The oceans cover more than 70% of the Earth’s surface and play a critical role in absorbing CO2 from the atmosphere. Estimates suggest that around a quarter of all anthropogenic CO2 emissions are taken up by the ocean, although recent research suggests the oceanic carbon sink could be even larger. The ability of the ocean and marine ecosystems to capture and store carbon has slowed the pace of global warming, although it has also led to ocean acidification, which can have a hugely negative impact on marine life.
CO2 from the atmosphere dissolves in the surface waters of the ocean. Much of it stays as dissolved gas, but some is turned into organic matter through photosynthesis by tiny marine plants (phytoplankton). Through an important process called the biological pump, this organic carbon is transferred from the surface to ocean depths when algal material or faecal pellets from fish and other organisms sink. The daily migration of fish to and from the depths also contributes organic carbon particles, along with excreted and respired material.
70% of the earth’s surface is water and plays a critical role in absorbing CO2
While fish are the dominant vertebrates in the ocean, the scale at which they help to sequester atmospheric CO2 remains poorly understood. A study carried out in 2021 estimated that carbon in fish faeces, respiration, and other excretions makes up about 16% of the total carbon that sinks below the ocean’s upper layers. That equates to roughly 1.5 billion tonnes every year – or an amount of carbon equivalent to twice the CO2 emissions of all 27 Member States of the European Union.
Protecting marine fish, about 90% mesopelagic can sequester 5.50 GtCO2 per year. That is twice of the total fossil fuel emissions by EU-27 in 2022 (2.76 GtCO2/y) or 3.9x total Africa in 2022 (1.42 GtCO2/y) ¹
More research needs to be carried out to understand exactly how climate change and seafood harvesting impacts the role of fish in oceanic carbon cycling. A recent study has shown that bottom-trawling releases as much carbon from the seabed as the entire aviation industry. This is another reason why marine protected areas (MPAs) – which currently cover a fraction of the ocean floor – need to be extended. More (properly enforced) MPAs could lead to enhanced carbon sequestration by increasing populations of fish and other marine species, as well as boosting fishery yields and food security.
Research has also revealed that 43.5 percent of the blue carbon extracted through commercial fishing between 1950 and the present day has come from areas of the ocean that would have been unprofitable to fish without subsidies. Limiting blue carbon extraction by fisheries, particularly in unprofitable areas, would not only reduce CO2 emissions from fuel use, but help to sequester more atmospheric carbon by enabling fish stocks to recover, thereby boosting the biological carbon pump.
1.5 billion tonnes of carbon contained in fish faeces, respiration, and other excretions sinks below the ocean’s upper layers every year.
The oceans cover more than 70% of the Earth’s surface and play a critical role in absorbing CO2 from the atmosphere. Estimates suggest that around a quarter of all anthropogenic CO2 emissions are taken up by the ocean, although recent research suggests the oceanic carbon sink could be even larger. The ability of the ocean and marine ecosystems to capture and store carbon has slowed the pace of global warming, although it has also led to ocean acidification, which can have a hugely negative impact on marine life.
CO2 from the atmosphere dissolves in the surface waters of the ocean. Much of it stays as dissolved gas, but some is turned into organic matter through photosynthesis by tiny marine plants (phytoplankton). Through an important process called the biological pump, this organic carbon is transferred from the surface to ocean depths when algal material or faecal pellets from fish and other organisms sink. The daily migration of fish to and from the depths also contributes organic carbon particles, along with excreted and respired material.
How do marine fish help store carbon?
- Excretion and Respiration: Fish contribute significantly to the oceanic carbon pump by excreting and respiring organic material. Around 1.5 billion tonnes of carbon sinks below the ocean’s upper layers each year through fish faeces and other excretions.
- Biological Pump: The carbon-rich particles fish excrete are part of the biological pump, a vital process that transfers carbon from the ocean’s surface to its depths. This helps lock away atmospheric CO2 for long periods, mitigating the effects of global warming.
- Daily Migration: Many fish species migrate between the ocean’s surface and its deeper layers. During this movement, fish contribute to the transport of organic carbon particles, further supporting carbon sequestration.
Current situation – what we need to do
Protecting marine fish populations, especially mesopelagic species, is essential to enhance carbon sequestration in the ocean. If managed effectively, fish could help sequester 5.50 GtCO2 per year, surpassing the total fossil fuel emissions of the EU. Expanding marine protected areas (MPAs) and limiting harmful practices like bottom-trawling are crucial steps to avoid disrupting this natural carbon sink. It’s important to understand how fisheries and climate change affect fish populations to prevent harm to the ocean’s ability to capture carbon.
(1) Note: These species data are from Table 1 and the Supplementary Appendix 1 of Schmitz, O.J., Sylvén, M., Atwood, T.B. et al. Trophic rewilding can expand natural climate solutions.
Nat. Clim. Chang. 13, 324–333 (2023). https://doi.org/10.1038/s41558-023-01631-6
Next steps
- Return to the main Climate Heroes page.
- Read all the original Climate Hero case studies in ‘Animating the Carbon Cycle: Supercharging Ecosystem Carbon Sinks to Meet the 1.5°C Climate Target’. Download the pdf here.
- Curious to learn more? Watch the video from the UN Climate Change Side Events at COP28: Bringing elephants into the room – why UNFCCC needs to put animals on the table.