The ocean as a carbon sink threatened by climate change

The ocean covers 70% of the Earth's surface and is home to a large marine biodiversity. It is in constant exchange with the atmosphere, storing and redistributing heat. In addition to producing half of our oxygen, the ocean regulates the Earth's climate and temperature by absorbing large amounts of atmospheric CO2.

As Ocean Climate points out, temperature increases from greenhouse gases affect the entire thermal mechanics of the Earth, including the ocean and the atmosphere which are constantly interacting.

The ocean as a carbon sink

A carbon sink is a natural or artificial reservoir that absorbs and stores carbon from the atmosphere through physical, chemical and biological mechanisms. On Earth, there are several natural carbon sinks such as soils, forests and oceans.

The processes associated with the marine environment constitute an "oceanic carbon pump". These pumps can be split into several parts: the biological pump, the physical pump and the chemical pump. The biological and physical carbon pumps are mainly those that store the most CO2.

Biological carbon pump

The biological pump transfers carbon from the surface to the seabed through photosynthesis and phytoplankton. During this process, the atmospheric CO2 is dissolved in the ocean. Plankton absorb the CO2 and release oxygen through photosynthesis. This produces organic matter and the chemical reaction that follows from the CO2 dissolution process releases carbonates that plankton can use for their shells. When organisms die, the carbonate that makes up their shells is deposited on the ocean floor, a process known as sedimentation. As the carbon is deposited on the ground, it is sequestered in the form of limestone.

The biological pump reacts quickly to disturbances, while the physical pump is the result of ocean circulation and acts over a longer period.

Physical carbon pump

For the physical carbon pump, atmospheric CO2 is dissolved in the ocean, especially at the poles. At the poles, the water is denser and sinks to the depths, taking the CO2 with it. Subsequently, the CO2 is stored in the deep waters thanks to water movements that promote exchange.

Chemical carbon pump

In addition to the biological and physical pump, there is a chemical pump that also participates in the absorption of CO2 in the oceans. The weathering of silicate rocks, such as olivine, increases the alkalinity of the oceans. Alkalinity is the ability to neutralise acids. Indeed, the increase in greenhouse gases, especially the CO2 present, acidifies the oceans. In order to increase alkalinity, CO2 dissolved in the ocean interacts with minerals released by the weathering of rocks. This chemical reaction converts the CO2 into bicarbonate and carbonate, so that carbonate rocks (limestone) can sequester the CO2.

Climate change and oceans

Climate change is threatening marine biodiversity and the oceans through the increase of greenhouse gases. So far, the ocean has absorbed more than 90% of excess heat and 30% of carbon dioxide (CO2) induced by human activities.

The ocean helps to slow the warming of surface waters and land surfaces through its ability to absorb heat and store anthropogenic (human activity related) CO2 emissions. However, climate change is having significant impacts on the ocean and marine biodiversity, in particular :

  • Increasing greenhouse gas emissions are acidifying the oceans. The acidification of the oceans endangers the marine ecosystem and risks causing the migration and disappearance of certain species. The pH is a measure of the acidity of the ocean and has been decreasing by about 0.02 per decade since 1910.

  • Climate change also threatens the supply of oxygen to the ocean. The exchange between warm surface waters and cold deep waters is no longer as easy. Warmer surface waters are less dense than cold deep waters, water circulation is more difficult and oxygen is depleted.

  • There is an impact on the melting of glaciers and ice caps, with a reduction of almost 40% in the ice pack since 1970, the disappearance of icebergs, which leads to a rise in sea levels, particularly with flooding on the coasts, the reduction in drinking water resources, which are fed by glaciers, and also the disappearance of biodiversity such as polar bears.

  • The global temperature of the oceans is increasing. For example, the average temperature of the surface ocean - between 0 and 75 m deep - has increased since the 1970s by 0.11°C per decade. Even a slight increase in temperature can potentially unbalance and rapidly melt large areas of ice.

The consequences of climate change for the environment and for humans are critical. The ocean, which is a carbon sink, is a key resource for carbon dioxide (CO2) reduction. In order to prevent these consequences from worsening, scientists are developing technologies to remove the millions of tons of CO2 released into the atmosphere and subsequently dissolved in the oceans.

Boosting natural pumps and avoiding worsening climate impacts

As the ocean covers a large majority of the Earth's surface, it is a key element in the removal of CO2. Research has led to technologies to boost the ocean's natural pumps. The platform Ocean CDR presents different approaches:

  • Seaweed or floating sargassum culture: the seaweed feeds on CO2, exports organic carbon and the biomass sinks into the deep sea. The seaweed on the surface that has helped to sequester CO2 can then be removed from the oceans and stored on land.

  • Artificial fertilisation of phytoplankton. This method aims to accelerate the biological pump for the sedimentation of dead organisms whose shells are made of carbonates.

Mangrove picture by Muhammadh Saamy
  • Protecting and restoring ecosystems through blue carbon. Blue carbon refers to the carbon sequestered, stored and released by coastal and marine ecosystems. Ocean or coastal ecosystems have a higher sequestration potential per hectare than terrestrial forests. For example, it can be 3 to 5 times higher for mangroves. According to the IPCC, greenhouse gas emissions from the degradation of these ecosystems represent between 0.1 and 1.46 GtCO2 per year, or up to 14% of the CO2 emissions from global deforestation.

  • Recovering liquid CO2 on land from industry and storing it in geological formations or deep in the ocean.
Olivine sand by Project Vesta
  • The enhanced weathering method of improving the alkalinity of the oceans by adding alkaline and silicate rocks such as olivine to the beaches or oceans. This is a technique for removing carbon dioxide by dissolving the minerals in the rocks. For example, the Vesta project aims to dissipate crushed olivine on beaches in order to turn beaches and the sea into carbon sinks.

These approaches could have a high potential for CO2 sequestration. Indeed, assessments have been made in terms of impacts, costs and efficiency. However, before they can be implemented, it is necessary to develop cross-sectoral collaborations, for research to be carried out to test and deploy innovations, and for adequate governance to be put in place to promote these innovations that aim to eliminate CO2.

It remains to be seen whether these consequences and uncertainties will be devastating or whether nature will adapt. Studies and experiments are being conducted every day to find solutions and combat climate change.