Antarctica paves the way for the largest waterfall in the world. The action takes place below the surface of the ocean. Here, trillions of tons of cold, dense, oxygen-rich waters gush from the continental shelf and plunge to great depths. This Antarctic “bottom water” then spreads north along the sea floor in deep ocean currents, before slowly rising, thousands of kilometers away.
In this way, Antarctica drives a global network of ocean currents called the “cyclic upwelling” that redistributes heat, carbon, and nutrients around the globe. This inversion is crucial to maintaining the stability of Earth’s climate. It’s also the main way oxygen reaches the ocean depths.
But there are signs that this circulation is slowing and is happening decades earlier than expected. This slowdown has the potential to disrupt the connection between the Antarctic coasts and the deep ocean, with catastrophic consequences for Earth’s climate, sea level, and marine life.
our New searchpublished today in the journal The nature of climate change, uses real-world observations to decipher how and why the ocean depths around Antarctica have changed over the past three decades. Our measurements show that the overturning circulation has slowed by about a third (30%) and oxygen levels in the deep ocean are declining. This happens even earlier than climate models and expect.
We found that melting Antarctic ice disrupts the formation of Antarctic bottom waters. Melt water makes Antarctic surface waters clearer and less dense, and thus less likely to sink. This puts the brakes on the circulatory coup.
Why is this important?
As bottom water flow slows, oxygen supply to the ocean depths decreases. The shrinking, oxygen-rich lower water layer is then replaced by warmer water with less oxygen, lowering the oxygen levels.
Ocean animals, large and small, respond to even small changes in oxygen. Deep ocean animals are adapted to low oxygen conditions but still have to breathe. Loss of oxygen may prompt them to seek refuge in other areas or to adapt their behaviour. Models indicate that we are Imprisoned This results in a shrinking of the ‘viable’ environment available to these animals with an expected decrease of up to 25%.
Deceleration may also be in overturning Intensification Global Warming. The inverted circulation transports carbon dioxide and heat to the ocean depths, where it is stored and hidden from the atmosphere. As the ocean’s storage capacity decreases, more carbon dioxide and heat is left in the atmosphere. This feedback accelerates global warming.
Signs of a worrying change
Making bottom water observations is challenging. The Southern Ocean is remote and home to the strongest winds and biggest waves on the planet. Access is also restricted by sea ice during the winter, when bottom waters form.
This means that observations about the depths of the Southern Ocean are sparse. However, repeated full-depth measurements taken from the ships’ voyages have provided glimpses of the changes underway in the ocean depths. bottom water layer It becomes warmer, less voluminous and thinner.
These signs point to a worrying change, but there are still no direct observations of deep inversion.
What have we done?
We collected different types of notes in a new way, taking advantage of each of their strengths.
Full-depth measurements collected by the ships provide snapshots of ocean density, but are usually repeated about once per decade. Anchored instruments, on the other hand, provide continuous measurements of density and velocity, but only for a limited time in a particular location.
We have developed a new approach that combines ship data, docking records and high-resolution digital simulations to calculate the strength of water flows at the bottom of the Antarctic and how much oxygen it carries to the ocean depths.
Our study focused on a deep basin south of Australia that receives bottom water from several sources. These sources are located downstream from large meltwater inputs, so this area is likely to provide early warning of climate changes in the deep ocean.
The results are amazing. Over three decades, between 1992 and 2017, the overturning circulation in this region slowed by about a third (30%) causing less oxygen to reach the depths. This slowdown was due to regeneration near Antarctica.
We found that this resuscitation reduces the density and volume of the water formed at the bottom of Antarctica, as well as the speed of its outflow.
The observed slowdown would have been greater if it were not for a short-lived weather event that triggered A Partial and temporary recovery of bottom water formation. The recovery, driven by increasing salinity, demonstrates the sensitivity of bottom water composition to salinity changes over the Antarctic continental shelf.
Worryingly, these observations show that changes It is expected to happen by 2050 Already underway.
Ice loss from Antarctica is expected to continue, and even accelerate, as the world warms. we It will almost certainly cross 1.5 ℃ global warming threshold by 2027.
More ice loss means more recovery, so we can expect circulation to slow and deep oxygen loss to continue.
The consequences of slowing down will not be limited to Antarctica. The overturning circulation extends throughout the global ocean and influences the pace of climate change and sea level rise. It would also be disabling and harmful to marine life.
Our research provides yet another reason to work harder – and faster – to reduce greenhouse gas emissions.
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