The ocean may have removed much less carbon dioxide from the atmosphere than expected in 2023, mainly due to high ocean temperatures. If climate change continues to rapidly weaken the planet鈥檚 ocean and land carbon sinks, it will be even more difficult to avoid dangerous levels of warming.
The ocean currently absorbs a quarter of the world鈥檚 CO2 emissions from the atmosphere each year. However, warming temperatures are expected to weaken this natural carbon sink and observations from 2023 鈥� the hottest year on record 鈥� suggest this process may be happening even faster than anticipated.
鈥淎s the world warms, the wonderful free service that is being provided to us by the ocean and the land will decline in strength, so the fraction of CO2 from fossil fuels that are stored in the atmosphere will increase,鈥� says at Colorado State University, who wasn鈥檛 involved with the new research.
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at ETH Z眉rich in Switzerland and his colleagues modelled the flux of CO2 from the oceans to the atmosphere in 2023, excluding the polar oceans. They based the model on direct measurements at different parts of the ocean, as well as observations of sea surface temperature, wind speed and the activity of plankton.
Overall, the researchers found that the ocean carbon sink was about 10 per cent weaker than expected given a strong El Ni帽o weather pattern emerged early in the year. Historically, a shift to warm El Ni帽o conditions helps the tropical Pacific absorb more carbon dioxide by suppressing the deep waters upwelling to the surface there, limiting how much CO2 they release into the air.
But in 2023, this effect was diminished by an increase in CO2 released from the Atlantic and Pacific Oceans outside the tropics. This researchers attributed the rise to record-high sea surface temperatures in large areas of both oceans, which reduced the water鈥檚 capacity to absorb CO2.
鈥淭he CO2 un-dissolves,鈥� says Denning. 鈥淚t comes out of solution and degasses into the atmosphere.鈥�
The waning of Earth鈥檚 carbon sinks will make it even more difficult to reduce emissions fast enough to meet climate targets. In addition to the weak ocean sink, 2023 saw the near-total collapse of the land carbon sink due to drought and wildfires. But it is unclear whether the weakened sinks were blips or signs of a more lasting change.
鈥淭here鈥檚 an intriguing possibility that we could interpret the results from the ocean and the land in an unusually warm year as a harbinger of things to come,鈥� says Denning. 鈥淚t鈥檚 hard to know whether the response to a short-term warming will resemble the response to a long-term warming.鈥�
at the University of East Anglia, UK, says it would be 鈥渨ay too early鈥� to conclude the ocean sink is weakening faster than expected on the basis of a single year, especially given the complexity of all the factors involved. 鈥淚 don鈥檛 think we can really say the models are underestimating the effect of climate at this stage,鈥� she says.
However, continued high sea surface temperatures in 2024 have been concerning for the carbon sink, says at the US National Oceanic and Atmospheric Administration. These temperatures, combined with a shift to La Ni帽a conditions 鈥� which boost the upwelling of CO2-rich waters 鈥� could lead to an even weaker ocean carbon sink this year. 鈥淲e鈥檙e just waiting to see what happens,鈥� says Feely.
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