"Bioenergy stakeholders at different levels should consider that the climate impact of bioenergy from forest wood is difficult to quantify and highly site-specific, i.e. impacts depend on the type of biomass species being cultivated, its geographical location of production, the time it takes to produce the biomass, the relevant types of climate forcing agents affected, and the kinds of climate metrics employed to understand the impacts from these agents," Francesco Cherubini told environmentalresearchweb.

Cherubini said that environmental-impact studies and climate-accounting mechanisms such as the Kyoto Protocol and its successor should transparently acknowledge these issues and urgently adapt to incorporate improved global-warming impact characterizations. Otherwise the direct climate impact of bioenergy could be over- or underestimated by up to one order of magnitude, potentially resulting in very ineffective, if not counterproductive, investments for climate-change mitigation.

The Norwegian team analysed temporary changes in atmospheric carbon-dioxide concentration and surface albedo as part of a lifecycle assessment for forest wood in the Pacific Northwest US, the US state of Wisconsin, Canada and Norway, and for fast-growing species like eucalyptus in Brazil and willow in the European Union and US.

"Specific bioenergy case studies are selected to show the importance of these factors, which far outweigh traditional greenhouse-gas emissions, especially for short time horizon and slow-growing biomass, and the grossly under- or over-estimation of the impact of bioenergy when improper characterization is carried out," said Cherubini. "In addition, the format and style of the results allows an application of these climate metrics on a routine basis, enhancing their inclusion in international standards and accounting mechanisms."

The team defined biogenic carbon dioxide as carbon dioxide sequestered as the biomass grows and resulting from oxidation of carbon in biomaterials harvested for energy, both at the conversion plant and during other life-cycle stages, and from the decomposition of dead organic matter. The vegetation generally absorbs carbon dioxide until it is harvested, a period that can range from one year for annual crops up to several decades. Then, when used as an energy source, the plant material releases carbon dioxide to the atmosphere.

The effects of biomass growth and harvesting were extremely site-specific, the researchers found. In areas that see snow in winter the felling of trees for biomass can increase surface reflectivity (albedo) by providing a flatter surface to reflect solar radiation back into space, and so create a cooling effect. The decay of carbon-dioxide emissions from combustion of forest biomass was slower than the decay of fossil-fuel emissions over the first few decades after release because of additional emissions from the forest site after harvest.

Where the albedo effect did not provide much cooling, bioenergy systems initially showed a higher net impact than fossil systems. This effect lasted a few years for fast-growing biomass species and for several decades for slow-growing biomass. "From the medium term (60–80 years) and beyond, the instantaneous climate impact of bioenergy systems gradually decreases and becomes smaller than that of fossil reference systems, even for transportation biofuels produced from forest wood that do not have the benefit of cooling effects from albedo," wrote the researchers in Environmental Research Letters (ERL) .

The treatment of plant-related carbon-dioxide emissions from bioenergy in government regulations is currently under review in the US.

"The prevailing challenge of the recent discussion around bioenergy in the scientific and policy community lay in the methodology used to assess the climate impact of biogenic carbon-dioxide emissions and the related bioenergy product," said Cherubini. "The importance and urgency of the topic is measured by the ongoing debate in the EU and US, where the EPA officially opted for a three-year deferral of the application of the final greenhouse-gas tailoring rule to biogenic carbon-dioxide emissions, with the aim to effect a survey of the science associated with treatment of biogenic carbon-dioxide emissions."

According to Cherubini, the production of bioenergy can also affect local climate through changes in surface-energy budget such as surface roughness, sensible and latent heat fluxes, which in turn can cause feedbacks on the global climate. "Linking these aspects to forest management for bioenergy is still in its scientific infancy, and further detailed research is needed to get a comprehensive understanding of the dynamics and consequences for the local climate and forest ecosystem, both as a single stand and at a landscape level," he said.