From the beginning of the industrial revolution to the mid-1990s, the ocean absorbed around one-third of man-made carbon dioxide (CO2) emissions, according to a new study published on 15 March in Science (1). However, regional differences in CO2 uptake by oceans suggest this route of CO2 absorption may not be robust enough to be guaranteed in the future. Moreover, dramatic increases in CO2 are leading to acidification, which may have severe consequences for marine life.
Oceans and other land ecosystems absorb CO2 from the atmosphere, in effect, providing a crucial service. Wherever CO2 meets water, it is absorbed onto the surface and in the case of the ocean, currents and circulation patterns carry the dissolved CO2 deep into the ocean’s interior where it builds up over time. Without this important process, atmospheric CO2 levels would be much higher.
An international team of scientists led by Prof Nicolas Gruber from ETH Zurich examined data based on a global survey of CO2 and other chemical and physical properties in the oceans collected during more than 50 research cruises up to 2013. Data were measured from the surface to depths of up to 6 kilometres. A newly developed statistical method allowed them to distinguish between changes in man-made and natural CO2 (the CO2 present in the ocean before industrialisation).
The researchers estimate that between 1994 and 2007, the ocean absorbed around 34 billion tonnes of man-made CO2 from the atmosphere, accounting for around 30 per cent of all human-produced CO2 emissions during this time period. The reconstructed changes in CO2, therefore, suggest a “continuing strong role of the ocean in the recent global carbon budget,” according to the authors. But at some point, will the oceans become saturated?
Up to now, the percentage of CO2 uptake by oceans has been stable. In other words, the more CO2 humans produce, the more the oceans absorb. However, the study did find regional differences in CO2 uptake. For example, the North Atlantic Ocean absorbed 20 per cent less CO2 than expected during the examined time period. But this was balanced by increased absorption in the South Atlantic. The researchers observed similar fluctuations in other oceans, including the Southern, Pacific, and Indian Oceans. So, there also seems to be complex feedback systems in play.
Furthermore, while the uptake of CO2 provides an important service for humanity, it comes with the cost of increasing acidification. Calcium carbonate, the main component of seashells, dissolves in acidified environments. This poses a threat to mussels, corals, and many other sea creatures. Moreover, changes in the ocean’s chemical composition can also affect fish.
“Documenting the chemical changes imparted on the ocean as a result of human activity is crucial, not least to understand the impact of these changes on marine life,” says Gruber. Most of the man-made CO2 in the atmosphere comes from burning fossil fuels. Perhaps, humans should help out the oceans and its abundant marine life by reducing CO2 emissions and therefore, the immense burden on the world’s oceans.
(1) Gruber, N. et al. The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science (2019). DOI: 10.1126/science.aau5153