The Balearic Islands hold Spain’s largest blue carbon reserve

The waters of the Balearic Islands conceal one of the Iberian Peninsula’s greatest allies in the fight against climate change. A study led by researcher Nerea Piñeiro-Juncal of IMEDEA reveals that marine meadows, especially those formed by Posidonia oceanica, are part of a natural system capable of storing the equivalent of nearly 25% of the annual emissions of Spain and Portugal.

The study, entitled “Blue carbon inventories of Spain and Portugal for their inclusion in national climate mitigation strategies”, quantifies for the first time how much CO₂ is stored in coastal ecosystems known as “blue carbon,” such as seagrass meadows and salt marshes, in these two countries. Taken together, these areas store an enormous amount of carbon, with the western Mediterranean playing a particularly important role.

In this context, the Balearic Islands play a decisive part. Posidonia oceanica meadows, which are especially extensive across the archipelago, account for around 40% of the total seagrass meadow area analysed. This plant, found only in the Mediterranean, stores carbon in marine sediments for centuries, turning the seabed into a genuine natural reservoir.

In the words of Nerea Piñeiro-Juncal, “Posidonia meadows are long-term carbon sinks. In the Balearic Islands, their extent makes them the main blue carbon sink in Spain, contributing significantly to climate change adaptation.” In fact, the report led by the IMEDEA researcher indicates that, of the 1,976 km² of meadows and marshes that store CO₂ in Spain and Portugal, the highest proportion corresponds to the seabeds of the Balearic archipelago, with 650 km².

Spain and Portugal are home to two of the main types of blue carbon ecosystems: seagrass meadows and salt marshes. To date, no comprehensive national assessment of blue carbon ecosystem reserves had been carried out in Spain and Portugal. The study by Nerea Piñeiro’s team assesses the scale of the carbon sink associated with these ecosystems across the entire Iberian Peninsula and the Spanish island territories, as well as the potential CO₂ emissions resulting from their degradation.

It is estimated that biomass ecosystems in the study area store 95 Tg of CO₂ in biomass and in the first metre of soil, equivalent to approximately 25% of the CO₂ emissions of Spain and Portugal in 2022. It should be remembered that 1 Tg is equal to 1 million metric tonnes, so 95 Tg of CO₂ amounts to 95 million tonnes of CO₂.

The average rate of soil organic carbon accumulation was estimated at 0.15 Tg of CO₂ per year, equivalent to 0.04% of the annual emissions of the two countries in 2022.

This “climate treasure,” however, is far from guaranteed. The study warns that the deterioration of these ecosystems over the last century has already released large amounts of CO₂ into the atmosphere. According to the study, between 11 and 27 Tg of CO₂ may have been released over the past century, and projections suggest that, unless protection measures are strengthened, much more could be emitted in the coming decades. Specifically, it is estimated that between 1.3 and 5.6 Tg of CO₂ will be released over the next 30 years. This underlines the urgent need to intensify conservation and restoration efforts.

In the Balearic Islands, tourism pressure, boat anchoring, pollution and coastal development are among the main threats to posidonia. Its degradation not only means a loss of biodiversity, but also the release of carbon accumulated over hundreds or thousands of years, as well as the loss of its capacity to sequester carbon, which helps offset part of the emissions generated by the islands’ population.

According to Piñeiro-Juncal, “the value of these ecosystems goes far beyond carbon. They are essential for the health of the coastline and for key sectors such as tourism.” Indeed, beyond their climatic role, posidonia meadows are vital for the archipelago: they protect beaches from erosion, improve water quality and provide shelter for numerous marine species.

The study opens the door to integrating blue carbon into climate and conservation policies. The data obtained can be incorporated into national emissions inventories and serve as a basis for restoration projects and carbon offsetting initiatives. In this regard, the study makes Spain and Portugal two of the leading countries within the European Union in this area.

In the Balearic Islands, where posidonia already enjoys specific legal protection, the findings reinforce the need to intensify conservation measures.

In a context of climate crisis, the message is clear: protecting posidonia not only preserves the landscape and biodiversity of the archipelago, but also contributes directly to slowing global warming.