Blue carbon ecosystems against climate change

22/07/2024

Almost everyone knows that terrestrial vegetation captures carbon dioxide from the atmosphere and returns oxygen to us. But the role of seas, oceans and coastal ecosystems in the uptake and retention of this compound that contributes so much to global warming is less known. A team, which includes a CEAB-CSIC researcher, has explained it in a scientific and informative article, recently published in the journal Frontiers for Young Minds.

Trees and terrestrial plants use CO2 to carry out photosynthesis and accumulate carbon in the leaves, stems and trunks, as well as in the soil. This carbon they retain is known as ‘green carbon’.

Aquatic plants – those found under the sea and in coastal areas – do exactly the same thing. They capture CO2 and use it to carry out the process that allows them to obtain energy. They store carbon in their leaves, rhizomes and roots, and bury significant amounts of this element in their sediments, in the soil in which they are rooted. It is the so-called ‘blue carbon’.

The ecosystems that perform this function are known as ‘blue carbon ecosystems’ and have the particularity of retaining the captured CO2 for thousands of years. This family of ecosystems includes:

Marine meadows or ‘marine forests’: extensions of plants to the coastal seabed such as, for example, Posidonia oceanica, Zostera marina, Zostera noltii, Cymodocea nodosa or Halophila stipulacea.
Wetlands: very flat coastal lands that remain flooded or are periodically flooded as a result of the ebb and flow of the tides or the filtration of sea water.
Mangroves, which are found in tropical and subtropical latitudes, at the mouths of rivers or streams into the sea. The species that inhabit it are aquatic plants and trees that resist the high salinity of the marine waters that mix with the fresh waters.

Field work. Mediterranean sea. Author: L.Carrau/CEAB-CSIC

Natural CO2 stores

Blue carbon ecosystems cover less than 2% of our planet’s surface, as they have been severely mistreated throughout history – and still today – by humans. They have dried out, they have been built on top of, they have been destroyed by activities such as trawling or anchoring boats on seagrass beds… However, they continue to play a very important role in the carbon cycle. Not so much because of the greenhouse gases they capture (about 2% of the CO2 that humans emit annually), but above all because they are very efficient at retaining it.

Scientists explain that the habitats formed by these aquatic plants are authentic “carbon storage or sewers.” Although they capture it more slowly than forests, they are much more effective at retaining it.

Researchers from the CEAB-CSIC aquatic macrophytes group Miguel Àngel Mateo and Òscar Serrano (co-author of the article in Frontiers) detail that “unlike many terrestrial ecosystems, in which matter decomposes quickly, in blue carbon ecosystems ( due to the conditions of the soils, which are permanently covered with water), it takes much longer and the carbon is stored with great efficiency. They store more and more, for as long as that ecosystem remains alive and healthy. We have dated blue carbon deposits up to 12,000 years old.”

Mateo, who is at the same time the head of the Spanish Group of Experts on Blue Carbon Ecosystems, remembers that the stock of carbon stored by these habitats is equivalent, at least, to an entire year of anthropogenic emissions. However, “We continue to mistreat them. Each year, globally, about 300 million tons of carbon dioxide are released due to the deterioration or destruction of blue carbon ecosystems. When we destroy one, the carbon it retained is remineralized and returns to the biosphere, aggravating climate change.”

The researchers remember that, in addition to their role in the carbon cycle, these ecosystems are valuable for many other reasons: aquatic vegetation is the first link in the food chain, it creates key spaces for biodiversity – they are the habitat or refuge of many species – filter the water, eliminating pollutants and protect the coasts from erosion and extreme phenomena, increasingly frequent with climate change.

Therefore, taking care of it provides us with many benefits, beyond its role in carbon capture and retention. Preserving them and, sometimes, restoring them therefore becomes a natural solution to confront climate change.