Short version: Carbon credits must be associated with the permanent avoidance or permanent enhanced removal of GHG emissions. If a project that only temporarily stores carbon (e.g., by sequestering it in trees or soils) substitutes for activities that permanently avoid carbon emissions (e.g. by reducing fossil fuel use), environmental integrity will be undermined.
Long version: One challenge with using carbon credits to compensate for CO2 emissions is that the effects of CO2 emissions are very long-lived. Most of the carbon in a tonne of CO2 emitted today will – eventually – be removed from the atmosphere. However, around 25% remains in the atmosphere for hundreds to thousands of years.[1] To physically compensate for CO2 emissions, carbon credits must be associated with avoided emissions or enhanced removals that are similarly permanent.
The problem is that the effects of some types of projects can be reversed. A “reversal” occurs if carbon stored by a project is later emitted, resulting in no cumulative change in atmospheric carbon over time. For many kinds of carbon offset projects, reversals are either physically impossible or extremely unlikely. The greatest risk occurs with projects that store carbon in reservoirs (like trees) that may be subject to future disturbances. The classic example is a forestry project that keeps carbon in trees and soils (and adds to those carbon stores over time, as the forest grows). If a fire later burns down the project’s trees – or the trees are cut down to make way for new development – some, or all, of the carbon may be (re)emitted, leading to a reversal.
One common misunderstanding is that – for carbon credits – “permanent” means something less than hundreds or thousands of years. A standard convention, for example, is that carbon only needs to be kept out of the atmosphere for a few decades (e.g., 40 years) to be considered “permanent.” Such compromises are frequently made in the context of carbon crediting programs seeking to balance technical requirements (i.e., storing carbon indefinitely) with practical constraints (i.e., realistically, crediting programs can provide only a finite guarantee). But, scientifically, anything less than a full guarantee against reversals into the indefinite future is not “permanent.” Buyers of carbon credits subject to reversal risk should bear this in mind and recognize the potential liability that reversals could pose in the future – even after the minimum “permanence” period guaranteed by crediting programs. Strictly speaking, such credits do not fully offset fossil CO2 emissions.
[1] Technically, the individual molecules of CO2 emitted may cycle back and forth between the atmosphere and terrestrial reservoirs multiple times, but atmospheric concentrations of CO2 will remain elevated by an amount equal to about 25% of the original mass emitted after 1,000 years (Joos et al. 2013).