Friday, September 18, 2026

Keeping an eye on orcas : Portuguese Navy launches official warning map

Photo: gettyimages


From Yacht by Ursula Meer
 
An orca and a boat: once an awe-inspiring sight, now a frightening one.
To help sailors on the Iberian Peninsula plan their routes, the Portuguese Navy has therefore now published a map showing sightings and attacks. 

A new official Orca map produced by the Portuguese Navy is designed to help sailors plan their routes. It brings together sightings and attacks along the Portuguese coast.
However, when looking at the figures in particular, it remains true that the maps provide a valuable overview of the situation, not a comprehensive set of statistics

On 8 September 2026, the Instituto Hidrográfico, the Portuguese Navy’s Hydrographic Institute, launched an online platform for orca sightings.
The Official Orca Portal displays reported sightings and interactions on maps, highlights nautical warnings and accepts new sighting or attack reports via an online form.
Anyone who registers can also receive notifications by email.
 

 
For sailors, this is more than just another map.
For the first time, a Portuguese authority is bringing all this information together in an official service. However, it does not replace your own route planning, nor does it replace ongoing warnings or radio contact whilst at sea. 
What the new chart offers, where its limitations lie and how it fits into the existing information landscape.

The eighth sunken boat

​On 23 July 2026, the sailing yacht "Idem" sank off Estaca de Bares.
The orcas had damaged the rudder so badly that water poured in and the boat was beyond saving.
The two sailors on board were rescued by a nearby motorboat – unharmed, but without their boat.
It is the latest in a series of eight documented total losses since 2022.

Incidents such as this one demonstrate why reliable real-time information is no mere academic exercise for sailors in Orca Alley.
This makes it all the more significant that the Instituto Hidrográfico, the Portuguese Navy’s Hydrographic Institute, launched a platform for orca incidents on 8 September 2026.
The official Orca portal displays reported sightings and interactions on maps, provides links to nautical warnings and accepts new reports of sightings or attacks via an online form.
Anyone who registers can also receive email notifications.

For sailors, this is more than just another chart.
For the first time, a Portuguese authority has brought all this information together in an official publication.
However, it does not replace your own route planning, nor does it replace ongoing warnings or radio contact whilst at sea.

What's happening right now

​Anyone scrolling through the Facebook pages of the relevant orca groups these days will quickly get a sense of the scale of the problem.
Between 1 August and 7 September alone, attacks were reported: twice off Huelva, once off Tarifa, near Setúbal, off Laxe, Vigo, Cabo de São Vicente and Penedo da Saudade.
In between, there have been repeated sightings – and reports of crews who escaped unharmed.

One of these reports comes from sailor Elin Power.
In the ‘Orca Attack Reports’ group, she wrote about an attack that lasted around 10 to 20 minutes and involved more than ten strikes against the boat.
She said she had followed the standard procedures: engine off, steering released, pots banged against steel parts.
Then the orcas abruptly swam off.
Her conclusion: “I think we had a lucky day.”
The rudder was intact, and no water had got in.
A nearby motorboat skipper had stayed close by – that gesture alone had given her a sense of security.

At the same time, sailors repeatedly report harmless or even no sightings of orcas at all.
This shows that an encounter is possible, but not inevitable.

Official map and private initiative

​Anyone wishing to sail safely through this dangerous area should avoid any encounter in the first place. That is why various organisations have been collecting data for years on the orcas and their routes along the Iberian Peninsula – some for scientific purposes, others to protect sailors.
One of the most important private providers in this regard in recent years has been Rui Alves with his website orcas.pt.

His website collects reports from a large network of sailors, observation boats and other contacts, and publishes the latest events, including the location, time and classification as a sighting or incident.
In response to a query from YACHT, Alves explains how he goes about his work: every report is checked individually.
He interviews skippers in person, compares reports from crews who were in the area on the same day, and requests photographs. 
Identifiable orcas are matched to individual animals using photographs in order to understand their movement patterns. 
“This allows me to verify 80 to 90 per cent of cases,” he says.

But Alves takes it a step further: every day, he posts movement forecasts for well-known orca pods on Facebook.
These are based on the last reported position, combined with an assumed average speed of two to three knots.
This makes it possible to estimate when a group might pass a particular stretch of coastline – not real-time tracking, but a concrete planning guide.
These alerts are also sent to his Telegram community, which now comprises more than 3,000 sailors.

The Instituto Hidrográfico explains that its mapping module also displays events that extend beyond orcaiberica.org and reported to orcas.pt. 
Orcaiberica.org is run by the Grupo de Trabajo Orca Atlántica (GTOA), a Spanish network of scientists which has been collecting data since the incidents began and publishes monthly interaction maps.

Rui Alves describes the collaboration in more detail in response to a query from YACHT: According to him, he regularly sends the Instituto Hidrográfico – albeit with a slight delay – all sightings and interactions collected on orcas.pt. 
The institute presents this data and supplements it with its own reports. 
“The Institute uses my data as a basis and also supplements it with its own reports,” writes Alves.

This means that the official map is not a completely separate second data source.
It is a consolidated representation of official warnings, reports from orcas.pt, information from orcaiberica.org and other data.
The Instituto Hidrográfico expressly points out that an event may appear multiple times due to the consolidation of external sources.
Time discrepancies are also possible, as Alves, according to his own statement, does not transmit his data without delay.
In terms of interpretation, this means that the map provides a broad, useful overview for route planning, but is not a finalised set of overall statistics covering all incidents.

​Not a real-time image in the strict sense

The Navy refers to updated, georeferenced information.
This is useful for route planning – but it is not a real-time picture in the technical sense.
Time can elapse between observation, reporting, plausibility checks and publication.
Furthermore, not all encounters are reported.

Then there is the question of terminology.
A sighting initially only indicates that orcas have been observed.
An interaction or attack describes contact with a boat.
Damage to the rudder, loss of manoeuvrability and a distress situation are yet further categories. Anyone looking at a map must therefore always check what the respective point actually documents.

This distinction is also important because the causes of the behaviour have not yet been conclusively established. Experts debate whether it is play, exploration, learnt behaviour or other explanations
A recently published scientific review also emphasises, that the well-known approaches explain various aspects of the phenomenon, but do not provide a definitive answer.

​In recent years, several organisations have established themselves around Orca Alley off the Iberian coast, collecting, analysing and disseminating sightings and incidents.
They adopt different approaches, collect different data – and do not always cooperate with one another.
An overview:
  • ​orcas.pt (Rui Alves, Portugal) The most comprehensive private platform featuring a real-time map, daily movement forecasts and a Telegram community of over 3,000 sailors.
Every report is checked individually: Alves interviews skippers in person, compares reports from the same area and requests photos.
The data is regularly passed on to the Instituto Hidrográfico.
Reports can be submitted via orcas.pt/orcasreport or directly via Telegram.
  • Hydrographic Institute (Portuguese Navy, since 8 September 2026) The new official portal brings together data from three sources: reports from the national maritime warning system ANAV, entries from orcaiberica.org and data from orcas.pt. It is therefore not an independent data collection effort, but rather a consolidated presentation – with the explicit note that duplicates may occur. Useful as an official overview of the Portuguese coastline, supplemented by an email alert service and a dedicated reporting form.
  • Cruising Association (CA, United Kingdom) The CA operates its own Orca information and reporting portal at theca.org.uk/orcas in four languages – English, French, Portuguese and Spanish.
The Orca project group is led by Paul Lingard.
The CA collects not only reports of attacks but also of uneventful encounters – a methodologically important distinction that makes comparisons possible in the first place. The data collected is shared with marine biologists and behavioural researchers.
The reports form the basis of an ‘Interaction Comments Library’, which categorises and evaluates various defence strategies.
In addition, the CA publishes historical comparative statistics by year, month and coastal section.
The GTOA publishes monthly interaction charts providing a geographical overview of Orca activities – including a traffic-light system for the individual sea areas.
The charts are an important basis for route planning, but are not considered to be complete, according to the organisation’s own assessment.
The GT Orca AIS app, which provides real-time data, is available as a supplementary resource.

Has 2026 been a particularly strong year so far?

The recent spate of reports suggests as much, but there is no definitive answer.
The most comprehensive publicly available comparison table of all orca incidents is provided by another organisation: the British Cruising Association.
It analyses the monthly reports from the Grupo de Trabajo Orca Atlántica.
These record a total of 54 interactions for the period from 1 January to 13 July 2026.
During the same period, there were 58 in 2025 and 83 in 2024.
On this basis, therefore, there is no question of this being an exceptionally high year.

However, this limitation is already stated in the source itself.
According to its own explanation, the historical CA analysis is the most comprehensive overview available; however, the underlying GTOA maps are not considered to be 100 per cent complete.
The figure 54 is therefore not an exact total of all encounters.
It is a comparative figure derived from a specific reporting chain.

At the same time, the latest reports suggest that it is too early to give the all-clear.
With ten attacks and nine sightings, it is one of the busiest months recorded on the map since the earliest period shown, from July 2022.
It is thought that three different orca groups may currently be active at the same time.
This indicates a lively late-summer period, but does not allow for a reliable extrapolation to the whole season.

Links :
  • Book recommendation : Yacht accidents Jan-Erik Kruse categorises typical risks on sailing yachts based on analysed accident reports. When sailing through an orca-inhabited area, it is particularly helpful to consider preparation and the ability to take appropriate action: the book covers distress signals, AIS and radar, as well as risk management and the loss of the rudder and keel. It is no substitute for up-to-date local sailing information, but it does sharpen the focus on what the crew and skipper should clarify before a serious incident occurs.
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Thursday, September 17, 2026

The deepest dive ever made in American Waters



From World Atlas 
 
On May 7, 2019, Victor Vescovo and marine scientist Alan Jamieson descended into Sirena Deep aboard the submersible Limiting Factor.
The dive reached 10,714 meters, or 35,151 feet, inside the U.S. Exclusive Economic Zone associated with Guam.
Vescovo piloted the vehicle and Jamieson served as chief scientist.
They spent 176 minutes on the seafloor, filming, surveying, and collecting material.
Federal Mariana Trench management documents identify Sirena Deep as the deepest feature in the U.S.-associated Trench Unit.
The site sits far beyond Guam's 12-nautical-mile territorial sea.
For a science paper, the precise description is "deepest documented crewed dive within the U.S.
Exclusive Economic Zone." 
 
Sirena Deep Reached 35,151 Feet 
 
Illustration of the Mariana Trench, home to Sirena Deep, in the western Pacific.

The Five Deeps Expedition reached Sirena Deep during its Mariana Trench campaign in May 2019.
Expedition records give the May 7 descent a depth of 10,714 meters, with an uncertainty of about 10 meters.
The submersible carried Vescovo and Jamieson to the trench floor, where they remained for 176 minutes.
Their work began after a descent lasting several hours through the western Pacific water column.

The crew recorded video, examined rock exposures, searched for biological activity, and recovered samples.
Federal monument records later repeated the 35,151-foot depth and documented the first crewed descent to the site.
One recovered specimen was described as mantle-derived rock collected from the western slope of the Mariana Trench.
Expedition accounts also record geological and biological observations made during the bottom period.
The May 7 dive was the fifth Mariana mission conducted by Limiting Factor during that campaign.

Sirena Deep Sits Inside The U.S. EEZ 
 
 
The shoreline of Guam, the U.S. territory whose Exclusive Economic Zone contains Sirena Deep.

Sirena Deep lies south of Guam near 12 degrees north latitude and 144 degrees east longitude.
NOAA places it within the Mariana Trench Monument's Trench Unit, which follows U.S.
maritime boundaries around Guam and the Northern Mariana Islands.
An Exclusive Economic Zone can extend up to 200 nautical miles from a coastal baseline.
The United States exercises defined resource rights and jurisdiction within that zone.
Guam's territorial sea extends 12 nautical miles from its baseline.
Sirena Deep lies well offshore, inside the EEZ linked to Guam.
Federal management maps place the trench unit within those limits and identify Sirena Deep as its deepest feature.
The phrase "American waters" can cover several legal maritime zones.
In this case, "within the U.S. Exclusive Economic Zone" identifies the jurisdiction precisely.

Limiting Factor Was Engineered For 11,000 Meters 
 
 
The bathyscaphe Trieste, an earlier crewed submersible built for extreme ocean depths.

Limiting Factor is a two-person Triton 36000/2 submersible rated to 11,000 meters.
Its pressure hull is a titanium-alloy sphere with walls 90 millimeters thick.
Triton states that the sphere was machined to 99.933 percent of a true sphere.
The crew sits inside that compact metal pressure vessel during the entire descent.

A spherical hull spreads external pressure around the crew compartment.
The vehicle was pressure-tested to the equivalent of 14,000 meters.
Triton lists its normal endurance at 16 hours and its maximum operating depth at full ocean depth.
The design also uses syntactic foam for buoyancy.

High-definition cameras, wide-angle cameras, exterior lights, navigation equipment, and sampling systems are built into the submersible.
The equipment supported 176 minutes of work on the Sirena Deep floor.
Limiting Factor completed repeated hadal dives during the Five Deeps Expedition, using the same crew sphere across several ocean basins.

HOW FAR DOWN THE WATER GOES The deepest oceanic trenches on Earth
 
Pressure At The Bottom Approaches 108 Megapascals
 
 
A submersible in deep water, where hull design must withstand crushing pressure.

At 10,714 meters, the water column places immense force on a submersible hull.
A basic hydrostatic calculation using seawater density near 1,025 kilograms per cubic meter gives about 108 megapascals.
NOAA commonly describes pressure in the deepest Mariana Trench as roughly a thousand times atmospheric pressure at sea level.
The pressure acts over every exposed part of the vehicle, including viewports, penetrations, joints, and external equipment.

Precise depth estimates use pressure readings alongside water compressibility, temperature, salinity, local gravity, tides, and atmospheric pressure.
Sonar measurements also depend on the speed of sound through seawater.
Small uncertainties in those variables can move a calculated depth by several meters.
The reported Sirena Deep figure carries an uncertainty of about ±10 meters.
Full-ocean-depth expeditions calibrate their instruments carefully because a few meters can alter a published extreme-depth measurement.

Mantle Rock At The Southern Mariana Trench 
 
Diagram of a subduction zone, the tectonic setting that shapes the southern Mariana Trench.

Sirena Deep occupies the southern Mariana subduction system, where the Pacific Plate bends beneath the Mariana region.
The trench floor there is cut by major faults and steep slopes.
NOAA locates Sirena Deep near the meeting point of the trench axis and the East Santa Rosa Bank Fault.
A 2020 Deep-Sea Research Part I study examined rock and imagery collected near 10,677 meters.
Researchers documented ultramafic rock altered by serpentinization, a water-rock reaction involving mantle minerals such as olivine.
Earlier work in the southern Mariana forearc also recovered strongly serpentinized peridotite.
Geologists studying these samples can examine material associated with the upper mantle and deep crustal processes.
During the 2019 crewed descent, Vescovo and Jamieson collected material from the western slope.
Federal management documents later recorded a mantle-derived specimen among the recovered samples.

Microbial Material Has Been Found Near 10,677 Meters 
 
Giant tube worms clustered around a deep-sea hydrothermal vent in the Mariana Trench.

A 2012 lander deployment at Sirena Deep photographed filament-like material on exposed rock and talus near 10,677 meters.
Researchers analyzed the images and samples in a 2020 peer-reviewed study.
The material occurred on rock surfaces in permanent darkness, cold water, and hadal pressure.

The authors interpreted the filaments as a possible microbial mat.
They also documented serpentinized ultramafic rock at the site.
Serpentinization can generate chemical compounds that some microorganisms use as energy sources.
The authors kept the biological identification tentative because direct sampling at this depth remains difficult.

Other Mariana Trench studies have documented hadal amphipods and distinctive microbial communities under similar pressure.
Research on Hirondellea gigas found specialized microbial associations in animals collected within the trench system.
Specimens examined in that work came from the Sirena and Challenger areas of the Mariana Trench.

Challenger Deep Falls In Micronesian Waters 
 
Sonar map of the Challenger Deep with annotated dive history.
Editorial credit: Vlvescovo, CC BY-SA 4.0, via Wikimedia Commons.

Challenger Deep contains the lowest measured seafloor in the Mariana Trench.
A NOAA-affiliated study reported 10,935 meters, with a 95 percent confidence interval of ±6 meters.
The site lies roughly 300 kilometers southwest of Guam.
NOAA research places it within waters of the Federated States of Micronesia.
The United States and Micronesia established a formal maritime boundary in this part of the western Pacific, defining the neighboring maritime zones.

Vescovo descended to Challenger Deep during the Five Deeps Expedition and completed a separate full-ocean-depth mission there.
Sirena Deep sits within the U.S. EEZ associated with Guam.
The reported depth difference between the two sites is about 221 meters.
Their maritime locations place the 10,714-meter Sirena descent in the U.S.-jurisdiction record.

The Puerto Rico Trench Came First 
 
Diagram showing the Puerto Rico Trench, the deepest part of the Atlantic Ocean.

Vescovo completed an earlier U.S.-associated hadal dive on December 19, 2018, in the Puerto Rico Trench.
Limiting Factor reached a verified depth of 8,375.1 meters during the first major descent of the Five Deeps Expedition.
The mission was the first crewed visit to the deepest point of the Atlantic Ocean.
U.S.
Geological Survey mapping places major portions of the trench inside the U.S.
Exclusive Economic Zone north of Puerto Rico.
The dive put the new submersible through a complete hadal mission several months before the Mariana campaign.
Limiting Factor later entered deep sites in the Southern, Indian, Pacific, and Arctic oceans during the same expedition.
Sirena Deep reached 10,714 meters in May 2019.
Expedition documents preserve both depths and dates in the mission chronology. 
 
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Wednesday, September 16, 2026

Rough weather, real risk: what 8,300 ship accidents reveal



CHIRP Maritime analysis of 16 years of SAFETY4SEA incident reports shows how weather conditions influence maritime risk

Every sailor has a story about storms.
But how much of the risk at sea is actually linked to weather?

To explore this question, CHIRP Maritime examined 16 years of accident reports published by SAFETY4SEA, mapping more than 8,300 incidents worldwide between 2010 and 2026 and matching each one with the weather recorded at that location on the day of the incident.

CHIRP Maritime works with the University of Leeds MSc Business Analytics and Decision Sciences course and annually sponsors one or more graduates to analyse maritime incident data.
For this study, graduate Rakib Hossain analysed incident reports published on the SAFETY4SEA website.

The key test was simple: each accident day was compared with conditions at the same location one and two weeks before and after the incident.
The waters and season remained broadly the same; what changed was the weather.
Five findings stand out.

Rough weather genuinely raises the risk.
Accidents were 62% more likely to occur on rough days than on calm days at the same location.
When wave heights exceeded 2.5 metres, the risk more than doubled.
 
Accidents were markedly more likely on rough-weather days – and likeliest of all in heavy seas / 
Image Credit: CHIRP Maritime

Yet most accidents happen in ordinary conditions.
The typical accident day saw gusts of just 38 km/h, while severe storms featured in fewer than 2% of cases.
Crews may successfully avoid the worst weather, but more routine rough conditions can still catch ships out.

Weather picks its targets.
Ships left adrift or disabled encountered rough weather on almost one in four accident days, with sinkings and groundings close behind.
Fires and collisions showed little change, suggesting that weather plays a much smaller role in these incidents.
 
Rough weather hits disabled ships, sinkings and groundings hardest; fires and collisions barely respond. / Image Credit: CHIRP Maritime

Cargo ships feel it most. Dry-cargo vessels — including bulk carriers, container ships and general cargo ships — were 84% more likely to experience an accident on rough days, representing the clearest ship-type effect.
Passenger ships showed the largest increase overall, although this was based on fewer cases, while tankers showed comparatively little change.Dry-cargo ships show the clearest weather effect; the passenger-ship rise is striking but rests on fewer cases. / Image Credit: CHIRP Maritime

The reports broadly reflect the recorded conditions.
When an article attributed an incident to weather, the meteorological records usually supported that assessment.
In storm-force gusts, seven in ten reports referred to weather as a factor, compared with around one in three during light winds.
 
Weather gets blamed more often as the measured wind gets stronger — the reports track reality. / Image Credit: CHIRP Maritime

The message is clear: weather is not the whole story when it comes to maritime accidents, but it represents a real and measurable part of the risk – particularly during winter, in heavy seas and for vessels carrying the world’s dry cargo. 
 
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Tuesday, September 15, 2026

Super El Niño could have unexpected effect on UK winter


A winter storm batters England’s south coast in January 2025
Mark Passmore/Alamy

From New Scientist by Madeleine Cuff

An El Niño is usually expected to herald a cold end to winter in the UK – but this year could be different

The unprecedented El Niño gathering strength in the Pacific Ocean could upend forecasts for UK weather this winter, bringing continuously wet, stormy conditions until spring.
El Niño is a natural climate phase that happens when easterly trade winds weaken, allowing the warm water they had piled up in the western Pacific to wash back across the central and eastern Pacific.
This swathe of warm water heats the atmosphere and weakens the winds further in a feedback loop.
The phenomenon’s impact on the weather in the UK and north-western Europe is notoriously tricky to predict, because weather patterns in this region are strongly affected by weather phenomena in the North Atlantic.

Under an El Niño event of normal intensity, the UK is more likely to experience wetter, windier, stormier weather in the late autumn and early winter, potentially followed by a colder, drier end to winter.

However, the sheer size of the current El Niño – already one of the largest on record and still intensifying – could mean that the end of winter and start of spring continue to be wet and stormy, says Adam Scaife at the Met Office, the UK’s national weather service, who leads the agency’s long-range forecasting.
“We found that the big events show a slightly different response over the Atlantic,” he says.
El Niños usually increase the likelihood of cold easterly winds prevailing over the UK during late winter.
But a very strong El Niño may reverse that trend, leading to prevailing westerlies late in the season.
The weather under those conditions is “still cold, but it’s not extreme”, he says.
Scaife first reported this effect of strong El Niños in a 2006 research paper he co-wrote with Thomas Toniazzo, now at NORCE Research in Norway.
The study assessed the weather in the North Atlantic during El Niño years and found that very strong El Niños produced this mild and wet pattern in late winter.

 
The El Niño in 1997-1998, for example, was one of the most powerful on record.
It saw high pressure developing in the North Atlantic to the west of the UK over the late winter, leading to westerly prevailing winds that blew milder, wetter, stormier air over the UK and northern Europe.
The same pattern appeared in the 2015-2016 El Niño, when the UK had one of its mildest and wettest winters on record.
“We think that’s a real effect [of strong El Niños],” says Scaife.
“We’re looking for that at the end of this winter.”
A prolonged wet and stormy winter could saturate soils and make parts of the UK more susceptible to severe flooding.
But it would also offer an opportunity to replenish reservoirs and groundwater after an exceptionally dry summer.

However, Scaife warns it is very difficult to tease apart the effect of a strong El Niño compared with that of a standard-strength one, in part because there are only a few very strong events in the historical record for which we have good data.

El Niños – and their opposite pattern, La Niñas – also vary hugely from year to year and decade to decade, so it can be fiendishly difficult to separate out a signal from the noise in the system.
What’s more, it is very difficult to predict weather patterns months in advance, and there is still huge uncertainty over what late winter will hold for the UK.
This week, the World Meteorological Organisation (WMO) warned that the El Niño is still intensifying in the tropical Pacific and is set to be the largest on record, fuelling drought, floods and extreme temperatures around the world.
In most cases, a stronger El Niño brings more pronounced impacts globally, such as severe drought in Australia, the Amazon and Indonesia, and heavy rain in parts of South America, East Africa and the southern US.
The pattern is likely to peak in November, but will persist until at least February 2027, the WMO said, bringing extreme impacts that will last well into next year.
 
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Monday, September 14, 2026

The world’s biggest carbon sink may start emitting CO2 after net zero

Storms in the Southern Ocean help to remove carbon dioxide from the atmosphere
Hemis/Alamy
 
From New Scientist by Alec Luhn

If the carbon dioxide in the atmosphere declines, the Southern Ocean is likely to begin emitting more carbon than it absorbs, prolonging global warming

The Southern Ocean draws down more carbon than any other single region, absorbing about 10 per cent of human emissions.
But if the world reaches net-zero emissions and carbon dioxide concentrations fall, it will start releasing CO2 into the atmosphere.
This massive release of carbon will prolong global warming for centuries, even if humanity artificially removes gigantic amounts of CO2 from the atmosphere, a study has found.
The results show how difficult it will be to reverse climate change, even as countries place ever-larger hopes on carbon dioxide removal.
“That impact can sustain much, much longer than we expected,” says Yechul Shin at Seoul National University in South Korea.
“Let’s say we try to reduce the CO2… this sink-to-source transition in the Southern Ocean still acts as an obstacle to reduce the atmospheric CO2 concentration.”

The notoriously strong winds around the “roaring forties” and “furious fifties” latitudes of the southern hemisphere stir up waves and encourage the water to absorb CO2.
The water cools and sinks, storing that carbon away in the deep ocean for centuries or millennia.
This carbon uptake has been increasing as humanity pumps ever more greenhouse gas into the atmosphere, creating a higher concentration of CO2 in the air than in the surface water.
But if the atmospheric CO2 concentration begins falling, that ratio will flip, and the Southern Ocean will start releasing rather than absorbing CO2.
“It’s like you’re constantly pressing a piston into the ocean,” says Nicolas Gruber at ETH Zurich in Switzerland, who wasn’t involved in the study.
“If you pull the piston up again, then the CO2 comes back out.”

The researchers modelled a scenario in which the world reaches net zero in 2125 and the concentration of atmospheric CO2 starts to gradually decline from 700 parts per million.
They also modelled one in which humanity drastically decreases atmospheric CO2 after net zero through carbon dioxide removal, a range of approaches ranging from widespread tree planting to machines that filter CO2 out of the air.
In both scenarios, the Southern Ocean goes from absorbing about 3 grams of carbon per square metre per year now to emitting 8 to 9 grams of carbon per square metre by the year 2400.
Under both scenarios, atmospheric CO2 is projected to fall, but this decline gradually levels off after 2200 as the Southern Ocean emits more CO2.
In the carbon-removal scenario, atmospheric CO2 actually rises again for 50 years before levelling off at 425 parts per million – roughly the current concentration.
Without carbon removal, it levels off at 590 ppm.

The scenarios aren’t necessarily realistic, though.
Many countries, including the UK and members of the European Union, aim to reach net zero as soon as 2050.
Moreover, the amount of carbon dioxide removal remains minuscule compared with emissions, which are still increasing.
But regardless of when humanity reaches net zero, the ocean as a whole is likely to become a net source of CO2 as it begins releasing all the carbon that major sinks like the Southern Ocean stored, according to Alberto Naveira Garabato at the University of Southampton, UK.
“Anything that changes in terms of the way the ocean interacts with the atmosphere could be like adding a massive country to the world’s carbon budget,” he says.
While scientists largely expect the Southern Ocean to begin emitting CO2 at some point, the new research finds that two major feedbacks may amplify this release, says Gruber.
The first is driven by the enormous amount of heat that the Southern Ocean transfers to the depths alongside the carbon.
Warm water can hold less gas.
So, as that stored heat gradually returns to the surface, the Southern Ocean will lose more CO2, just like a glass of warm soda loses its fizz more rapidly than a glass of cold soda.

The second feedback is essentially an increase in ocean acidification.
Many of the plankton in the Southern Ocean form alkaline shells of calcium carbonate.
When they die and sink, they transfer alkalinity to the deep ocean.
But as the surface water warms, it will become less dense.
As a result, it will mix less with cold, dense and more-alkaline deep water.
With less alkalinity returning to the surface, less CO2 – which is slightly acidic – will be absorbed by the ocean.
The sooner humanity reaches net zero and stops heating the Southern Ocean, the less CO2 these waters will ultimately emit, says Shin.
This month, the UN said the world will fail to meet the Paris Agreement goal of keeping global warming below 1.5°C and should instead focus on ways to reduce global temperatures later on, including through carbon removal.
But the new study shows this may be harder to do than we expect, says Gruber.
“It’s like crashing a car into the wall, and then you back up the car.
It’s not the same car any more,” he says.
“We can’t just simply take out CO2.
There are consequences, and some of these consequences are long-lasting.”

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