Counting (on) Blue Carbon - Challenges and Ways forward for carbon accounting of ecosystem-based carbon removal in marine environments

The latest IPCC assessment report highlights once more the need for negative emissions via carbon dioxide removal (CDR) measures to reach ambitious mitigation goals. In particular ecosystem-based CDR measures are currently the focus of national net-zero strategies and novel carbon crediting efforts. Using ecosystem-based carbon removal measures in marine environments as an example, we here highlight key challenges concerning the monitoring and evaluation of blue carbon fluxes for carbon crediting. Challenges specific to ecosystem-based CDR measures are i) the definition of baseline natural carbon fluxes, which is necessary for ii) clear anthropogenic CDR signal attribution, as well as iii) accounting for possible natural or anthropogenic disturbances of the carbon stock and hence an assessment for the durability of the carbon storage. In addition, the marine environment poses further monitoring and evaluation challenges due to i) temporal and spatial decoupling of the carbon capturing and sequestration processes, combined with ii) signal dilution due to high ecosystem connectivity, and iii) large pre-existing carbon stocks which makes any human-made increase in carbon stocks even harder to quantify. To increase the scientific rigor behind issued carbon credits, we propose a concentration of monitoring efforts on carbon sequestration rather than capturing processes, and baseline establishment for natural carbon sequestration in diverse ecosystems. Finally, we believe that making carbon credits subject to dynamic adjustments over time, will increase their credibility.