review, considering the full breadth of the latest papers that consider the net emissions from woody biomass.

Overall, these papers found both good news and bad news.

For example, changing harvest practices in order to extract more biomass may have long-term effects on nutrients and productivity, which impact the ability of the forest to grow and draw down CO2. Use of logging residues are likely to have long-term impacts on soil productivity if too little logging debris is left in the forest.

The type of fossil fuel avoided and the fossil fuels consumed by vehicles and other machinery involved in the processing and transportation of the biomass can also alter the emissions profile: an electric truck delivering wood pellets to a nearby location will result in fewer emissions than a diesel truck and freighter delivering the pellets from Canada to Europe.

Using tree tops and branches for bioenergy that otherwise would have been burnt to reduce wildfire risk can indeed result in lower net emissions over a short period of time. On the other hand, the researchers warn, if those harvest residues were used in long-lived wood products such as panels instead of bioenergy, the carbon is locked away from the atmosphere for a much longer time, thus achieving greater climate mitigation effects.

Increased use of sawmill residues works well as a mitigation strategy, but the supply of such residues is very limited.

They conclude that all of these impacts need to be quantified to assess if the use of bioenergy causes a net reduction in emissions to the atmosphere and that the assumption of carbon neutrality is an over simplification.

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