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Policy Brief: Fossil and biological carbon: a tonne is not a tonne

Whether biological carbon credit should be traded in carbon markets is topical, with discussions ongoing in the UNFCCC, ICAO and the California Cap-and-Trade system. To date, compliance markets have rejected the eligibility of biological carbon offsets. They are right to do so. Fossil and biological carbon operate on different parts of the carbon cycle, and on very different timescales. Fossil carbon is permanent; biological carbon is potentially and frequently subject to rapid fluxes, whether natural or manmade. For these reasons, offset credits from REDD+, afforestation and reforestation or other biological systems should not be treated as fungible with fossil carbon, but should instead be addressed through other, appropriate, policy measures.


One of the problems in addressing the climate change crisis is that there are a number of different radiative forcers involved including greenhouse gases, black carbon and aerosols, coming from a wide variety of sources, which each have different chemical and physical characteristics. A rational policymaking approach to address climate change would therefore be to put in place tailored measures to address this diversity in ways most appropriate to different forcers and sources.

The use of biological carbon offsets in carbon markets, trading it against fossil emissions, is seen by some as an opportunity to find the needed finance to address emissions from land use changes, including from deforestation and forest degradation. Other promoters see it as a political bargaining chip to overcome the resistance of vested fossil fueled interests to decarbonize in their sectors, rather than as the most effective means to address emissions from these sources. As a result of such views, there is pressure from some quarters to include credits from ‘reducing emissions from deforestation and forest degradation’ (REDD+) in carbon markets.

In practice, purchasers have been notably reticent to buy the few types of eligible biological carbon credits, because of some of the practical difficulties associated with, including biological carbon in the markets. In the compliance market, eligible land use credits are temporary and so have to be replaced. Biological carbon is subject to far greater concerns on permanence than fossil carbon. For example, credits from absorbing carbon through planting trees may be reversed due to insect infestation, fires or harvesting. There are also technical concerns defining realistic baselines in the land use sector against which credits can be issued. Concerns about conflicting priorities for land use, including food security are also relevant for certain types of biological carbon.

Whatever the rationale brought forward for including biological carbon in carbon markets, the assumption is made that fossil and biological carbon are indeed fungible: that one tonne of avoided emissions from not burning fossil fuels is equivalent to avoiding carbon emissions from biological stores. This, however, is not the case. This briefing outlines some key scientific differences in the carbon cycles from fossil and biological sources and sinks that makes them inherently non-fungible, and thus explains why offset credits from REDD+, afforestation and reforestation or other biological systems should not be used to offset any use of fossil carbon.

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