DEEP TROUBLE
How climate change has reached the world’s end
Words: Jamie Morton
Editor: Andrew Laxon
Design and graphics: Paul Slater and Ben Cummins
Scientists have observed startling changes in the vast and wild ocean beneath New Zealand and issued an urgent call to slow the warming of our planet. JAMIE MORTON explains why these dramatic shifts in our deep south should worry all of us.
Professor James Renwick. Photo / Colin McDiarmid
Professor James Renwick. Photo / Colin McDiarmid
You’re Professor James Renwick. You’re a climate scientist who’s spent most of his decades-long career watching the single largest annual change on our planet. You’ve seen the seasonal swelling and shrinking of Antarctica’s sea ice, as if breathed in and out from a gargantuan pair of lungs, from every angle a scientist can.
From a constant trickle of peer-reviewed research papers.
From the satellite data you download each morning to your Victoria University office.
From the ice itself nearly 20 years ago, when you camped out on it amid white-out blizzards.
You’ve become attuned to its ebbs and flows.
How, in the pitch-black months of the polar winter, it balloons outward to blanket a vast expanse of ocean.
You’ve seen those small anomalies – those years when that extent might be two or three per cent more or less than the baseline – and you’ve seen the bigger ones.
Each time, you’ve tried to work out why – and natural drivers like shifts in wind or climate regimes have usually offered the answer.
But here in 2023, there doesn’t seem to be such a reassuring explanation for what’s on your computer screen.
In what’s been considered a one-in-7.4 million statistical aberration – an event so freakish and alarming that it’s stoked a chorus of concern from climate scientists – the Southern Ocean is suddenly missing about 10 New Zealand’s worth of sea ice.
While more data is needed to be sure the dramatic drop-off isn't just some random outlier, scientists suspect our own planet-heating pollution is to blame.
Some have even suggested two words that none of us want to hear: tipping point.
“We’ve been getting a fairly good handle on what’s happening with Antarctic sea ice, but what's happened this year? Different kettle of fish,” Renwick says.
“Personally, I think this must be a sign of the warming of our climate finally making it to the Antarctic ocean.”
THE WORLD’S END
For those who’ve never ventured into it, the beauty and hostility of this wildest, windiest, most remote corner of the world is difficult to imagine.
Mammoth swells, built by westerly flows that roar over endless expanses of water, crash down upon the bows of large ships with terrifying force.
In 2017, just a few years after the HMNZS Wellington was forced to turn back en route to the subantarctic islands, its 1900-tonne sister ship Otago came close to capsizing amid 20m-high waves.
As one Ocean Race veteran put it, this isn’t a place where humans belong – yet most of us have no idea how much we all depend on it to keep our planet liveable.
That’s come to be appreciated since early explorers first ventured into its depths in search of a Terra Australis: a mythical land mass thought to have existed since the time of the Roman Empire.
One of them, Captain James Cook, instead met only sea ice.
But he correctly hypothesised an Antarctic land lay beyond, and that waters encompassed the entire southern latitudes.
Māori voyagers similarly made their way south to find a frozen world deep below Aotearoa.
“We as Māori have narratives, or what we call kōrero tuku iho, that speak to Māori and Polynesian voyages across the Pacific arriving into the southern oceans,” says Waikato University’s Associate Professor Sandy Morrison.
That of course didn’t mean they reached the place that we now call Antarctica, Morrison says, but there’s certainly evidence to show they’d journeyed into a land of ice.
Ngāti Rārua, and Te Āti Awa from Te Tau Ihu o te Waka a Māui, called it Te Tai Uka a Pia - translating to “sea covered with foam like arrowroot”.
Waikato University Associate Professor, Sandy Morrison, Photo / Ben Fraser
Waikato University Associate Professor, Sandy Morrison, Photo / Ben Fraser
“We as Māori have narratives, or what we call kōrero tuku iho, that speak to Māori and Polynesian voyages across the Pacific arriving into the southern oceans”
Centuries later, there’s still debate whether a title for this Southern Ocean – only officially recognised by the world two years ago – is even needed.
Some descriptions chart its northern boundary as lying as far beneath us as 60 degrees south latitude, while others place it as high as even 40 or 30 degrees south.
Niwa oceanographer Dr Craig Stevens. Photo / Aitana Forcen Vazquez
Niwa oceanographer Dr Craig Stevens. Photo / Aitana Forcen Vazquez
Niwa oceanographer Dr Craig Stevens, who camped out with Renwick back on that trip to the ice, would tell you he can gaze at it from his Wellington office - if he leans hard enough out the window.
Wherever one draws the boundaries, he says, it’s clear it’s unique in connecting all the world’s great ocean basins.
As the planet’s only band of water to run a full longitudinal circuit, it acts like a gigantic oceanic roundabout, linking together the Atlantic, Indian and Pacific.
South to north, the Southern Ocean spans from the white reaches of Antarctica to the Subtropical Front - an invisible boundary that flows around the bottom of the South Island, where water from the tropics and the polar region collide.
It plays a critical role as the interface between Antarctica’s vast ice sheet - and the nearly 60m of equivalent global sea level rise stored within it - and the rest of the globe.
This critical relationship goes two ways: Antarctica’s state fundamentally depends on how it interacts with the Southern Ocean’s gradually warming water and, in turn, the natural regulation of our ocean system is profoundly influenced by the continent.
Take the way its polar winds roar outward to freeze the ocean’s cold and salty surface water – as much as 250 trillion tonnes of it each year.
The water drops down to the ocean floor, taking with it vast amounts of oxygen and carbon dioxide.
From here, it’s slowly spread across all the Earth’s basins, ending up mixed in with surface water in far-flung corners of the globe, centuries later.
“This is critical as it ventilates the deep ocean with oxygen, locks away some of the CO2 we’ve added to the atmosphere, and supplies nutrients to the rest of the ocean,” Stevens says.
It’s estimated the Southern Ocean has already swallowed into its dense water an incredible 40 per cent of the CO2 we’ve produced.
In short, it’s been helping save us from ourselves.
Coming to understand these large-scale functions, and how they’ve shifted in step with Earth’s climate over millions of years, hasn’t been simple for scientists.
They typically reconstruct past changes in oceans using ancient records, like those stored in the remains of tiny organisms that once lived near the surface millions of years ago.
But the Southern Ocean is a relatively young body of water.
It’s estimated to have formed only 30 million years ago – compare that with the Atlantic’s 180-million-year existence – and its history has been shaped by the coming and going of ice ages.
In the most recent one 20,000 years ago, when much more ocean water was pent up in ice caps, sea levels were so low that one could’ve walked between the main islands of Aotearoa.
“This is the challenge for climate science, in that so much of what we know comes from records that stretch far back in time,” Stevens says.
In trying to understand the ocean’s modern era, scientists are more focused on what we can learn from its turbulence and the movement of its tides.
These secrets aren’t yielded easily.
“Modelling teams work to provide useful information on the future by finding the best blend of processes that are complex enough to have the right things going on, but not so intense that the computers cannot do the work fast enough,” Stevens says.
Staggering leaps have come with powerful new computer models, advances in satellite observation and insights from the global Argo network’s fleet of drifting data-harvesting robots.
The latest generation, called Deep Argo floats, dive thousands of metres below the surface, collecting a trove of fresh information about temperature, salinity, oxygen and nutrients.
“A decade ago, we were relying on sparse ship measurements perhaps a decade or more apart,” Stevens says.
“Now, there are many hundreds of data profiles appearing online every 10 days, just for the Southern Ocean.”
A CHANGING OCEAN
All of this has been confirming several worrying things about what’s happening to the ocean. Studies drawing on billions of satellite observations show it’s growing only wilder, with its extreme winds having strengthened by 1.5m a second in three decades, and its extreme waves having climbed by an average 30cm.
Largely thanks to us, it’s becoming more acidic.
“Ocean acidification results from the extra CO2 we’ve dumped in the atmosphere, which dissolves in surface waters where it produces acid,” Niwa principal scientist Dr Cliff Law says.
“In the past, natural processes in the surface ocean have neutralised this acid, but now these can’t keep up with the rate we are producing CO2.”
We often think of ocean acidification as a change in pH levels - the lower the value, the higher the acidity – but there’s much more to the picture.
It also means a drop in the dissolved carbonate that many marine organisms use to build their shells and skeletons - along with a rise in dissolved CO2, which can benefit things that photo-synthesise, like phytoplankton and seagrasses.
Plankton. Photos / Tara Expeditions
Plankton. Photos / Tara Expeditions
“This could see more seaweed and less kina, paua and pipis on our beaches,” Law says.
“It’s now recognised that the larval stages of these organisms are the weak spot; they have to grow rapidly but will struggle to produce their carbonate shells as the ocean acidifies.”
Around our own coasts, waters have acidified by much as 8.6 per cent within just the last three decades.
“Considering the increase over the last 250 years since we started driving motor cars was only around 30 per cent, it’s clear this rate is increasing,” Law says.
“What happens in the future depends on what we do now; acidification won’t stop, but cutting our emissions will slow the rate and give marine organisms more time to adapt.”
With this acidifying, of course, has come warming, with that slab of ocean between 500m and 2000m heating by roughly 0.0020C every year.
That might sound marginal, but incremental shifts can have big consequences.
“What happens in the future depends on what we do now; acidification won’t stop, but cutting our emissions will slow the rate and give marine organisms more time to adapt.”
Ocean layers are set apart by how much heat and salt they contain, and it doesn’t take much change to reconfigure how they slide past one another - or how oxygen and nutrients move through the ocean basins.
As this water is transported around the planet by the global ocean conveyor, all the absorbed heat is eventually released back into the atmosphere, until our climate reaches a new equilibrium state.
Even if our soaring global carbon emissions magically ceased today, the heat we’ve already put into our oceans has now committed our descendants to a long future of warming.
Those studies about the Southern Ocean growing windier matter deeply here.
By bringing deep, carbon-rich waters up to the surface, the same westerly flows that help regulate the ocean’s storing capacity are now threatening its future as a giant CO2 bank.
MELTING ICE, RISING TIDES
Satellite observations are showing this warming water taking giant bites out of Antarctica’s land ice, of which an incredible 150 billion tonnes is now entering the ocean each year.
“Most of the ice mass loss to date is from the edges of the Antarctic continent where land ice flows into and onto the ocean forming large floating ice shelves and ice cliffs,” Victoria University glaciologist Professor Tim Naish says.
“As the Southern Ocean continues to take up heat and warm, and as wind patterns change, more and more warm ocean water is upwelling and melting the ice shelves and the margins of the ice sheet.
“This causes the edge of the ice sheet to retreat inland, and the ice sheet deflates as it thins around the edges.”
While large parts of the sheet are still buffered by vast, wide shelves, that’s not the case around the Amundsen Sea, where Antarctica’s fastest melting is unfolding.
The irreversible retreat in this area spells unavoidable, multi-metre sea level rise over coming centuries.
“If humans keep global warming below 2C, in line with the Paris climate target, then we will still get 50cm of sea-level rise by the end of the century – this cannot be avoided.”
Even by the end of this one, Antarctica may have contributed between 5cm and 45cm of the 50cm to two metres of total global sea level rise projected by the UN’s latest major science stocktake.
The uncertainty in these ranges doesn’t just reflect the fact scientists are still racing to understand how the ice sheet will respond, but that it isn’t clear whether the world will decarbonise fast enough.
“If humans keep global warming below 2C, in line with the Paris climate target, then we will still get 50cm of sea-level rise by the end of the century – this cannot be avoided,” Naish says.
“But if certain processes play out, such as rapid ice shelf disintegration, the runaway retreat of West Antarctic glaciers and domino-style ice cliff collapse, then we could have up to 2m.”
An even more frightening scenario - assuming a worst-case scenario where global heating climbs as high as 5C - is the 19m to 22m of sea level rise that could occur by the year 2300.
That’d be enough to put cities, and some entire countries, underwater.
Such worries were recently underscored by a study indicating that ocean warming in the West Antarctic has already locked in at about triple the historical rate of melting.
That means that even heroic efforts to slash emissions may only have limited power in slowing warming in the Amundsen Sea over coming decades.
In that time, Kiwis can expect to see impacts start really hitting home.
Storm surges brought flooding to Tamaki Drive, Auckland. Photo / Jason Oxenham
Storm surges brought flooding to Tamaki Drive, Auckland. Photo / Jason Oxenham
For many parts of our coast, 30cm of sea level rise is considered a threshold for extreme flooding - above which a 100-year coastal storm becomes an annual event.
That could be crossed as early as 2040, by which time, many exposed homes will likely have already become uninsurable.
By the turn of the century, seas in populated places like Petone or Auckland’s Viaduct Harbour might have risen by more than 80cm - threatening billions of dollars of property and infrastructure with more frequent flooding.
Scientists further point out that today’s changes in the Southern Ocean won’t just make coastal storms more frequent – but much worse.
Flooding in Petone, Wellington. Photo / Mark Mitchell
Flooding in Petone, Wellington. Photo / Mark Mitchell
Across the latitudes that New Zealand occupies, climate change-driven shifts will equate to stronger gradients, more energy and more propensity for the atmosphere to take up moisture from the ocean.
“New Zealand is therefore in the firing line of a more energetic ocean-atmosphere system, capable of delivering more intense storm and rain events, with increasing frequency,” Niwa Antarctic oceanographer Dr Natalie Robinson explains.
On top of this melting, warming, rising and acidifying, scientists are tracking another alarming trend: the ocean is losing its breath.
Measured global oxygen levels have fallen by about two per cent since the 1950s, with rising temperatures increasingly limiting our oceans’ ability to suck O2 in from the atmosphere, Niwa physical oceanographer Denise Fernandez says.
“New Zealand is therefore in the firing line of a more energetic ocean-atmosphere system, capable of delivering more intense storm and rain events, with increasing frequency.”
“The decline in ocean oxygen content, known as ocean deoxygenation, is also coupled with nutrient pollution from sewages and river discharges which exacerbates plant growth causing further oxygen loss.”
Because around half of Earth’s oxygen is produced in the ocean, a decline in oxygen content – and it may ultimately tumble by up to four per cent by the end of this century - will threaten marine life, ecosystems and the resources we reap from our big blue backyard.
Within the Southern Ocean itself, scientists still have a poor understanding of the mechanics driving this deoxygenation, and, critically, what this means for ventilating the depths of the global ocean.
THE DAY AFTER TOMORROW
Those of us who saw the 2004 apocalyptic blockbuster The Day After Tomorrow might recall its CGI super-storms, the flash-freezing of Manhattan and a young Jake Gyllenhaal.
Critical as climatologists remain about the scientific accuracy of its premise – a global warming-driven collapse in ocean circulation triggering an instant ice age – it did shine some much-needed light on the importance of the Earth’s major currents.
And in the aftermath of one particularly troubling study projecting a dramatic slowing of the current that swirls around Antarctica, many media reports reached for that Hollywood reference.
Powered by the Southern Ocean’s strong westerly winds, the Antarctic Circumpolar Current is the Earth’s strongest, longest surface oceanic current.
On a map, it can appear as someone’s roughly ringed Antarctica with a marker pen to illustrate a single, steady, continuous flow.
Rather, this current is very much a sum of its parts: an endless number of whirling, small-scale eddies, like rapids in a river, which continuously ferry water and energy around the continent.
In the space of just a decade, scientists have developed an entirely new appreciation of its immense complexities – all of which remain key to answering many of oceanographers’ biggest questions about climate change.
To Niwa’s Dr Erik Behrens, one of the most pressing is how much more excess heat the Southern Ocean can absorb before Antarctica loses the protective buffer that this current and its eddies form.
Behrens describes the picture as a fine balancing act between winds, meltwater, surface heating and how all of these elements interact with the current’s smallest eddies.
While the Southern Ocean’s big winds and storms churn up water to help take up heat, he says that benefit is being increasingly countered as Antarctica’s ice sheets and shelves melt faster than ever before.
Sitting in the middle of this balance are mesoscale eddies, which take up energy from accelerating winds and carry icebergs and meltwater around.
Photo / 123RF
Photo / 123RF
Using high-resolution models, Behrens and his global counterparts have turned to these eddies to build a clearer picture of what future shifts in this ongoing push-and-pull process will mean for deep ocean currents.
Each new study has brought more concerning insights: and one recent landmark paper in the journal Nature offered a particularly dire prognosis.
It involved what’s called overturning, where dense surface water at high latitudes (closer to the North and South poles) descends into the deep ocean, generating a flow ultimately connecting every ocean.
If global emissions kept their course, it found, then overturning of Antarctica’s dense, cold, salty bottom water could slow by more than 40 per cent in just the next 30 years.
That could put it on a pathway to potential collapse – contributing to those rising sea levels, changed weather patterns and loss of vital nutrients for marine ecosystems.
Even the physical oceanographer who led the study, the University of New South Wales’ Professor Matthew England, was taken aback by the modelling results.
“It dawned on me that the very textbook description of the ocean circulation that I had learnt about as an undergraduate is set to be forever changed, as we perturb these giant ocean circulation patterns with a rapidly changing climate,” he says.
“In the very worst case, he says, the bottom layer of the ocean may become stagnant, without any deep circulation - much like the isolated depths of an anoxic lake.”
“Scientifically, it was an exciting discovery, because there are fascinating ocean dynamics at play - but when we put aside our oceanographic interests and thought about changes to Earth’s climate, we were confronted by the pace of projected change.”
To appreciate the significance of this change it's important to understand what overturning in the Southern Ocean actually is.
“It’s part of the global overturning, which connects deep and surface circulation in all ocean basins with each other, like a conveyor belt and regulating the earth’s climate,” Behrens explains.
“Changes to this part of the conveyor belt will have consequences for the other parts, as they depend on each other and influence how heat and nutrients are exchanged between the tropics and higher latitudes.”
In the very worst case, he says, the bottom layer of the ocean may become stagnant, without any deep circulation - much like the isolated depths of an oxygen-depleted lake.
Behrens lays out the cascading, compounding effects in play.
More meltwater reduces the density of the ocean’s surface, making it more stratified.
Strong winds are then less able to stir the ocean up and form dense water, the key driver of over-turning.
Instead of sinking to the bottom of the ocean, this water moves around at mid-depth, effectively isolating the deepest, coldest layer.
“This would dramatically change the entire climate, since the cooling effect of the ocean would be largely reduced, and we’d feel the full impact of increasing greenhouse gas emissions.”
AN AGE OF EXTREMES
Even today, there’s no shortage of signs that this fragile part of the world is in serious trouble. Renwick’s worries about this season’s striking lack of sea ice are backed by a major new study warning that a regime shift may have already begun, permanently leaving the ocean with lower coverage.
One its authors, Dr Edward Doddridge of the University of Tasmania, says there used to be an annual “reset” in sea ice, where the ocean’s memory was effectively wiped every winter, with no discernible relationship between the summer minimum and winter maximum.
But since 2016, that pattern appears to have broken down to a point from which the ice won’t be able to recover – an eventuality long predicted by climate models, now becoming reality.
The importance of this sea ice can’t be overstated: it helps regulate the Earth’s climate by reflecting sunlight back into space, drives ocean circulation and shields ice shelves.
It’s also critical to a plethora of species, from microscopic organisms to Antarctica’s most famous resident, the emperor penguin, most of whose colonies could become virtually extinct this century.
Three emperor penguins pose for photographs on the McMurdo Sound sea ice, Antarctica. Photo / Mark Mitchell
Three emperor penguins pose for photographs on the McMurdo Sound sea ice, Antarctica. Photo / Mark Mitchell
Even discounting what’s been missing at sea, it’s estimated the continent lost about 2,100 gigatonnes of glacial ice last year, most of it coming from a rapidly retreating West Antarctic Ice Sheet.
Another recent study laid out plenty more extreme occurrences, including a moisture-laden “atmospheric river” that last year led to record temperatures in central East Antarctica.
All these observations – and the disturbing likelihood that the picture is even more grave than our best science can tell us – underscore why local scientists just mounted an emergency summit in Wellington.
One of them, Stevens, was also among 300 international experts who recently sounded a call for more science in our deep south.
“From the perspective of a modest island economy close to the Southern Ocean, we must sustain our observations of it,” he says.
“If we don’t, we risk having far less warning of future changes that will be felt throughout the globe.”
Antarctica New Zealand’s chief scientific advisor, Professor Jordy Hendrikx, credits our own experts for having shed invaluable light on the picture.
“We are supporting science that has, amongst many other discoveries, observed and improved our understanding of ocean currents, improved our understanding and population dynamics of marine species and documented Antarctic sea ice extent at new lows,” he says.
“We’re also supporting ambitious work to drill into sediment below the Ross Ice Shelf to understand what happened in our geological past when global temperatures resembled what we are expecting to witness in the coming decades with climate change.”
Still, he acknowledges the Southern Ocean remains “chronically under-sampled” - and therefore exceedingly challenging to understand and manage.
For the rest of us, the scientists’ message is just as it was after the Northern Hemisphere’s hellish summer, and a July that went down as the planet’s hottest month in 120,000 years.
Only rapid decarbonisation can change those projections that’ve come to so worry experts like Renwick and Naish.
England laments how his team’s own startling discovery caused a stir when their study hit print – yet new oil and gas drills continue to be green-lit around the world.
“The right response to the scientific results coming out at the moment, not just from our study but also from a suite of new publications and recent observations, should be an urgent end to fossil fuel extraction,” he says.
“We have the means to solve this problem, what we need now is action from business, government, and industry.”
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