Antarctic alarm bells: observations reveal deep ocean...

时间:2023-05-27 10:45:00  热度:0°C

Antarctica sets the stage for the world’s greatest waterfall/ The action takes place beneath the surface of the ocean/ Here/ trillions of tonnes of cold/ dense/ oxygen-rich water cascade off the continental shelf and sink to great depths/ This Antarctic “bottom water” then spreads north along the sea floor in deep ocean currents/ before slowly rising/ thousands of kilometres away/

In this way/ Antarctica drives a global network of ocean currents called the “overturning circulation” that redistributes heat/ carbon and nutrients around the globe/ The overturning is crucial to keeping Earth’s climate stable/ It’s also the main way oxygen reaches the deep ocean/

But there are signs this circulation is slowing down and it’s happening decades earlier than predicted/ This slowdown has the potential to disrupt the connection between the Antarctic coasts and the deep ocean/ with profound consequences for Earth’s climate/ sea level and marine life/

Our new research/ published today in the journal Nature Climate Change/ uses real-world observations to decipher how and why the deep ocean around Antarctica has changed over the past three decades/ Our measurements show the overturning circulation has slowed by almost a third (30%) and deep ocean oxygen levels are declining/ This is happening even earlier than climate models predicted/

We found melting of Antarctic ice is disrupting the formation of Antarctic bottom water/ The meltwater makes Antarctic surface waters fresher/ less dense/ and therefore less likely to sink/ This puts the brakes on the overturning circulation/

Now that’s a waterfall/ dense water flowing from the continental shelf into the deep ocean in the Ross Sea/ Consortium for Ocean-Sea Ice Modelling in Australia (COSIMA) and National Computational Infrastructure/

Read more/ Torrents of Antarctic meltwater are slowing the currents that drive our vital ocean overturning – and threaten its collapse

Why does this matter?

As the flow of bottom water slows/ the supply of oxygen to the deep ocean declines/ The shrinking oxygen-rich bottom water layer is then replaced by warmer waters that are lower in oxygen/ further reducing 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/ Losses of oxygen may cause them to seek refuge in other regions or adapt their behaviour/ Models suggest we are locked in to a contraction of the “viable” environment available to these animals with an expected decline of up to 25%/

Slowdown of the overturning may also intensify global warming/ The overturning circulation carries carbon dioxide and heat to the deep ocean/ where it is stored and hidden from the atmosphere/ As the ocean storage capacity is reduced/ more carbon dioxide and heat are left in the atmosphere/ This feedback accelerates global warming/

Reductions in the amount of Antarctic bottom water reaching the ocean floor also increases sea levels because the warmer water that replaces it takes up more space (thermal expansion)/

Freshening of shelf waters reduces the flow of dense water and slows the deepest parts of the overturning circulation while also reducing deep oxygenation/ Kathy Gunn/ Author provided

Signs of a worrying change

Making observations of bottom water 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 winter/ when bottom water forms/

This means observations of the deep Southern Ocean are sparse/ Nevertheless/ repeated full-depth measurements taken from ship voyages have provided glimpses into the changes underway in the deep ocean/ The bottom water layer is getting warmer/ less dense and thinner/

Satellite data shows the Antarctic ice sheet is shrinking/ Ocean measurements taken downstream of regions of rapid melt show the meltwater is reducing the salinity (and density) of coastal waters/

Antarctic ice mass loss over the last few decades based on satellite data/ showing that between 2002 and 2020/ Antarctica shed an average of ~150 billion metric tonnes of ice per year/ adding meltwater to the ocean and raising sea-levels (Source/ NASA)/

Read more/ Antarctica s heart of ice has skipped a beat/ Time to take our medicine

These signs point to a worrying change/ but there are still no direct observations of the deep overturning circulation/

What did we do?

We combined different types of observations in a new way/ taking advantage of each of their strengths/

The full-depth measurements collected by ships provide snapshots of ocean density/ but are usually repeated about once a decade/ Moored instruments/ on the other hand/ provide continuous measurements of density and speed/ but only for a limited time at a particular location/

We developed a new approach that combines ship data/ mooring records/ and a high resolution numerical simulation to calculate the strength of Antarctic bottom water flow and how much oxygen it transports to the deep ocean/

Our study focused on a deep basin south of Australia that receives bottom water from several sources/ These sources lie downstream of large meltwater inputs/ so this region is likely to provide an early warning of climate-induced deep ocean changes/

The findings are striking/ Over three decades/ between 1992 and 2017/ the overturning circulation of this region slowed by almost a third (30%) causing less oxygen to reach the deep/ This slowing was caused by freshening close to Antarctica/

We found this freshening reduces the density and volume of Antarctic bottom water formed/ as well as the speed at which it flows/

The observed slowdown would have been even greater if not for a short-lived climate event that drove a partial and temporary recovery of bottom water formation/ The recovery/ driven by increased salinity/ further illustrates the sensitivity of bottom water formation to salinity changes on the Antarctic continental shelf/

Worryingly/ these observations show that changes predicted to occur by 2050 are already underway/

Abyssal ocean warming driven by Antarctic overturning slowdown/ Credit/ Matthew England and Qian Li/

What next?

Ice loss from Antarctica is expected to continue/ even accelerate/ as the world warms/ We are almost certain to cross the 1/5℃ global warming threshold by 2027/

More ice loss will mean more freshening/ so we can anticipate the slowdown in circulation and deep oxygen losses will continue/

The consequences of a slowdown 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 will also be disruptive and damaging for marine life/

Our research provides yet another reason to work harder – and faster – to reduce greenhouse gas emissions/

Read more/ Record low Antarctic sea ice is another alarming sign the ocean s role as climate regulator is changing

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