When a source of emissions and an emissions sink correspond in terms of their warming impact, and in terms of the timescale and durability of carbon storage. For example, fossil carbon is stable in the lithosphere over millennia if it is not extracted and burned, therefore mitigating measures (e.g. offsets) that aim to neutralize the effect of these emissions must persist for a comparable, geological-timescale. Although all CO2 once emitted, whether originally sourced from the lithosphere or biosphere, persists in the active carbon cycle for centuries to millennia, it may be appropriate to balance shorter-duration carbon released from biogenic carbon stocks (e.g. forests and soils) with comparably temporary storage in like stocks. The variable risks of reversal of different carbon stocks must also be considered, for example forests may suffer from unforeseen anthropogenic (e.g. illegal logging), non-anthropogenic (e.g. disease and disaster), or climate change-induced (e.g. warming) reversal risks.
