The ocean's ability to absorb carbon is weakening as climate change intensifies, a new United Nations report warns ahead of the COP31 climate conference in November. The ocean absorbs around 30 per cent of human-caused carbon dioxide emissions and 90 per cent of the excess heat trapped by greenhouse gases, but climate change has already reduced its capacity to absorb carbon by approximately six per cent, UN News reported on Wednesday.

Source: UN News

"For a long time the ocean has been doing us an enormous climate service, but that service is under increasing pressure," Sara Traerup, lead author of the report, told UN News. Rising temperatures and changes in ocean chemistry, circulation and ecosystems are weakening its ability to absorb and store carbon, leaving more CO2 in the atmosphere, and the process "can definitely accelerate" in the foreseeable future, she warned.

The report was a collaboration between the UN's environment agency, UNEP, and the UN climate secretariat, UNFCCC, which co-organises COP31. The UNFCCC's 2026 Climate Technology Progress Report sets out the same ocean figures — absorbing about 30 per cent of global CO2 emissions and more than 90 per cent of excess heat — and warns that warming, acidification and deoxygenation are weakening the ocean carbon sink.

Source: UNFCCC Climate Technology Progress Report 2026

~30%Share of human-caused CO2 the ocean absorbs
90%Share of excess greenhouse heat the ocean takes up
~6%Fall in ocean carbon-absorption capacity already recorded
407 million tonnesCO2 sequestered each year by blue carbon ecosystems

From sensors to seagrass

The report calls for stronger "ocean carbon intelligence" — better monitoring, sensors, data and modelling — because reliable data is lacking in the Southern and Indian Oceans, the Arctic and many coastal regions. "As of now, we don't have full information about the ocean, and what we don't know, we cannot manage and protect," Traerup said. Blue carbon ecosystems such as mangroves, seagrasses and salt marshes already sequester around 407 million tonnes of CO2 a year, while marine carbon-removal technologies remain highly uncertain and need further research and regulation, the report said.

Small Island Developing States contribute less than 1 per cent of global emissions yet are "on the front line of ocean and climate impacts," Traerup noted. The report cited Seychelles' 2015 debt-for-nature swap and Pacific work in Fiji, Papua New Guinea, Solomon Islands and Vanuatu, where more than 100 people were trained in blue carbon assessment. "We shouldn't treat ocean-based technologies as a shortcut," Traerup cautioned — they must complement, not replace, emissions reductions and ecosystem protection.