Photo by Leon Rohrwild

The world’s oceans absorbed around 1 billion tonnes less carbon dioxide in 2023 than expected — a drop of about 10% compared with previous years — according to new research led by ETH Zurich and published in Nature Climate Change. The team reported that record sea surface temperatures and shifting climate patterns weakened one of the planet’s most important natural carbon sinks.

What changed in 2023

The study found that exceptionally high sea surface temperatures in the North Atlantic and other extratropical regions reduced the solubility of CO₂, meaning less of the gas could be absorbed. At the same time, a strong El Niño event in the tropical Pacific altered circulation, limiting the usual upwelling of cooler, carbon-rich deep waters. While El Niño years often increase CO₂ uptake in the tropics, in 2023 the warming elsewhere effectively cancelled out that effect.

Mechanisms that cushioned the blow

The authors emphasised that several processes helped prevent an even steeper decline. Stratification of the water column reduced mixing and the release of CO₂ from deeper layers. The biological pump — phytoplankton absorbing carbon through photosynthesis and transferring it to the deep sea as they die and sink — continued to function. In addition, reduced accumulation of dissolved inorganic carbon in surface waters lessened the risk of CO₂ outgassing back into the atmosphere.

Implications for climate and conservation

The weakening of the carbon sink suggests the global carbon budget — the amount humanity can emit while still meeting climate targets — may be more limited than previously thought. If oceans absorb less, more CO₂ remains in the atmosphere. Scientists warned that this not only accelerates climate change but also compounds stress on marine ecosystems already facing deoxygenation and acidification.

For conservation and policy, the findings highlight the importance of protecting biological processes that underpin carbon uptake, such as plankton productivity and healthy food webs. These ecosystems play a critical role in stabilising the ocean’s capacity to absorb CO₂.

Questions ahead

Researchers cautioned that it remains unclear how long the compensating mechanisms observed in 2023 will hold. If marine heat waves become more frequent and intense, as climate models project, episodes of weakened carbon absorption could occur more regularly. The study concluded that the North Atlantic in particular is emerging as a hotspot for sink loss, suggesting the need for closer regional monitoring and tailored policy responses.