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The renewal of deep waters in the North Atlantic has slowed over the past three decades. This is shown by a new study from the GEOMAR Helmholtz Centre for Ocean Research Kiel, now published in Nature Communications. The study demonstrates that the “age” of water masses in the North Atlantic has been increasing continuously since the 1990s, indicating a weakening of the Atlantic circulation system. The results suggest that this trend cannot be explained by natural variability alone, but instead represents a signal of anthropogenic climate change. A slowdown in ocean circulation has far-reaching consequences for climate regulation, the ocean’s oxygen supply, and its uptake of carbon.
The ocean is continuously ventilated when surface waters sink and transport, for example, oxygen and carbon to greater depths. The efficiency of this process can be estimated using the so-called water age, which describes the time elapsed since a water mass last was in contact with the atmosphere.
Water age as a measure of ocean ventilation
Researchers at the GEOMAR Helmholtz Centre for Ocean Research Kiel have investigated how ocean ventilation in the North Atlantic has changed over recent decades. The North Atlantic Meridional Overturning Circulation (AMOC), which operates in this region, is responsible for the ventilation of large parts of the global ocean.
Their results show that North Atlantic waters are ageing overall. By integrating observational data with model simulations, the researchers conclude that this trend is more consistent with climate change than with natural variability alone. The study has now been published in Nature Communications.
Fluorinated greenhouse gases as time stamps
For their analysis, the researchers used measurements of the long-lived industrial trace gases CFC-12 and sulphur hexafluoride (SF₆). These gases, some of which escape unintentionally into the atmosphere, have been detectable in the ocean since the 1980s. Acting as chemical time stamps, they allow scientists to reconstruct the time when water was last in contact with the surface. In addition, the team analysed simulations from seven coupled Earth system models.
Consistent ageing of water masses since the 1990s
The analysis reveals a clear picture: across the entire North Atlantic, water masses are on average significantly older today than they were 30 years ago. Between the 1990s and the 2010s, the mean water age increased by more than ten years. At the same time, Apparent Oxygen Utilization (AOU) also increased. AOU describes the difference between the oxygen concentration that seawater would theoretically have at the surface in gas exchange with the oxygen-rich atmosphere, and the oxygen concentration actually measured at depth. A larger difference indicates a longer time span over which oxygen has been consumed by respiration within the ocean interior. Increasing water age goes hand in hand with declining ventilation, meaning that less oxygen-rich water is transported to depth.
The trend is particularly pronounced outside the Labrador Sea. Despite strong natural variability, driven for example by changing wind and weather patterns, this variability is overlaid by a large-scale, long-term ageing of water masses.
“Our results show that ventilation of the deep North Atlantic is weakening – even though individual regions may respond more strongly or more weakly at times,” says first author Haichao Guo, who completed his doctoral thesis last year in the Biogeochemical Modelling research unit at GEOMAR.
Climate signal rather than natural variability
A key result of the study lies in the interpretation of this trend. The researchers compared the observations with climate model results that include both natural variability and anthropogenic influences. All models consistently show increasing ageing of the North Atlantic. While natural variability can explain regional and short-term changes, it cannot account for the multi-decadal ageing trend observed across all ocean basins. This indicates that the detected trend represents a robust climate signal rather than a coincidental outcome of natural variability.
Why weaker ventilation is a concern
Declining ocean ventilation indicates a weakening of the North Atlantic circulation, which plays a key role in the climate system. It determines how much heat and carbon dioxide (CO2) the ocean can absorb and how well the deep ocean is supplied with oxygen. If this process weakens, the ocean’s long-term capacity to absorb heat and CO2 from the atmosphere is reduced. At the same time, the oxygen concentrations in deeper water layers decline, with potentially serious consequences for marine ecosystems.
Models indicate that changes in deep-water renewal are extremely sluggish: once ventilation has weakened, this state can persist for centuries – even if anthropogenic greenhouse gas emissions were to decrease.
Uncertainties and open questions
The study focuses on water masses with ages of up to around 200 years, as the industrial trace gases used in the analysis only allow reliable age estimates of such relatively young waters. Different tracers and longer time series would be required to investigate deeper and older ocean layers. The results also indicate that current climate models underestimate the magnitude of the observed changes. This is caused most likely by a too simplistic representation of deep mixing processes in the ocean in numerical models. However, the direction of the trend – increasing water age – is consistent across all models and observations, leading the researchers to conclude that it is robust.
“The combination of observations and models provides a consistent picture: North Atlantic waters are ageing – and this is in line with the expected weakening of North Atlantic circulation as a consequence of global warming,” says co-author Professor Andreas Oschlies, head of the Biogeochemical Modelling research unit at GEOMAR.
Guo, H., Koeve, W., Kriest, I., Frenger, I., Tanhua, T., Brandt, P., He, Y., Xue, T. & Oschlies, A. (2026): North Atlantic ventilation change over the past three decades is potentially driven by climate change.Nature Communications, 17, 200.
https://doi.org/10.1038/s41467-025-67923-x
https://www.geomar.de/n10141 – images for download
https://www.geomar.de/en/research/fb2/fb2-bm/overview Biogeochemical Modelling Research Unit
https://www.geomar.de/en/discover/ocean-and-climate/ocean-observation/gulf-strea... The Gulf Stream and Atlantic Circulation
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