The ocean plays a fundamental role in regulating Earth’s climate by acting as a significant carbon sink, absorbing a substantial portion of atmospheric carbon dioxide. This vast marine environment helps moderate global temperatures by removing CO2, a potent greenhouse gas, from the atmosphere. The ocean has absorbed approximately 25-30% of all carbon dioxide emissions generated by human activities, along with about 90% of the excess heat from these emissions. This capacity to draw down and store carbon helps maintain the planet’s climatic balance.
Ocean’s Primary Carbon Absorbers
The most significant direct absorbers of carbon dioxide in the ocean are microscopic organisms known as phytoplankton. These single-celled marine algae and cyanobacteria are abundant in the sunlit surface waters and utilize sunlight to convert dissolved carbon dioxide into organic matter through photosynthesis.
Phytoplankton form the base of the marine food web and are responsible for roughly half of the planet’s primary production, absorbing as much carbon as all land plants and trees combined despite their much smaller biomass.
Different types of phytoplankton contribute to this process, including diatoms, dinoflagellates, and coccolithophores. Their rapid growth and short lifespans mean they have a high turnover rate, continuously absorbing CO2 from the water. As phytoplankton absorb carbon dioxide from the surface waters, it lowers the CO2 concentration in the water, allowing the ocean to absorb more CO2 from the atmosphere to maintain equilibrium. While macroalgae, or seaweeds, also perform photosynthesis and absorb carbon, their global contribution to carbon absorption is considerably smaller compared to the widespread and numerous phytoplankton.
Building Blocks of Carbon Sequestration
Beyond direct photosynthetic absorption, other marine organisms contribute to carbon sequestration by incorporating carbon into their physical structures. Corals, mollusks, foraminifera, and certain types of coccolithophores build shells or skeletons primarily composed of calcium carbonate (CaCO3). This process, called calcification, removes dissolved inorganic carbon from seawater.
Corals, for example, build vast reef structures by secreting calcium carbonate, which can store carbon for extended periods. Mollusks, such as clams and oysters, also form shells from calcium carbonate, with some studies estimating significant carbon deposition in mollusk shells over millennia.
While calcification itself releases some CO2 into the surrounding water, the overall effect of these organisms is to lock away carbon in solid forms that can eventually sink and become part of marine sediments. This removal of dissolved CO2 indirectly enhances the ocean’s capacity to absorb more atmospheric CO2.
The Journey of Carbon to the Deep Ocean
The carbon absorbed by surface-dwelling organisms embarks on a journey to the deep ocean, a process often referred to as the “biological pump.” This pump is a biologically driven mechanism that transports organic carbon from the surface to the ocean interior and seafloor sediments.
When phytoplankton and other marine organisms die, or when they are consumed and their waste products are excreted, this organic matter sinks through the water column. This sinking material, often called “marine snow,” carries carbon to deeper waters, effectively sequestering it from the atmosphere for hundreds to thousands of years.
While much of this organic matter is recycled in the upper layers, a small fraction reaches depths below 500 meters, where it remains sequestered. Some carbon eventually gets buried in marine sediments on the seafloor, providing long-term storage. This deep-ocean transport and burial contributes to the ocean’s long-term carbon storage.
Global Importance of Ocean Carbon Absorption
The ocean’s capacity to absorb and store carbon dioxide is important for global climate regulation. Marine organisms and the processes they facilitate help mitigate the impacts of rising atmospheric CO2 levels by removing vast quantities of the greenhouse gas.
However, the ocean’s ability to continue this function is influenced by changes such as warming temperatures and ocean acidification, which can affect the efficiency of these carbon-absorbing processes. The ongoing health of these marine ecosystems is directly linked to the planet’s climatic stability.