Blue carbon inventories of Spain and Portugal for their inclusion in national climate mitigation strategies.

The fast changes in climate are driving global efforts to reduce greenhouse emissions and offset those that cannot be avoided. Interest in vegetated coastal ecosystems, known as Blue Carbon Ecosystems (BCEs), has grown rapidly due to their potential contribution to global carbon sequestration. Spain and Portugal host two of the main BCEs types; seagrass meadows and tidal marshes. To date, no comprehensive national assessment of BCEs carbon stocks has been conducted for Spain and Portugal. We have assessed the magnitude of the carbon sink associated with them across the entire Iberian Peninsula and insular Spanish territories and the potential CO2 emission resulting from their degradation. The BCEs in the studied area are estimated to store 95 Tg CO2-eq in the biomass and top meter of soil, equivalent to about 25% of the CO2 emissions of Spain and Portugal in 2022. The average rate of accumulation of organic carbon to the soil stock was estimated at 0.15 Tg CO2-eq y-1, equivalent to 0.04% of the annual anthropogenic CO2eq emissions of these two countries (in 2022). Additionally, the loss of BCEs in Spain and Portugal over the last century may have released 11-27 Tg CO2-eq, whereas we predicted that 1.3-5.6 Tg CO2-eq will be released over the next 30 years. Which underscores the urge to increase conservation and restoration efforts. This study provides the first comprehensive Spanish and Portuguese national blue carbon inventory for its inclusion in NDCs, providing baseline data for the implementation of blue carbon offsetting projects.