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IMG_8786a © ThomasVignaud
Marine and coastal ecosystems play a vital role in regulating the climate and mitigating the impacts of climate disruption. Storing carbon, disseminating heat, combating erosion, reducing the impact of tsunamis... an overview in images of the various “services” provided by the oceans

This article was published on the website www.afd.fr

Did you know that the oceans absorb nearly a quarter of anthropogenic CO2 emissions?

Coastal and marine areas then play an essential role in regulating the climate, at the same time sheltering unique ecosystems essential to regional development.

Come and discover the extraordinary regulatory capability of these ecosystems, and the complexity of their interactions and biodiversity, which contribute to the stability and resilience of our planet. We focus on plankton in all its diversity, mangrove swamps and coral reefs in tropical regions, and seaweeds.

These photographs and researchers’ statements are taken from the collective work “Les Écosystèmes marins dans la régulation du climat” (“Marine ecosystems in climate regulation”) coordinated by the FFEM (French Facility for Global Environment), in partnership with the Tara Foundation expeditions.

  • The oceans, reservoirs of life

“The oceans represent the biggest habitat on the planet, currently covering 70.8% of Earth’s surface. With an average depth of some 3,800 metres, it’s better to consider the oceans in terms of volume, which is around 1.37 billion km3. The main characteristic of this immense habitat is its continuity, and so its connectivity.“


Gilles Bœuf



Sponges and anemones (Yasawa, Fiji) © Thomas Vignaud

The stability of the oceans, at least for the last 100 million years, is absolutely extraordinary: pH, osmotic pressure and salinity, temperature, dissolved gas content.

The marine biodiversity they foster, still largely unknown to us, is much more than just a list of species; it’s an interconnected whole of interactions between living organisms and their environment.
Humankind has been fishing this biodiversity since ancient times, certainly for hundreds of thousands of years.

Initially, life was exclusively marine and today’s oceans maintain their primordial role in the evolution of life and the climate.

  • The oceans, reservoirs of heat

“The climate machine is driven by solar energy, and the oceans are the main receptor: they absorb almost 60% of the sun’s rays entering the climate system. The atmosphere, which is fairly transparent to the sun’s rays, absorbs half as much.“


Bruno Voituriez


South channel, Fakarava, French Polynesia © Thomas Vignaud


The constant exchanges of energy and heat between atmosphere and oceans affect both temperature and salinity. The variations in density generate circulation in the depths, known as “thermohaline”: the denser water sinking as the lighter water rises.

These ocean currents redistribute the excess heat from the tropics towards the poles.

  • The oceans, pivotal in climate change  

“Pivotal because the oceans have contributed greatly to reducing the extent of contemporary climate change, absorbing 93% of the heat accumulated in the atmosphere via the greenhouse effect, by capturing more than a quarter of anthropogenic CO2 emissions since 1750, and by taking in almost all the water released from melting glaciers.“


Jean-Pierre Gattuso et Alexandre Magnan

Rangiroa lagoon at low tide © Thomas Vignaud


The oceans are also victims of climate change. Increased concentration of CO2 dissolved in seawater lowers pH and reduces available carbonate ions. This phenomenon, known as ocean acidification, threatens every marine organism that possesses a calcareous skeleton.

The temperature of oceanic surface waters is rising. That has serious consequences: migration of species, disturbance of oxygen exchange, bleaching of coral reefs. Rising sea levels will also result in varying degrees of temporary coastal plain submersion and increased salt concentrations in coastal soils.

  • Phytoplankton, origin of life on earth

“Plankton, atmosphere and climate have been intimately linked since the first traces of life emerged on earth. The story began more than three billion years ago, when photosynthetic cyanobacteria, drawing energy from sunlight, started injecting oxygen into the primitive atmosphere where previously there was none.“


Christian Sardet

A medley of protists, Toba bay (Japan) © Christian Sardet / Tara Oceans / CNRS / Chroniques du Plancton (Plankton Chronicles)


Microscopic plankton able to photosynthesise (phytoplankton) are major climate regulators through their ability to produce biomass, so absorbing and regulating atmospheric CO2.

As it dies this organic matter forms a sediment of corpses, detritus and waste, feeding the creatures of the abysses. For millions of years, this “marine snow” has carried an enormous amount of carbon to the bottom of the oceans. The whole process comprises the “carbon pump”.

  • Mangrove swamps, where fresh and sea water mingle

“The red mangrove, characterised by its large stilt-like roots, grows well along the sea’s margins on soil frequently saturated with water, making it an iconic species for trapping and storing atmospheric CO2.“


Cyril Marchand

Fadiouth island (Petite-Côte, Sénégal) and watercolour © Valérie Fakir - watercolour © Céline Bricard


Mangrove swamps are communities of trees, shrubs and sea-grass beds along tidal zones in tropical regions. The mixture of salt and fresh water, coupled with high  evapotranspiration, creates a wide range of salinities which result in mangrove swamps becoming zoned into more or less distinct tracts, parallel to the coastline, each dominated by a different plant species.

Mangroves are able to transform very large amounts of atmospheric CO2, “fixing” it into organic matter, via photosynthesis.

In addition to this fixing, very significant quantities of carbon, estimated at 10 t CO2-equivalent/ha/yr, are stored in mangrove swamp soils. These soils are actually saturated with water, and the decomposition process for the plant debris is very slow due to lack of the oxygen needed for the organisms (bacteria and fungi) which perform the task.

  • Coral reefs, protective ramparts

“Formed by the piling-up of coral skeletons consolidated by the biological activity of organisms such as calcareous algae, coral reefs constitute real barriers that protect coasts and their inhabitants from storms, cyclones and tsunamis.“


Serge Planes et Denis Allemand

Platygyra acuta under an electron microscope (Red Sea) © Éric Tambutté - Monaco Scientific Centre (CSM)


A reef can absorb up to 97% of wave energy and reduce wave height by 85%, so helping to offset the impact of extreme climatic events. They also represent a considerable source of revenue for island nations.
An estimated 30 million people worldwide depend on reefs for their food.

Coral reefs are facing dramatic losses: nearly 25% have disappeared over the last 20 years.

  • Under the canopy of great seaweed forests

“Seaweeds are omnipresent in coastal habitats, ranging from Laminaria beds and giant kelp forests, through to coral reefs dominated by calcareous species, by way of the Fucale (Sargasso) beds that form a canopy beneath the surface of the water. Many studies have highlighted the capacity of seaweeds to reduce greenhouse gases.“


Claude Payri


Laminaria beds (Brittany, France), Roscoff Biological Station © Sandrine Ruitton


Seaweed forests and beds grow along rocky coasts and occupy nearly 58,000 km of coastline, from high latitudes through to the tropical regions.

Their rates of primary production and renewal are very high but, unlike beds of phanerogams (seed-producing plants) and mangrove swamps, there is little or no sequestration of organic matter.

When subjected to stress, some seaweeds will rapidly release volatile iodide ions.

Under the effect of light (photolysis) this iodine complexes with ozone in the air to produce particles onto which water vapour condenses, leading to the formation of clouds such as stratocumulus.