The flow of Antarctic bottom waters along the ocean floor is slowing, an unseen shift with potentially vast consequences. Between 1992 and 2017, the strength of this deep, cold, oxygen-rich current declined by roughly 30 percent. That change curtails oxygen supply on the seafloor and gradually disconnects portions of the deep ocean from the rest of the globe. The slowdown matters for climate, sea level, and the life that depends on the ocean below.
The brake on this Antarctic undercurrent, cited as one of the climate-change risks, appears to be advancing earlier than expected. A study from an international team warns that interrupting this flow could trigger a cascade of effects across the planet, a caution echoed by Nature Climate Change (Nature Climate Change).
Antarctic bottom waters move trillions of tons of cold, dense, oxygen-rich water from the continental shelf to the ocean depths, behaving like a vast underwater waterfall.
All ocean currents are interconnected. Nature.
From there, the Antarctic bottom water spreads northward through the Pacific, Indian, and Atlantic basins, hugging the seafloor and circulating in deep currents before gradually resurfacing thousands of kilometers away.
Just as lungs push oxygen into the blood, Antarctic bottom waters redistribute heat, carbon, oxygen, and nutrients around the globe. Their proper functioning helps stabilize climate, sustains deep-sea ecosystems, and they act as a key conduit for oxygen reaching the deepest zones.
The consequences of this shift
The slowdown carries multiple consequences. It can disrupt the climate system, shrink habitable space for marine life, and may limit the ocean’s capacity to absorb carbon, with ripple effects for ongoing climate change.
Scientists had warned this outcome was possible under climate change, but the pace at which it could unfold startled many observers.
“The slowdown could sever the link between the Antarctic coast and the ocean depths,” the study authors noted.
Already reduced by 30 percent
Data show roughly a one-third decline in the circulation of this current over the past three decades. The slowdown already has tangible effects, with deeper oxygen levels dropping in ways models had anticipated.
Results influence the entire planet, as agencies stress.
The underlying cause lies in regional warming and ice melt tied to climate change. Meltwater alters surface temperatures and water density, making colder waters less prone to sink and thus slowing deep circulation. In effect, the deep, oxygen-rich layer is being replaced by warmer water with lower oxygen levels.
Without oxygen and with more CO2
The oxygen deficit affects all marine life. While some deep-sea species tolerate low oxygen, a sustained shortage reduces habitable zones and may force species to adapt, migrate, or perish.
The consequences extend beyond the Southern Ocean. If the current further weakens or stalls, it could feed back into global climate dynamics.
In addition to transporting heat and oxygen, the current carries carbon dioxide from the depths. There, CO2 is stored; as the ocean’s storage capacity declines, more CO2 remains near the surface, potentially accelerating warming.
Antarctic ice loss is expected to continue as the planet warms. Some researchers indicate a high likelihood of crossing the 1.5°C global warming threshold by 2027, based on the study’s signatories.
More ice loss means altered surface cooling and, in turn, less mixing of surface waters. The slowdown of this current, and its knock-on effects, could intensify these changes.
Researchers emphasize that the consequences of slowing will not be confined to Antarctica. A disruption of this circulation touches the entire global ocean, and a broader collapse could worsen the climate crisis.
Connection: Nature Climate Change, s41558-023-01667-8.
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The broader response from researchers highlights the need to monitor how rapidly warming affects ocean circulation and to understand potential feedbacks to climate, sea level, and marine life. The findings underscore the interconnected nature of global systems and the importance of preserving ocean health in the face of change.