Envelope 1/2011

March 2011

Weaker carbon sink capacity of forests undermines the majority of climate benefits from forest energy

Increased energy production would cut down the carbon sink capacity of Finnish forests. Photo: Tuomo Björksten
If forest energy production is increased as planned by the Government, the carbon sink capacity of Finnish forests will suffer. A weaker carbon sink capacity will cut actual reductions in greenhouse gas emissions achievable through forest energy in Finland by 60-80 per cent by 2020. A study commissioned by the Ministry of the Environment from the Finnish Environment Institute, published in February, examines the climate impacts of, and particulate matter emissions attributable to, rapidly increasing forest energy production.

The aim of using wood as a source of energy is to reduce carbon dioxide emissions from energy production and to increase the proportion of renewable sources. The quantity of wood biomass harvested from Finnish forests for energy production has increased to five million cubic metres in the 2000s, and according to the Government’s bioenergy policy, will be increased further to 13.5 million cubic metres by 2020.

Wood can replace fossil fuels and reduce emissions from them, but the harvesting of wood biomass from forests simultaneously weakens their carbon sink capacity.

Carbon sink changes are insufficiently accounted for in many calculations used in climate policy, which can easily lead to the climate benefits achieved through forest energy being overstated.

The weaker carbon sink capacity of forests diminishes emission reductions

According to the study, the increase in forest energy production will lower the annual carbon sink capacity of Finnish forests by 6.2 million tonnes of CO2 by 2020. The living tree biomass will continue to be a carbon sink but due to the reduced accumulation of organic matter the soil will change from a carbon sink to a carbon source and its carbon reserves will begin to dwindle.

Forest energy can replace an estimated 10.7 million tonnes of carbon dioxide emissions from coal, 8.7 million tonnes from heating oil or 7.6 million tonnes from natural gas. Taking into account both emissions and changes in the carbon sink, Finland’s net emissions to the atmosphere will decrease by 1.4 – 4.5 million carbon dioxide tonnes, depending on which fossil fuel is replaced. Hence the difference between fossil fuel emission reductions achieved through forest energy, and actual net reductions in emissions, is significant.

 - Forest energy is not as low in emissions as is generally assumed. Harvesting of wood from forests reduces the quantity of atmospheric carbon accumulated in forests, even though growing forests do take up carbon from the atmosphere. Logging residue, such as branches, wood from first thinnings and tree stumps, would store coal for a long time if left to rot in the forest. The climate benefit achieved by carbon storage is similar to that of, for instance, long lasting products made of wood, explains Leading Researcher Jari Liski of the Finnish Environment Institute.

Forest energy emissions decline with time

Emissions due to the reduction of carbon reserves in forests are highest when the use of forest energy commences or the quantity used increases. As a result, a changeover to forest energy is not a particularly quick way to reduce emissions from Finland’s energy production. If the aim is to reduce emissions quickly to a level sufficient to tackle climate change (for instance to the warming by two degrees as outlined at the Cancun climate conference) energy production emissions must be reduced in other, faster ways in addition to a shift to forest energy.

If tree biomass, such as branches and wood from first thinnings, that forms the most short-lived carbon storage in forests and rots the fastest, is used for energy production, forest energy emissions would remain significantly lower as production continues. When wood that decays slowly, such as thick dead logs and large stumps, is used  as a source of energy, emissions remain high for decades, because if left in the forest, it would have stored carbon for a long time. If the forest energy use is based heavily on slowly decaying components the emissions would remain comparable to those of using fossil fuels for decades. When rapidly decaying wood, such as branches, is used the emissions  would decrease remarkably faster.

Hence, emissions from forest energy can be substantially reduced by targeting production at those parts of trees with more advantageous climate impacts, such as branches and wood from first thinnings. If, however, production targets for bioenergy are set high, climate wise less favourable wood may have to be used in order to achieve them. This will undermine the overall positive effects  of forest energy on climate change.

Climate policy calculation formulae overestimate the climate benefits of forest energy

Many  calculation formulae for emissions that have been internationally approved for climate policy, do not pay sufficient attention to the impact of forest energy on the carbon sink of forests, while reductions in fossil fuel emissions achievable through the use of renewable wood are accounted for in full. For instance, Finland’s emissions calculated on the basis of the Kyoto Protocol do not include any adjustment for changes in the carbon sink of forests caused by the use of wood as an energy source. The European Union’s calculation formulae for biofuels only take account of changes in carbon sinks where the production of biomass transforms forest into another form of land use. By contrast with these calculation formulae, the annual greenhouse gas inventory conducted under the UN Climate Convention, which is not applicable to obligatory emission restrictions, takes full account of forest energy impacts on the carbon sink of forests. This inventory provides thus information on the actual net emissions caused by using forest bioenergy.

Because changes in the capacity of forests to act as carbon sinks is not adequately accounted for, many current climate policy calculation formulae overestimate actual emission reductions and climate benefits achievable through forest energy. As a result, the emission figures used in climate policy for forest energy are not always comparable with the emission levels used in climate research.

Further information

  • Publication: Metsäbiomassan energiakäytön ilmastovaikutukset Suomessa
    (in Finnish with abstracts in English and Swedish)
  • Jari Liski, Leading Researcher, Finnish Environment Institute (SYKE)
    tel. +358 (0)40 748 5088
  • Niko Karvosenoja, Senior Researcher (particulate matter emissions), Finnish Environment Institute (SYKE) tel. +358 (0)400 148 768

Back to the newsletter