The deepest layer of the Southern Ocean, known as Antarctic Bottom Water (AABW), is shrinking and warming at an accelerating rate, according to research published in Geophysical Research Letters. A circumpolar analysis conducted between 2002 and 2023 shows the volume of this critical water mass fell by 3 percent, with the pace of decline quadrupling after 2015.
The Role of Antarctic Bottom Water in Climate Regulation
Antarctic Bottom Water acts as a fundamental component of the global climate system. According to the Australian Antarctic Program Partnership (AAPP), this cold, dense water mass accounts for up to 40 percent of the total volume of the world’s oceans. It functions as a primary buffer against global heating by sequestering heat and carbon deep beneath the surface.
Beyond thermal regulation, AABW is a central driver of the global “conveyor belt” of ocean circulation. It moves oxygen-rich water from the surface to the abyssal depths. As the volume of this water shrinks, the efficiency of this planetary circulation system faces potential disruption.
Did you know?
Antarctic Bottom Water is so dense that it sinks to the very bottom of the ocean basins, effectively “locking away” heat and carbon dioxide.
Evidence of Deep-Ocean Overturning Slowdown
The study reveals a convergence of physical changes in the deep ocean. Researchers observed simultaneous warming, a reduction in salinity, and a loss of oxygen within the AABW. These combined factors indicate a significant slowdown of deep-ocean overturning, which is the process that pushes surface water into the abyss.
Professor Nathan Bindoff of UTAS, a co-author of the study, stated that these multiple signs provide strong evidence that climate change is already altering the deepest parts of the world’s oceans. These shifts carry long-term consequences for how the planet transports heat and stores carbon.
Comparing Rates of Ocean Change
The acceleration of these changes is a critical focus for climate scientists. While the overall volume decline was recorded at 3 percent over a 21-year period, the transition in the rate of loss is stark. The decline in volume occurred at a rate four times faster in the years following 2015 compared to the period between 2002 and 2015.
| Period | Observation |
|---|---|
| 2002–2015 | Initial volume decline observed. |
| 2015–2023 | Rate of volume decline accelerated fourfold. |
Frequently Asked Questions
Why is Antarctic Bottom Water becoming “freshener”?
The freshening refers to the water becoming less salty.
How does this affect global sea levels?
The sources do not provide information on the effect on global sea levels.
Can this process be reversed?
The study identifies these trends as evidence of climate change altering deep-ocean dynamics.
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