The ocean has absorbed roughly 30% of human-caused carbon dioxide emissions over the industrial era, acting as the largest active carbon reservoir on Earth. The ocean’s capacity to absorb atmospheric carbon dioxide makes it a crucial natural defense against the rapid accumulation of greenhouse gases. The primary function of this oceanic system, known as the carbon sink, is to move carbon from the surface layer into the deep sea, where it can be stored for long periods.
The Physical Engine: Solubility Pump
The ocean absorbs carbon dioxide directly from the atmosphere through a physical and chemical process referred to as the solubility pump. This process is driven by the basic principle that gas solubility in water increases as the water temperature decreases. Surface waters at high latitudes, such as those in the North Atlantic and Southern Ocean, are significantly cooler and can therefore dissolve a greater amount of atmospheric carbon dioxide than warmer equatorial waters.
As this cold, dense surface water cools further and becomes saltier, it increases in density and sinks into the deep ocean. This downward flow is a key component of the thermohaline circulation, a global system of ocean currents. The dissolved carbon is then carried with the sinking water mass into the ocean interior, sequestering it far from the atmosphere for centuries or even millennia.
Carbon dioxide reacts with water to form dissolved inorganic carbon species, including bicarbonate and carbonate ions. This reaction allows the water to hold more carbon than if the gas simply remained dissolved. The solubility pump is the main mechanism responsible for transporting human-caused carbon emissions to the deep ocean, particularly in regions where deep-water formation occurs.
The Living Engine: Biological Pump
The biological pump operates alongside the physical process, driven by marine organisms that move carbon from the surface to the deep ocean. This process begins in the sunlit surface layer where microscopic marine plants, called phytoplankton, absorb dissolved carbon dioxide. Through photosynthesis, these organisms convert the inorganic carbon into organic matter, forming the base of the marine food web.
Zooplankton, small marine animals, consume the phytoplankton, transferring the carbon up the food chain. When these organisms die, or when zooplankton excrete waste, the carbon-rich material aggregates into larger, denser particles. This material, often described as “marine snow,” constantly falls through the water column under the force of gravity.
While much of this organic matter is broken down and recycled by bacteria and other organisms in the twilight zone (down to about 500 meters), a small fraction makes it to the seafloor, potentially locking away the carbon for thousands of years. The efficiency of this pump is directly tied to how quickly the particles sink and how much is remineralized before reaching the deep ocean.
The Hidden Cost: Ocean Acidification
The ocean’s absorption of excess atmospheric carbon dioxide comes at a steep price, leading to a phenomenon known as ocean acidification. When carbon dioxide dissolves into seawater, it initiates a chemical reaction with water to form carbonic acid. This weak acid then dissociates, releasing hydrogen ions and causing the pH of the seawater to decrease, making the ocean more acidic.
This shift in ocean chemistry has a profound effect on the availability of carbonate ions. The excess hydrogen ions bond with available carbonate ions, effectively reducing the concentration of carbonate that marine life needs to survive. This reduction directly impacts calcifying organisms, which rely on carbonate ions to build and maintain their shells and skeletons.
Organisms such as corals, oysters, mussels, and a type of plankton called pteropods struggle to build their calcium carbonate structures in waters with lower pH. They must expend more energy to extract the necessary ions, leaving less energy for functions like growth and reproduction. In severe cases, the decreased availability of carbonate ions can cause existing shells and skeletons to dissolve, threatening the survival of these species and disrupting entire marine food webs.
Measuring the Sink’s Capacity
Scientists quantify the ocean carbon sink, which currently absorbs approximately 2.5 billion tonnes of carbon annually. Measurements of the partial pressure of carbon dioxide in surface waters, collected by research vessels, moorings, and autonomous platforms, are compiled into global databases like the Surface Ocean CO₂ Atlas (SOCAT). These data are then used with mapping methods and biogeochemical models to estimate the air-sea carbon dioxide flux.
These models and observations help to track the invasion of human-caused carbon into the ocean interior and are used to project future absorption rates. A growing concern is the finite capacity of the ocean sink and the potential for a reduction in its efficiency. Warming surface waters, a consequence of climate change, decrease the solubility of carbon dioxide, which directly weakens the solubility pump’s ability to take up gas.
Furthermore, rising temperatures can increase the stratification of the ocean, limiting the mixing of water layers and potentially slowing the biological pump. Understanding the limits of these processes and how they might change is essential, as a less efficient ocean sink would mean a greater fraction of carbon dioxide emissions would remain in the atmosphere, accelerating climate change.