Forests are widely praised for their ability to capture carbon dioxide, absorbing an estimated 7.6 billion tonnes of CO₂ each year. However, scientists say the world’s oceans surpass forests as the planet’s largest carbon sink, taking in roughly 30% of global carbon emissions annually.
While forests store carbon in trees, plants and soils through photosynthesis, the ocean operates on a far larger scale. Marine plants such as phytoplankton absorb carbon dioxide from the atmosphere, while tiny crustaceans like krill help transport it deeper into the ocean. When marine organisms die, their bodies sink to the seabed, effectively locking carbon away for centuries.
Coastal ecosystems including mangroves, seagrass meadows and salt marshes are often referred to as “blue carbon” habitats because of their exceptional ability to sequester carbon in sediments. Even marine wildlife plays a role. Large whales, for example, store significant amounts of carbon in their bodies, which can be transferred to the ocean floor after death.
Despite the ocean’s greater absorption capacity, experts caution that this process comes at a cost. As carbon dioxide dissolves in seawater, it increases acidity levels in a phenomenon known as ocean acidification. Lower pH levels make it more difficult for shell-forming organisms such as corals and molluscs to maintain their skeletons and exoskeletons, threatening marine ecosystems.
Forests, meanwhile, remain critical in regulating regional climates, supporting biodiversity and preventing additional emissions caused by deforestation. Tropical forests in particular are among the most effective terrestrial carbon sinks.
Researchers emphasise that while both forests and oceans play essential roles in slowing global warming, neither can offset continued high levels of greenhouse gas emissions. Cutting emissions at the source, they say, remains the most effective strategy in tackling climate change.
In the balance between land and sea, the ocean absorbs more carbon overall — but protecting both systems is crucial to stabilising the planet’s climate.