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So far, the ocean has helped to buffer global warming by absorbing more than 90 per cent of the excess heat trapped in the Earth system by the anthropogenic greenhouse effect. A new modelling study by the GEOMAR Helmholtz Centre for Ocean Research Kiel has now examined how the ocean might respond if atmospheric carbon dioxide was drastically reduced in the future. The results show that, after centuries of cooling, the Southern Ocean could trigger renewed warming by releasing the stored heat back into the atmosphere. Whether this would occur as a single major “heat burp”, in many smaller pulses, or continuously over centuries remains unclear. The study has now been published in AGU Advances.
Human emissions have pushed the Earth into an energy imbalance, the effects of which are most clearly felt in the ocean. Since the start of the Industrial Revolution, the ocean has absorbed more than 90 per cent of the surplus heat that could no longer be radiated into space. Consequently, the ocean has significantly mitigated atmospheric warming.
“The ocean is an extraordinarily large heat reservoir, and until now it has helped to soften the warming of the atmosphere,” says Dr Ivy Frenger, lead investigator of the ERC Starting Grant project OSTIA (The Ocean’s Role in Mitigating Climate Change) at GEOMAR. In a new modelling study, Dr Frenger and her team investigated how the ocean would respond if society managed to stop emitting greenhouse gases, subsequently cooling global temperatures by removing CO2 from the atmosphere. Their findings suggest that, after centuries of cooling, heat accumulated in the deep Southern Ocean could be released in the form of a “burp”, warming the atmosphere again.
Understanding feedbacks and their implications
Frenger explains: “At present, it is not at all foreseeable that CO₂ emissions will be reduced quickly enough, let alone that we will achieve sustained net-negative emissions. However, we still need to understand what will happen when we remove our substantial human disturbance of the carbon cycle – when we reach net zero CO₂ emissions or even reduce atmospheric CO2 concentrations back to pre-industrial levels. We need to understand how the climate system would react in such a case, identify the relevant feedback processes, and estimate their potential effects.”
Observational data show that the ocean has been warming at an unprecedented rate since at least the 1950s. The period from 2012 to 2024 was the warmest on record. This warming does not stop at the surface, but penetrates large parts of the ocean’s interior. The heat stored in the deep ocean makes long-term warming inevitable because deep water only returns to the surface after centuries or millennia.
“We certainly want to stop emitting greenhouse gases. Otherwise, atmospheric temperatures will continue to rise and even more heat will accumulate in the ocean,” says Frenger. “But we also need to know what would happen if human disturbance ceased, or if we attempted to reverse some of the anthropogenic climate change by removing CO2 from the atmosphere. Which feedback loops would then come into play?”
Earth system model provides a glimpse into a possible future
This is precisely the area that the new study focuses on. So far, little attention has been given to the question of what happens to the heat stored in the ocean once humanity manages to remove more carbon dioxide from the atmosphere than it emits.
To address this issue, the Kiel-based team employed an intermediate-complexity Earth system model: UVic v. 2.9, which was developed at the University of Victoria. This model combines representations of the ocean, sea ice and land vegetation with a simplified atmosphere. “Although the model is less spatially detailed than others and uses a simplified atmospheric component – because the focus is on the ocean and the carbon cycle, it allows simulations over periods ranging from centuries to millennia,” says Frenger.
The team ran an idealised climate scenario that is commonly used to test how the Earth system responds to the reversal of human-induced warming: emissions rise year on year until, after 70 years, atmospheric CO2 has doubled. Then, emissions are sharply reduced until more CO2 is removed from the air than is emitted, that is, net-negative emissions are achieved.
A sudden transfer of heat from the ocean back into the atmosphere
The result is striking: “After a long cooling phase, our simulations suggest that the Southern Ocean could effectively ‘burp’ out heat that built up during earlier warming,” Frenger explains. This release could continue for decades or even centuries, raising temperatures at rates comparable with those experienced during the past 150 years of human-driven warming.
Frenger adds: “The Southern Ocean acts as a kind of exhaust valve for the global ocean, allowing heat stored deep below the surface to eventually escape. This applies not only to the heat that has already accumulated there, but also to the heat that is transported from other regions by large-scale circulation.” In other words, the Southern Ocean provides a potential pathway by which stored heat can return to the atmosphere.
The deep Southern Ocean is a key region
The Southern Ocean, which surrounds Antarctica, plays a pivotal role in the global climate system. It connects all ocean basins. Under natural, pre-industrial conditions, heat that accumulates near the equator due to incoming solar radiation is transported southwards and released back into the atmosphere before being radiated into space.
The new study by Frenger, Frey et al. suggests that human-induced climate change, in the form of rising CO2 concentrations and atmospheric warming, has disrupted these flows. The ocean is therefore unable to release heat as efficiently as before. Consequently, more energy remains trapped in the ocean interior.
As the Earth system moves back towards energy balance following the cessation of anthropogenic emissions, the stored heat will gradually be released back into the atmosphere. The timing, intensity and spatial patterns of this release depend on changes in winds, ocean circulation and stratification. These factors will determine whether the heat is released gradually, thereby slowing the cooling rate, or whether more abrupt releases occur on geological timescales, as seen in the simulations.
No comparable release of CO2 in the model
Unlike heat, the model did not simulate a similar release of CO2 from the ocean. Preliminary results suggest that this is due to seawater chemistry retaining a large proportion of the dissolved carbon. This means that, even if significant heat is released in the future, a similarly large “CO2 burp” that would further amplify warming is not expected.
The team emphasises that the results must be interpreted with care. The scenario is idealised – a rapid shift to net-negative emissions is currently unrealistic. The model has limited spatial resolution and does not include processes such as ice-sheet melt.
Furthermore, the study only analyses one feedback mechanism, and other processes and natural variability are not taken into account.
Climate strategies must consider long ocean timescales
Despite the uncertainties involved, the results were robust: similar patterns emerged across different model configurations. The researchers agree that we must understand the Southern Ocean far better in order to assess future climate trajectories. “Because of its central importance to the global climate system, the Southern Ocean should be monitored continuously,” says Frenger. “To date, it has been studied far less than regions such as the North Atlantic, partly due to its harsh weather conditions.”
The findings also highlight the need for climate strategies aimed at the next ten generations to take the ocean’s long timescales into account. “It is important to think beyond the year 2100,” says Frenger. “The disruption we have caused, and continue to cause, will affect us for a very long time. The most important step right now is to reduce current CO2 emissions to net zero, in order to avoid a further perturbation to the climate system.”
https://doi.org/10.1029/2025AV001700 Frenger, I., Frey, S., Oschlies, A., Getzlaff, J., Martin, T., & Koeve, W. (2025). Southern Ocean heat burp in a cooling world. AGU Advances, 6, e2025AV001700
https://www.geomar.de/n10091 – images for download
https://doi.org/10.1029/2025EO250385 A companion article by science journalist Sarah Derouin
https://soccom.org/ SOCCOM project for large-scale observational deployment
https://www.antarctica-insync.org InSync field campaign, which will begin in 2027
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