The ocean is a key component of the global carbon cycle and plays a fundamental role in regulating the Earth's climate. It has slowed anthropogenic climate change by absorbing 91 % of the heat trapped in the system Earth between 1971 and 2018 and 26 % of anthropogenic carbon emitted since the pre-industrial era. Future climate will depend largely on its ability to continue this role.

The transfer of CO2 between the atmosphere and the ocean is primarily driven by two mechanisms: the solubility pump, which is related to CO2 solubility and circulation, and the biological carbon pump (BCP). The BCP involves the photosynthetic conversion of dissolved inorganic carbon (DIC) near the surface into organic matter, which is then exported to the ocean interior. This vertical redistribution of carbon effectively reduces surface DIC and increases remineralized DIC at depth, thereby affecting atmospheric CO2 levels. It is estimated that without remineralized carbon, atmospheric CO2 levels would be approximately 150-240 ppm higher at the new equilibrium state.

Until now, it has often been assumed that the BCP operates in a steady state and that the increase in current and future carbon uptake and storage was dominated by a physically driven solubility pump. The lack of observational evidence and knowledge of nonlinear interactions between the BCP and other components of the Earth system has led to uncertainty in projections of the effectiveness of the BCP.

Recent study using a fully interactive Earth system model, the Norwegian Earth System Model (NorESM2), examined the role of BCP in shaping the pre-industrial climate and quantified the impacts of its removal on future carbon sequestration and climate change. The model simulated a pre-industrial quasi-equilibrium and subsequent historical and future scenarios with and without marine organisms (REF) (Abiotic).

The results showed that in the pre-industrial steady state without BCP, atmospheric CO2 levels were more than 50 % (163 ppm) higher (445 ppm compared to 282 ppm in the REF scenario). This led to a warming of global average surface temperature by 1.6°C and sea surface temperature by 1.15°C. In this “Abiotic” scenario, the ocean released approximately 730 Pg C to the atmosphere, of which almost half (345 Pg C) was adsorbed by land, primarily due to CO2 fertilization-induced vegetation growth.

Projections for the future (1850–2100) showed that without ocean biology, climate change would accelerate. Atmospheric CO2 increased significantly faster in the Abiotic scenario than in the REF scenario in all three emission scenarios examined (SSP1-2.6, SSP2-4.5 and SSP5-8.5). The reason for this stronger increase in CO2 is the reduction in carbon absorption by land and ocean. In the Abiotic scenario, 68 to 83 % of fossil fuel emissions would remain in the atmosphere by 2100, compared to only 37 to 65 % in simulations including ocean biology.

The weakening of ocean carbon uptake despite higher atmospheric CO2 levels is attributed to higher ocean surface pCO2 and lower air-sea pCO2 imbalance. The missing pCO2 deficit caused by ocean production, together with warmer temperatures and a higher initial Revelle factor, reduces the ocean's ability to absorb anthropogenic CO2. Land carbon uptake is also reduced due to saturation of vegetation growth in a warmer, higher CO2 world.

The accelerated climate change in the Abiotic scenario was reflected in many components of the Earth system. According to the SSP5-8.5 scenario, the global average surface temperature in the Abiotic scenario increased by almost 5 °C by 2100, a warming rate 30 % faster than in the REF scenario. The removal of ocean biology led to the 2 °C warming threshold being exceeded more than a decade earlier.

The study's findings clearly show that biological processes in the ocean play a key role in the uptake of anthropogenic carbon in the contemporary ocean by modulating the spatial and temporal patterns of the surface pCO2 deficit. The greatest impact of the absence of ocean biology is seen in ventilation regions such as the North Atlantic and Southern Ocean, where long-term transport of carbon from the surface to the ocean interior is significantly reduced.

Maintaining healthy and well-functioning marine ecosystems is essential to maximizing the ocean's role in mitigating anthropogenic climate change. Better understanding and modeling of biological processes in the ocean, particularly in key carbon sink regions, is essential for improving estimates of future ocean carbon sinks and developing effective climate mitigation strategies. The study shows that the biological carbon pump plays an important role in creating the conditions (disequilibrium and Revelle factor) that determine the magnitude of this sink, challenging the notion that it has little or no role in sequestering excess anthropogenic carbon. Spring

The entire study was published in the journal Nature Communications .

Glossary of key terms

• Abiotic simulation: An extreme hypothetical experiment where marine organisms and primary production are completely removed from the Earth system model.
• Anthropogenic carbon (Cant): Carbon added to the Earth system by human activity, particularly the burning of fossil fuels and land use change.
• Atmospheric CO2: Concentration of carbon dioxide in the Earth's atmosphere. A key greenhouse gas.
• Biological carbon pump (BCP): The process by which photosynthesis near the ocean surface converts dissolved inorganic carbon (DIC) into organic matter, which is then exported to the deeper ocean.
• DIC (Dissolved Inorganic Carbon): The total amount of dissolved inorganic carbon in seawater, which exists in the form of CO2, bicarbonates, and carbonates.
• Earth System Model (ESM): A complex computer model that simulates interactions between different components of the Earth system, including the atmosphere, ocean, land, and ice.
• pCO2: Partial pressure of carbon dioxide. The pCO2 imbalance between air and sea controls the flow of CO2 across the interface.
• Reshaped DIC: The fraction of DIC in seawater that was present in surface water before it was submerged in the deeper ocean, unaffected by remineralization.
• Pre-industrial climate state (REF): A reference simulation representing the state of the climate and carbon cycle before significant human impacts, typically based on conditions around 1850.
• Revelle factor: A measure of the ability of seawater to absorb additional CO2 without significantly increasing the partial pressure of CO2. A higher Revelle factor means a lower buffering capacity.
• Remineralized DIC: The portion of DIC in the deeper ocean that was formed by the decomposition (remineralization) of organic matter that sank from the surface.
• Shared Socio-Economic Pathways (SSP) Scenarios: A set of possible future socio-economic pathways and associated greenhouse gas emission levels used for climate change projections (e.g. SSP1-2.6, SSP2-4.5, SSP5-8.5).
• Sequestration: The process of removing and storing carbon from the atmosphere.
• Solubility pump (solubility pump): A process in which the ocean absorbs CO2 directly from the atmosphere based on the physical principles of gas solubility and ocean circulation.
• SST (Sea Surface Temperature): Ocean surface temperature.