The ocean serves as a massive reservoir in Earth’s climate system, continuously absorbing carbon from the atmosphere. This absorption is driven by a complex series of natural mechanisms collectively known as the ocean carbon pump. These processes capture atmospheric carbon dioxide and transport it into the deep ocean, playing a significant part in modulating the planet’s climate.
The Biological Carbon Pump
The biological carbon pump is initiated in the sunlit surface layer of the ocean by phytoplankton, which are microscopic, plant-like organisms. Like plants on land, these organisms use photosynthesis to convert dissolved carbon dioxide into organic matter for their growth. This activity reduces the concentration of CO2 in the surface water, allowing the ocean to draw down more of it from the atmosphere.
This captured carbon is transported to the deep ocean primarily through “marine snow,” a continuous shower of sinking organic material like dead phytoplankton and waste. Small animals called zooplankton graze on phytoplankton, and their dense fecal pellets are a component of this marine snow, accelerating the carbon’s journey downward. As these particles sink, they are consumed and decomposed by other organisms, releasing some carbon back into the water. However, a fraction of this material successfully reaches the deep sea below 1,000 meters.
The Physical Carbon Pump
Distinct from the life-driven biological pump, the physical pump operates on chemistry and physics, beginning with the direct dissolution of atmospheric CO2 into seawater. The capacity of seawater to hold dissolved CO2 is strongly dependent on water temperature; cold water absorbs significantly more than warm water. This process is most pronounced in the cold polar regions, like the North Atlantic and the Southern Ocean, where frigid surface waters become enriched with dissolved CO2.
The final step involves large-scale ocean circulation, or the global conveyor belt. This system transports the cold, dense, carbon-rich water from the poles, causing it to sink into the deep ocean basins. This downwelling current injects dissolved carbon into the ocean’s interior, removing it from contact with the atmosphere.
Long-Term Carbon Storage in the Deep Ocean
Once carbon is transported to the deep ocean, it enters a long-term storage phase. At depths below 1,000 meters, the water is isolated from the atmosphere, preventing the carbon from easily escaping. Carbon that reaches these depths can remain there for hundreds or even thousands of years, circulating slowly with deep ocean currents. It only returns to the surface through upwelling, often far from where it originally sank.
For a small but persistent fraction of the carbon, storage becomes even more permanent. Some organic material that settles on the ocean floor is not decomposed and becomes buried in marine sediments. Over geological timescales, this buried carbon can be transformed into rock, locking it away for millions of years.
Human Influence on Ocean Carbon Sequestration
Human activities are pressuring the ocean’s ability to sequester carbon. Rising global temperatures warm the ocean’s surface, which weakens the physical carbon pump because warmer water holds less dissolved CO2. These temperature changes also disrupt marine ecosystems, altering the productivity of phytoplankton communities that drive the biological pump.
The continuous absorption of excess atmospheric CO2 is causing ocean acidification, a decrease in seawater pH. This changing chemistry threatens marine organisms that build shells or skeletons from calcium carbonate, such as certain plankton, corals, and mollusks. As it becomes more difficult for these organisms to thrive, their role in transporting carbon to the deep sea may be diminished.
While the ocean is absorbing more total carbon dioxide because there is more in the atmosphere, this comes at a cost. The increased uptake is altering the chemical environment of the entire ocean. This rapid change places immense stress on the natural regulatory processes of the carbon pump, and the long-term consequences for these systems remain under investigation.