Residence Times and Legacy of Blue Carbon Reservoirs

Quantifying the carbon sequestration potential of blue carbon reservoirs is critically dependent on residence times – how long carbon remains in these reservoirs before being re-exposed to the atmosphere. Simple mass balance considerations provides estimates – or at least constraints - for many of the major blue carbon reservoirs in the earth system. For vegetated coastal ecosystems (mangroves, sea grass meadows and salt marshes) constituting a global reservoir of about 25 PgC, this is of the order 1000 to 2000 years. For carbon sequestered by the biological carbon pump in the worlds oceans, a reservoir of about 1800 PgC, it is 700 to 1300 years. It is over these time scales that the these respective reservoirs reach equilibrium with influx and efflux in near balance. The degradation of habitats and the over-exploitation of marine living resources have tipped these natural processes out of balance, to the extent where these blue carbon reservoirs are now net emitter of legacy carbon back to the atmosphere. Nature-based solutions can be sought to address parts of this crisis, for instance in improving ecosystem health and biodiversity. However, it is unlikely that nature-based solutions to carbon management exist on a scale commensurate with anthropogenic emissions. The best we can do is to limit net carbon emissions by restoring what we can, and to ensure that future practices do not further tip blue carbon reservoirs out of balance.