On the science of carbon debt

The report does not support the view that biomass produced from ‘well managed’ forests is automatically a zero or low-emissions energy source, neither does it support the claim that biomass is invariably a ‘dirty’ energy source with CO₂ emissions being higher than from fossil fuels. Simple examples are used to show how supplying and using biomass from forests can lead to very high emissions (a ‘carbon debt’), negligible net emissions (‘carbon neutrality’) or net-negative emissions (a ‘carbon gain’). Decisions about forest management practices involved in supplying biomass, and about how biomass is utilised, determine which of these outcomes happens.

Scientific literature reports very wide-ranging estimates of CO₂ emissions from bioenergy resources. It is proving challenging for stakeholders to reach consensus on the implications of scientific findings for using biomass as an energy source or for other products. Individual scientific arrive at divergent findings because they: 

• look at differing types of forests and forest management practices; 
• assume different end uses and processing chains for the biomass; 
• apply differing (and sometimes inappropriate) calculation methods; 
• address differing research questions (that are not always explicitly stated).

The report, written in close cooperation with four scientists with a track record of research into the subject, warns against drawing generalised conclusions from the results of individual scientific studies or based on simplistic interpretations of forest carbon balances. Also, a chapter in the report is devoted to considering the implications for policies towards forest biomass supply, particularly for use as bioenergy. A tentative outline is provided of technical methods that could support the effective use of biomass, for energy and non-energy end uses, ensuring zero or low associated CO₂ emissions.