The solution to California's rampant sea urchin problem is to eat them. I gave it a try - The Guardian

“Babe! I sprayed mouth everywhere!”

I never thought I’d find myself screaming these words on a tranquil Sunday morning in my tiny San Francisco kitchen. Then again, I never thought I’d find myself staring at a sink full of spiky, purple aliens with a knife murderously clutched in one hand, the ethereal voice of Phoebe Bridgers softly crooning in the background.

But these vibrant little aliens – purple sea urchins, in actuality – have become a major headache for the Pacific west coast. Their population has exploded by 10,000% since 2014, with scientists blaming the decline of sea otter and starfish populations – two of the urchin’s natural predators.

Hundreds of millions of purple sea urchins now blanket the coast from Baja to Alaska, where they have been devouring the region’s vital kelp forests, doing untold damage to the marine ecosystem in the process. In California, it is estimated that 95% of the kelp forests, which serve as both shelter and food to a wide range of marine life, has been decimated and replaced by so-called “urchin barrens” – vast carpets of spiked purple orbs along the ocean floor.

That’s why marine biologists and chefs have teamed up to release a new predator into their natural environment: me.

Or, to be exact, me and all of you. There’s been a push for years to get the public to eat more sea urchin as a way to help curb the population and recover the kelp forests.

It shouldn’t have been a hard sell. Sea urchin, or uni in the sushi world, is considered a delicacy in the fine dining circles. “The two main descriptors I would use are sweet and briny, similar to an oyster, similar to a clam,” said culinary scientist Ali Bouzari. “They taste like the sea because they live in the sea. They’re sweet, umami and a little bit salty. The texture is very creamy. It’s very similar to room-temperature butter.”

During the pandemic, however, fine dining has been harder to come by. And the retail costs, which range from $9 to$12 per urchin at your local fishmonger, isn’t something every home cook can justify.

But what Bouzari, co-founder of culinary research and development company Pilot R&D, has been pushing for the last few years is that sea urchin cuisine doesn’t have to be particularly precious, or expensive. You can have it served on a half shell, topped with espresso-cream whipped potatoes and caviar – as they do at Michelin-star restaurant SingleThread in Healdsburg – or you can sauté it with some onion, sausage and day-old rice and make a dirty rice, one of Bouzari’s favorite recipes. And anyone with access to the coast can have sea urchin dirty rice on a dirty rice budget.

And this was how I found myself in my kitchen, covered in kelp and purple spines.

Sea urchin harvesting

The day before my exploding mouth episode, I stood on the beach of Timber Cove in Jenner, California, waiting as Bouzari and his friend Justin Ang, a Pilot R&D product manager, paddled up to shore atop some surfboards.

They had spent the morning spearfishing, coming in with some scallops, blue rock fish and a giant ling cod – and of course, sea urchin. A year-long fishing license in California costs $52.66, while my husband and I each paid a$10 fee for “taking anything from the ocean” at Timber Cove.

Bouzari came out of the water dressed like an underwater sea pirate, with weights around his waist like a gun holster and a knife strapped to his calf. But you don’t need a wetsuit or fancy gear to harvest sea urchin, he explained. Anytime at low tide on the edges of a cove, urchin – an intertidal species – should become visible.

Sure enough, within a minute of stepping on to the rocks, I spotted my first sea urchin, stuck to the side of a rock.

Sea urchins are essentially a ball of hard purple spikes containing five egg sacs, which is what we eat – in the culinary world, they’re described as the tongues, the roe, the uni. I call it the yum.

They have no eyes or brain, but they do have mouths, which they use to suck up everything in their way, kelp or otherwise. Bouzari calls them the “Roombas of the ocean”.

The sea urchin came loose when I twisted it like a doorknob. The triumph of my first harvest overtook any lingering sensations of pain from gripping its prickly spines. Still, I’d recommend gloves.

I had brought some salted sourdough toast from San Francisco, and Bouzari quickly scooped a fat, golden tongue out of the hardened purple spikes to lay on to the olive-oiled surface. I had enjoyed uni before at sushi restaurants, but never tasted anything quite like the briny creaminess of sea urchin fresh from the ocean, on toast warmed in the California sun. That one bite felt like a calm summer day, floating on a boat in the water.

Dragon eggs in the kitchen

If saving the kelp forest means eating more urchin, then we ostensibly need to prove that people can not only afford to eat more sea urchin, but prepare sea urchin easily on their own.

Meaning someone like me, the person who infamously vomited in the middle of the newsroom on the first day of her dream internship because she ate undercooked chicken the night before, can harvest these creatures – which resemble evil dragon eggs – and manage to make a delicious, home-cooked meal.

I was concerned about transporting a cooler full of purple sea urchins on the two-hour drive down Highway 1 back to San Francisco, but Bouzari assured me that sea urchin would keep for several days, especially if left in salt water. The best practice is to keep them refrigerated, Bouzari said, with a damp cloth over them. “When it gets old, it starts to smell like ammonia,” he said.

OK, ammonia. I woke up Sunday morning, preparing myself for ammonia, a urine-like smell. What I was not prepared for was the odor of farts. I opened the cooler on Sunday morning and was greeted with a cloud of flatulence. “Vivian!” my husband said, immediately blaming me.

Fart jokes aside, it was Sunday morning and we were hungry. I wanted to continue on a more simplistic, dirty rice trend, and dug up a scrambled egg recipe from Gordon Ramsay. I found a fairly simple sea urchin bruschetta recipe from Robert Irvine before I remembered that I hate cilantro and scallions and don’t own a lemon zester. “So you’re just making toast and sea urchin again?” my husband asked.

Back at Timber Cove, Bouzari had shown me how to get the yum out of the shells with the finesse of a man who has done this many times. In my kitchen, I went at the urchin with the grace of a drunk baby elephant that had somehow gotten hold of a knife.

The first move in preparing a sea urchin is to cut out its mouth, which is hard and shell-like. When you have practice doing this like Bouzari, you can swipe it all out in one go. When you’re me, you fling multiple mouths all over your kitchen, multiple times.

The next move is to empty the water. At the beach, we emptied the water into the ocean and then washed it again in the waves. At home, I ran tap water into the hole where the mouth was, cleaning out any sand and sediment.

Bouzari showed me a move where he cut the urchin in half elegantly so that you could use the shell as a bowl or a candle holder after removing the roe. I had not mastered that. Instead, I cut the urchin jagged down the middle, at times just using my hands to rip it apart, sending spines flying on to the floor and into my sink.

Bouzari used a knife to detach the digestive tracts at each point. It was very clean and streamlined and from there, he rinsed out the shell once more and all that was left was the roe, waiting to be spooned out.

In my kitchen, my two halves of shell looked completely different – more like two handfuls of crumpled spikes, kelp and roe oozing together. I couldn’t find the points to detach the digestive tracts, so I ended up just scooping everything out and washing the roe free of the kelp. Bouzari made sure to tell me that eating the digested kelp wouldn’t hurt anybody – it just doesn’t taste very good.

It took me two hours to collect enough yum for five small crostini and to make six scrambled eggs. It felt like I did a lot of work for very little output.

Bouzari had warned this could happen with some of the purple sea urchins. Though they’re plentiful, some are not growing large enough to be commercially viable. Companies like Urchinomics have begun collecting the purple sea urchins and then fattening them up for high-end sushi restaurants.

But even the worst chefs can make butter noodles, and for that reason I chose to freestyle the dish, without a recipe, for my last urchin attempt.

I prepared spaghetti noodles, and then made a simple sauce out of olive oil, butter, garlic, lemon and pasta water. I sprinkled in some oregano, basil, chili flakes and sea salt before taking it off the heat and stirring in the urchin paste I made by taking the collected roe and mixing it together. I tossed the spaghetti in the sauce and laid two tongues on top of each bowl.

I wasn’t expecting much. The fart smell from the cooler (which Bouzari later assured me was nothing to worry about) still lingered, even though we’d opened every window in our apartment, and I stepped on a piece of mouth while I was tossing the pasta. But these noodles were creamy and buttery with hints of ocean. Each bite bit back with brine and lemon brightness.

For someone who had been told her entire life that she could not cook and should not cook, for the gastrointestinal safety of herself and others, this dish felt life-changing. It felt like a shining beacon into a tastier world, a victory for the culinary-challenged everywhere.

My husband said it was pretty good.

Smelly, frustrating, messy and … fun

Even amid the fart smell, I had begun to have fun. At times it got frustrating, when all I managed to spoon out was a thin sliver after minutes of painstakingly picking through clingy kelp. But those moments always faded with the incredible bubbling satisfaction of scooping out that one fat, picture-perfect golden tongue.

I’m the type of eater who loves to work for my food, getting my fingers dirty, sauce all over my face, as I gnaw the meat off the bones. Some people don’t. But for those who do, I can see how fun an afternoon of shore-picking sea urchins and eating them over toast on the beach could be, especially when it comes with the feeling that you’ve done something good.

“This is the rare opportunity for our gluttonous instincts to have a redemption arc,” Bouzari said. “This is something where there are hundreds of thousands, if not millions more of these purple urchins than there should be. Go for it. You could harvest literally more than you could eat and honestly harvesting more than you can eat it is the responsible thing you can do. Just get them out of the water.”

What I learned about eating urchins is that you really can’t ruin something that’s intended to be eaten raw. Plus, if you have to break a few dragon eggs to make an omelet, that’s sort of the point. Getting urchins out of the ocean is the goal, so break as many dragon eggs as you want.

After a day and half of eating urchin, my husband asked that we not have them for a while. As for me, I’m already planning my next batch of butter noodles.

April 11, 2021 at 05:02PM
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The solution to California's rampant sea urchin problem is to eat them. I gave it a try - The Guardian

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U.S., Philippines discuss recent massing of Chinese vessels in South China Sea - KTLA

Defense Secretary Lloyd Austin on Saturday discussed with his Philippine counterpart China’s recent positioning of “militia vessels” near the Philippines in the South China Sea.

Austin spoke by phone with Philippine Secretary of National Defense Delfin Lorenzana while Austin was flying from Washington to Israel to begin an international trip.

Pentagon press secretary John Kirby said Austin and Lorenzana discussed the situation in the South China Sea and the recent massing of Chinese vessels at Whitsun Reef, which has drawn criticism from Manila.

China has said its vessels are there for fishing.

In their phone call, Austin proposed to Lorenzana several measures to deepen defense cooperation, including by “enhancing situational awareness of threats in the South China Sea,” Kirby said. He did not elaborate.

Kirby said earlier this week that the aircraft carrier USS Theodore Roosevelt and its strike group, as well as the amphibious ship USS Makin Island, are operating in the South China Sea.

The U.S. has no military forces based permanently in the Philippines but sometimes rotates forces to the country under the U.S.-Philippines Visiting Forces Agreement.

The recent gathering of Chinese vessels near the Philippines is among moves the United States has criticized as efforts by Beijing to intimidate smaller nations in the region.

April 11, 2021 at 10:01AM
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Man swept out to sea survives by treading water for hours until found by fishing boat - Hawaii News Now

[unable to retrieve full-text content]Man swept out to sea survives by treading water for hours until found by fishing boat  Hawaii News Now The Link Lonk

April 11, 2021 at 04:08AM
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Beachgoers asked to be mindful as sea turtle nesting season gets set to begin - FOX 13 Tampa Bay

Thousands of visitors flock to our pristine beaches every year. However, those sandy shores are more than just a destination because sea turtle hatchlings call it home.

"It’s important for people to recognize how we mark our nests. What you’re going to see when you come to the beach is a roped-off area with yellow stakes and some signs that clearly state not to disturb the nests. The reason for that is because we want to leave enough space for them to do what they have to do as they incubate under that sand," Yvette Fernandez, the senior sea turtle biologist with Clearwater Marine Aquarium.

With the start of sea turtle nesting season just a few weeks away, teams of volunteers with Clearwater Marine Aquarium hit the beach Saturday to make sure it was clean while educating beachgoers and the public about the incoming turtle residents.

"Sea Turtle Awareness Day is an event we’ve been doing for quite a while to educate and bring awareness to the public about the importance of sea turtles in our ecosystem," Fernandez shared.

Keeping the turtles safe is no small task, and for the most part, it should be left to the professionals. But tourists and locals alike still play a big role.

"You can always make a difference by picking up trash, by knocking down those sandcastles on the beach," explained Fernandez.

READ Florida beachgoers have rare encounter with 800-pound leatherback sea turtle nesting during daylight hours

Taking their efforts one step further, CMA and the Winter the Dolphin Beach Club have partnered together to help create the first sea-turtle-friendly hotel on Clearwater Beach. The hotel has put up turtle-friendly lights that won’t interfere when the hatchlings leave the nest, helping to combat one of the biggest obstacles the hatchlings face.

"Those baby sea turtles are going to emerge and they’re going to follow the brightest lights out there which will be the celestial lights, the moon and the stars," said Fernandez. "But of course, if we have brighter lights behind that nest towards the buildings, they are going to follow those lights because they’re stronger. So, that’s why they get disoriented and that’s why it’s becoming one of our biggest issues."

It takes everyone doing their part to keep these hatchlings safe. No matter how cute they might be, if you see a hatchling on the beach, Fernandez advised not to touch the turtle but to instead call FWC at *FWC or #FWC, or send a text to Tip@MyFWC.com.

WATCH: Video shows hundreds of sea turtles coming ashore for mass nesting on Costa Rica beach

Sea turtle nesting season begins in May and lasts through October.

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April 11, 2021 at 04:21AM
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Tenby 'tombstoning' rescue: Unconscious man pulled from sea, police say - BBC News

[unable to retrieve full-text content]Tenby 'tombstoning' rescue: Unconscious man pulled from sea, police say  BBC News The Link Lonk

April 11, 2021 at 04:24AM
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Earthquake of magnitude 6 strikes Celebes Sea, near Philippines - EMSC - Reuters

(Reuters) - An earthquake of magnitude 6.0 struck in the ocean 214 km (132 miles) south-southwest of Sarangi, Philippines, the European- Mediterranean Seismological Centre said on Saturday.

The quake was at a depth of 300 km (186 miles), the EMSC said.

Reporting by Vishal Vivek in Bengaluru, Editing by William Maclean

April 10, 2021 at 05:07PM
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Oldest Sea Otter At Aquarium Of The Pacific Dies At 20 - Gazette Newspapers

The Aquarium of the Pacific announced somber news Friday evening.

The aquarium's oldest otter, Maggie, passed away from health complications on Friday, April 9.

Maggie was rescued and deemed non-releasable to the wild. On April 9, 2010, she arrived at Long Beach's Aquarium of the Pacific at just 9 years old.

According to a release, the aquarium staff considered Maggie to be a "motherly sea otter with a kind and gentle nature."

When rescued sea otters would arrive in Long Beach, Maggie would eagerly share food and swim with them, the release said.

—Stephanie Stutzman

At age 20, Maggie, who had been displaying a recent decline in health, was nearing the maximum age of sea otters.

A necropsy is expected o broaden the aquarium’s base of knowledge on geriatric sea otters.

The Aquarium of the Pacific reopened its outdoor exhibits to the public in January and its indoor operations in March, after being shuttered since the coronavirus pandemic began.

April 10, 2021 at 08:35AM
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Earthquake of magnitude 6 strikes Celebes Sea, near Philippines - EMSC - Reuters

(Reuters) - An earthquake of magnitude 6.0 struck in the ocean 214 km (132 miles) south-southwest of Sarangi, Philippines, the European- Mediterranean Seismological Centre said on Saturday.

The quake was at a depth of 300 km (186 miles), the EMSC said.

Reporting by Vishal Vivek in Bengaluru, Editing by William Maclean

April 10, 2021 at 05:07PM
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A pandemic problem no one has solved: Marine workers stuck at sea - Minneapolis Star Tribune

Shipping-industry leaders are scrambling to solve the crisis of thousands of maritime workers who have been stranded because of the coronavirus pandemic. Global attention on the Suez Canal blockage last month gave them a new chance.

Unions, seafarer ministries and multinational ship owners and charterers — odd bedfellows in normal times — all seized the moment to raise awareness of the plight shipping workers face due to COVID-19 travel restrictions.

"We spend most of our time arguing with ship owners but the reality is on this one is we've really pulled together because it is a humanitarian crisis," said Stephen Cotton, general secretary of the International Transport Workers' Federation (ITF). "The irony of the Suez situation is one small incident generates enormous coverage."

As countries closed borders last March, many decided not to allow seafarers to get off ships at port. And the maritime workers who managed to get on land often weren't able to book plane tickets home because of other restrictions.

At the peak, about 400,000 maritime workers were unable to get off ships and get home, according to the International Chamber of Shipping. That number of workers struggling to repatriate now stands about 200,000.

Jan Dieleman, president of Cargill Inc.'s ocean transportation unit, took the opportunity in a recent Star Tribune interview to vent his frustration about the lack of interest in seafarers' well-being during the pandemic.

"Because they were not deemed essential workers, they've had to abide by every single country's laws like everybody else," Dieleman said. "Nobody has been able to solve this. Everyone is looking for someone to blame, but as an industry, as a society, we need to do better."

A complex patchwork of government rules limiting the cross-border movement of people continues to be a challenge, Cotton said. The growing prevalence of coronavirus variants is now leading to new lockdowns that could strand more workers at sea.

Compounding the issue, demand for shipping grew during the pandemic, said Jason Zuidema, executive director of the North American Maritime Ministry Association.

First came the surge in demand for personal protective equipment. Then came North American consumers and their online shopping that stressed the industry as their home, garden and office supplies filled containers floating aboard vessels from Asia to Los Angeles.

"[The companies] were still desperate to keep up the flow of essential goods and [personal protective equipment]," Zuidema said in an e-mail. "Keeping our store shelves stocked came at the expense of not allowing seafarers to go home for many additional months beyond the end of their contracts."

Three-month contracts turned to six months, six-month contracts turned into eight months. And even though 11 months is the longest amount of time a seafarer is supposed to be working aboard a ship, according to standards set by the International Labor Organization of the United Nations, there were cases of crew members unable to disembark for 17 months during the pandemic.

Cargill, which has between 1,400 and 1,500 seafarers working its chartered vessels, tried to redirect its ships to ports where crew changes were allowed, Dieleman said.

The shipping industry is filled with shadowy middlemen who aren't always held accountable, Cotton said. Many charterers refused to detour to crew-friendly ports, prioritizing on-time delivery of their cargo instead, he said.

And while some were stuck at sea, the restrictions put on crew changes meant other workers were unable to earn an income by not being able to board.

By mid-July 2020, 13 nations, including the United States, signed a joint statement acknowledging seafarers as key workers in the pandemic efforts and pledging to improve conditions for the workers.

The U.N. General Assembly in December adopted a resolution that all nations should extend that status to seafarers. The International Maritime Organization, the U.N.'s maritime agency, reported last month that less than 60 had done so.

"There is still a long way to go before we are back to a normal crew change regime," Kitack Lim, secretary-general of the IMO, said in a statement last month. "As vaccination is rolled out in many countries, I urge governments to prioritize seafarers in their national COVID-19 vaccination programs."

The shipping industry continues to press national governments on seafarer stranding and vaccination. More than 750 organizations signed the Neptune Declaration for Seafarer Wellbeing and Crew Change early this year.

"Seafarers should be recognized as essential workers by governments around the world," Zuidema said. "They need to get priority access to vaccines."

About a quarter of global seafarers who have so far responded to an ITF survey said they are considering leaving the industry after the negative experience of this past year.

"Some seafarers are reconsidering going back to sea," Cotton said. "How do you explain to your partner, children, parents that you don't know when you'll be home?"

Kristen Leigh Painter • 612-673-4767

April 10, 2021 at 08:02PM
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A pandemic problem no one has solved: Marine workers stuck at sea - Minneapolis Star Tribune

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US Carrier Strike Group, Amphibious Warships Massed in South China Sea as Regional Tensions Simmer - USNI News - USNI News

USS Makin Island (LHD-8) and the aircraft carrier USS Theodore Roosevelt (CVN-71) transit the South China Sea on April 9, 2021. US Navy Photo

A U.S. carrier strike group and amphibious ready group are in the South China Sea as tensions increase between Manilla and Beijing over a Chinese maritime militia incursion into the Philippines’ exclusive economic zone, the Navy announced on Friday.

The Theodore Roosevelt CSG and the Makin Island ARG – with the 15th Marine Expeditionary Unit embarked – are drilling in the South China Sea, according to U.S. 7th Fleet.

“Combining the capabilities of the carrier strike group with those of the Makin Island Amphibious Ready Group sharpens our tactical skills and demonstrates our continued dedication to the security and prosperity of the Indo-Pacific,” Rear Adm. Doug Verissimo, commander of Carrier Strike Group Nine, said in a Friday statement. “The combined Navy and Marine Corps team has been a stabilizing force in this region for more than a century and will continue to support all who share in the collective vision of peace, stability, and freedom of the seas.”

The exercises with the TR CSG and the three-ship ARG come as 44 Chinese maritime militia ships have remained massed off the coast of the Philippines in the vicinity of Whitsun Reef, Philippine officials said. Last month, 200 ships, identified by Chinese authorities as fishing vessels sheltering from bad weather, moved into the area around the reefs.

On Friday, Pentagon spokesman John Kirby said Defense Secretary Austin “remains concerned by the massing of Chinese maritime military vessels in the Union Bank area of the South China Sea and Chinese efforts of impeding the lawful rights of our treaty ally of the Philippines. The United States stands by our ally.”

The move of the U.S. ships into the region comes as officials in Manilla have raised alarms over Chinese behavior. This week a Philippines Department of National Defense spokesman said Manilla was in contact with Washington on the situation.

“We are continuously in talks with the U.S. on the matter of mutual defense,” Philippine defense spokesman Arsenio Andolong said in a statement reported by the Star Tribune newspaper in Manilla.
“Both parties are committed to undertake their obligations under the [1951] Mutual Defense Treaty so that neither stands alone in these issues involving the two states’ inherent right of self-defense.”

Late last month, Secretary of State Antony Blinken affirmed the U.S. commitment to defending the Philippines if it was attacked.

The “United States stands with our ally, the Philippines, in the face of the [People’s Republican of China]’s maritime militia amassing at [Whitsun Reef],” he said in late March.
“We will always stand by our allies and stand up for the rules-based international order.”

The maritime militia ships have been spotted operating with the China Coast Guard and People’s Liberation Army Navy ships.

Earlier this week, Philippine journalists approaching reefs in the South China Sea by boat were interdicted by two Type-22 Houbei PLAN catamarans.

April 10, 2021 at 04:56AM
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U.S. to send two warships to Black Sea, Russia voices concerns - Reuters

ANKARA/MOSCOW (Reuters) - The United States will send two warships to the Black Sea next week, Turkey said on Friday as Russia, which has boosted its military forces near Ukraine, accused NATO powers that do not have a coast line in the region of increasing naval activity.

The Pentagon declined to discuss Turkey’s comments but said the military routinely sends ships to the region.

“That’s not anything new,” Pentagon spokesman John Kirby said in Washington, referring to U.S. military ships in the Black Sea.

Washington says Russia has amassed more troops on Ukraine’s eastern border than at any time since 2014, when it annexed Crimea from Ukraine and backed pro-Russian separatists in the eastern Donbass region of Ukraine.

Violence has flared between Ukrainian troops and the separatists. Russian President Vladimir Putin, during a telephone conversation with Turkey’s President Tayyip Erdogan, accused Ukraine on Friday of “dangerous provocative actions” in the Donbass region.

Turkey, a NATO ally, said on Friday the United States would deploy two warships to the Black Sea from April 14-15.

“A notice was sent to us 15 days ago via diplomatic channels that two U.S. warships would pass to the Black Sea, in line with the Montreux Convention. The ships will remain in the Black Sea until May 4,” Turkey’s foreign ministry said.

U.S. warships are regularly in the Black Sea, including a cruiser and destroyer being there late last month.

The 1936 Montreux accord gives Turkey control over the Bosphorus and Dardanelles straits, which connect the Mediterranean to the Black Sea. It also limits access of naval warships and governs foreign cargo ships.

Russian Deputy Foreign Minister Alexander Grushko raised concerns on Friday over what he said was increasing Black Sea naval activity by powers that did not have a coast line in the region, an apparent reference to the United States.

“The number of visits by NATO countries and the length of the stay of (their) warships have increased,” he was quoted as saying by the Interfax news agency.

According to a Reuters witness who keeps track of ships passing through Turkey’s Bosphorus strait, the United States and NATO increased their presence in the Black Sea early this year, when U.S. President Joe Biden’s administration took power.

The Reuters witness said the level had reached that seen in 2014-2015 at the time of the Crimea annexation.

Ukraine’s President Volodymyr Zelenskiy was due to meet Erdogan in Turkey on Saturday on a previously scheduled visit.

Reporting by Tuvan Gumrukcu, Ezgi Erkoyun, Yoruk Isik in Turkey and by Maria Tsvetkova and Polina Ivanova in Moscow and Idrees Ali and Phil Stewart in Washington; Writing by Tom Balmforth; Editing by Gareth Jones and Grant McCool

April 10, 2021 at 03:04AM
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Ocean eddies strongly affect global mean sea-level projections - Science Advances

INTRODUCTION

Satellite-observed sea-level measurements indicate an ongoing increase in the global mean sea level since the satellite era (13). Global mean sea-level rise (GMSLR) threatens low-lying coastal regions, and these regions will benefit strongly from sea-level projections to adapt their coastal protection infrastructure (4, 5). Useful scenarios of future global mean sea-level change in the upcoming decades can only be made by state-of-the-art climate models. Current projections are based on climate models in which ocean-eddy processes are parameterized and the present-day state in these models strongly deviates from available observations, in particular, in the Southern Ocean (6, 7).

The climate models used in the fifth Intergovernmental Panel on Climate Change Assessment Report provide estimations of GMSLR in the upcoming decades (6). One of the greatest uncertainties at that time was the contribution by the Antarctic ice sheet (AIS) to GMSLR (8), and it was noted that marine ice-sheet instability of the West Antarctic ice sheet could change the GMSLR projections by several decimeters (9, 10). Since then, observational studies have shown that the Antarctic contribution to GMSLR is increasing over time, in particular, for western Antarctica due to basal melt near the grounding line of various ice shelfs and glaciers (1, 11, 12).

In a recent model study (7), 16 state-of-the-art AIS models subject to basal melt were used to determine the sea-level response by ice loss. Basal melt of ice shelfs was driven by subsurface ocean temperatures obtained from global climate model simulations within the Coupled Model Intercomparison Project phase 5 (CMIP5). However, the present-day state of the Southern Ocean and its responses to climate change strongly differ among the CMIP5 models (13, 14); this holds as well for the older CMIP phase 4 models (15, 16). Differences (e.g., temperature and salinity) in the mean state, compared to the available observations, have been attributed to the coarse horizontal (ocean model) resolution in these models (1719).

In ocean models with a higher spatial resolution, where eddies are partly represented, the present-day Southern Ocean surface temperature is much better represented compared to that in ocean models where eddies are parameterized (20). Explicitly representing eddies in one climate model also leads to substantially different regional dynamic sea-level projections for the Caribbean compared to version of the same model where eddies are parameterized (21). Motivated by these results, we here analyze GMSLR projections in two (noneddying and eddying) versions of the Community Earth System Model (CESM), taking into account Antarctic basal melt under a particular climate change scenario.

RESULTS

Climate model simulations

The CESM is a fully coupled state-of-the-art global climate model and is participating in the CMIP5 and CMIP6 efforts. The high-resolution version of CESM (HR-CESM) used here has an ocean component with a 10-km (0.1) horizontal resolution, capable of capturing the development and interaction of mesoscale ocean eddies (22), and an atmosphere component with a horizontal resolution of 50 km (0.5). Both the ocean and atmosphere component of the low-resolution version of the CESM (LR-CESM) have a horizontal resolution of 100 km (1). The ocean component of this low-resolution model cannot generate mesoscale ocean eddies. The high-resolution and the low-resolution versions of the CESM are spun up by 200 and 500 years, respectively, under a present-day (year 2000) forcing and then continued for 101 years under the same forcing to give the 101-year HR-CESM control and LR-CESM control simulations. The HR-CESM and LR-CESM simulations are initiated from the end of the corresponding spin up and are forced under a 1% pCO2 increase each year (model years 2000–2100). More details of the CESM simulations can be found in Materials and Methods. Both CESM versions have a volume conservation constraint for the ocean component and do not capture dynamical ice sheets. The contributions of the mass loss of glaciers and ice sheets to the GMSLR therefore need to be determined by postprocessing of the model results (see Materials and Methods). The CESM does not include any changes in the land-water storage.

Since observations of the Southern Ocean are limited, we compare the CESM control simulations results with one reanalysis product, the Mercator data, in which available observations are assimilated. Figure S1 (A, C, and D) shows the time mean (26 years) and depth-averaged (250 to 450 m) oceanic temperature fields for Mercator, HR-CESM control, and LR-CESM control, respectively. The depth range is based on the mean depth of the various Antarctic ice shelfs (7). The HR-CESM control is much better in agreement with reanalysis compared to the LR-CESM control. For example, the area-weighted correlation pattern [root mean square (RMS) deviation] over the 80S to 60S band is about 10% higher (45% lower) for the HR-CESM control compared to the LR-CESM control and is robust over the simulation period (fig. S1B). For the HR-CESM control, the largest temperature difference with respect from Mercator is found west of the Antarctic Peninsula with a magnitude of about −2C (fig. S1E). The LR-CESM control is warmer compared to Mercator in the Weddell Gyre and along the (eastern) Antarctic continental shelf (fig. S1F). The results in fig. S1 indicate that ocean subsurface temperature differences (with respect to Mercator) are reduced under a higher spatial resolution of the ocean model.

The RMS deviation is increasing over time, indicating that there is a slight drift in the control simulations (figs. S1B and S2, A and B). For the LR-CESM control, the temperature trends are persistent over the analyzed period. On the contrary, in the HR-CESM control, part of these (significant) trends is related to multidecadal variability in the Southern Ocean (23). This multidecadal variability is related to ocean-eddy interactions with the background flow (24), which are absent in the LR-CESM control.

Global mean sea-level rise

The GMSLR in the HR-CESM and LR-CESM simulations consists of four contributions, and these are shown over the 101-year period in Fig. 1 (A and B); details on the computation of each contribution can be found in Materials and Methods. The largest contribution to GMSLR is caused by (thermo)steric effects (adjusted for drift in each control simulation). The second largest and third (largest) contributions are related to melt by glaciers (fig. S3) and changes in the surface mass balance of the Greenland ice sheet (GrIS; fig. S4), respectively. These three contributions to GMSLR are fairly similar for both HR-CESM and LR-CESM (Fig. 1, A and B). The fourth contribution to GMSLR is due to changes in the surface mass balance and mass loss as a result of basal melt of the AIS (fig. S5). The Antarctic contribution strongly differs between the HR-CESM and the LR-CESM, where the LR-CESM projected value for Antarctica in the year 2100 is 9.3 cm higher compared to that of the HR-CESM. Note that the mass loss by basal melt is balanced by changes in surface mass balance in the HR-CESM (fig. S5A), resulting in a near-zero contribution for the AIS.

The patterns of sea-level change between two 30-year periods (2071–2100 and 2000–2029) are shown for HR-CESM and LR-CESM in Fig. 1 (C and D), respectively. Here, gravitational, rotational, and deformation [GRD; (25)] effects by mass loss of glaciers and ice sheets are taken into account (see also figs. S3 to S5). Ocean currents change under global warming, affecting the dynamic sea level. The largest dynamic sea-level changes over time are found in the Southern Ocean and near western boundary currents for both HR-CESM and LR-CESM (fig. S6). Dynamic sea-level changes are also included in the local sea-level rise (Fig. 1, C and D) but do not contribute to GMSLR since the ocean has a volume conservation constraint.

The local sea level is projected to rise over the 2000–2100 period for most of the ocean surface in both simulations (Fig. 1, C and D). In the south of Greenland, the sea-level rise displays a dipole pattern of relatively higher and lower sea-level rise compared to the surroundings. This is associated with a weaker Atlantic meridional overturning circulation in both simulations (21), consistent with currently available observations (26, 27). The sea-level changes in the Southern Ocean are mostly caused by dynamic sea-level changes (fig. S6); however, these patterns differ substantially between the HR-CESM and LR-CESM. For example, there is a notable difference in the South Atlantic and Weddell Sea regions partly caused by the effects of ocean eddies on the path of the Agulhas Current (28). Agulhas Current changes (causing dynamic sea-level changes) contribute up to 60% of the local sea-level rise for the HR-CESM, and this is only up to 25% in the same region for the LR-CESM.

Antarctic basal melt

To further analyze the differences in Antarctic basal melt between LR-CESM and HR-CESM, we follow the procedure outlined in Levermann et al. (7) (see Materials and Methods). Positive oceanic temperature anomalies drive the basal melt of the ice sheets, which are shown here using five different regions in the Southern Ocean (Fig. 2A). For each region, we determined the vertically averaged subsurface temperature over a range of 100 m [for the specific depth ranges of the region, see table 1 in Levermann et al. (7)]. The time series for the five regions are shown in Fig. 2 (B to F) for the CESM simulations; also, the Mercator time mean and variability between 1993 and 2018 is plotted. The HR-CESM control reasonably matches with Mercator for the five different regions. On the contrary, the LR-CESM control is about 0.5 to 1.5C warmer compared to Mercator for the five different regions.

In the second half of the simulation, the LR-CESM temperature starts to deviate from the LR-CESM control temperature. In all regions (Fig. 2, B to F), a warming occurs over time (see also fig. S2D). Significant and positive (lag-)correlations are therefore found between the 2-m global mean surface temperature (GMST) anomaly and the five Southern Ocean regions for the LR-CESM (fig. S7), similar to the ones in Levermann et al. (7). The temperature anomalies are deviations from the respective control simulation (so not with respect to Mercator) and are adjusted for any drift (see Materials and Methods). For the HR-CESM, only East Antarctica and the Ross region (Fig. 2, B and C, and fig. S2C) are deviating from the HR-CESM control and for these two regions, significant (lag-)correlations exist with the GMST anomaly (fig. S7, A and B). There are no positive temperature anomalies (apart from natural variability) for the other three regions over the 101-year period for the HR-CESM and, hence, the lag-correlations with the GMST anomaly for these three regions are much smaller than those in the LR-CESM (fig. S7, C to E). The Southern Ocean subsurface temperature anomalies at the end of the century are positive for the LR-CESM and the largest anomalies are found in the southwestern part of the Weddell Sea (fig. S2D). On the contrary, the HR-CESM displays both positive and negative temperature anomalies in the Southern Ocean (fig. S2C). A nearly linear (positive) dependence of the GMST anomaly on the Southern Ocean temperature anomalies, a typical CMIP5 response (7), does not hold for the HR-CESM.

Most models participating in CMIP6 (see Materials and Methods) have the same horizontal ocean model resolution as the LR-CESM (i. e. ,1, see Table 1). There is a large variety in temperatures of the five Southern Ocean Regions for the CMIP6 ensemble, in particular, for the GMST, Ross region, and Amundsen region (fig. S8). Note that the CMIP6 control simulations cannot directly be compared to our CESM simulations because they have a preindustrial forcing. However, the temperature response of the CMIP6 simulations under the 1% CO2 increase scenario can be compared. The CMIP6 subsurface temperature anomalies (with respect to control simulations) of the last 30 years are shown in fig. S9 and are similar to the ones in fig. S2 (C and D). There is a large variety of temperature responses among the CMIP6 models, and the largest differences between the CMIP6 models are found near the Antarctic continental shelfs. The CMIP6 model mean has a positive temperature response over the whole Southern Ocean. The HR-CESM temperature anomaly pattern (fig. S2C) is most closely correlated with that of the GFDL-CM4 and CNRM-CM6-1-HR patterns, with a spatial correlation pattern value of r = 0.40 (Fig. 3A). The GFDL-CM4 and CNRM-CM6-1-HR have one of the highest horizontal ocean resolution (25 km) of the suite of CMIP6 models and are ocean-eddy permitting. Low correlation pattern values (∣r ∣ < 0.3) are found between the HR-CESM and most CMIP6 models and including the LR-CESM (Fig. 3A). The LR-CESM temperature pattern (fig. S2D) has great similarities with that of the CMIP6 CESM models (CESM2, CESM2-FV2, and CESM2-WACCM), and relatively high correlation pattern values (r = 0.64 to 0.72) are found between the LR-CESM and CMIP6 CESM models (Fig. 3B).

Table 1 Overview of the CESM and CMIP6 models and the spatial resolution of their ocean component lon, longitude; lat, latitude.
View this table:

The GMST is increasing for all CMIP6 models and the HR-CESM and LR-CESM responses are fairly similar to most of the CMIP6 models (fig. S10A). For East Antarctica (fig. S10B) and the Ross region (fig. S10C), the CESM simulations have a similar response to the CMIP6 models, but the anomalies are mainly below the CMIP6 mean. The HR-CESM hardly shows any positive temperature anomalies and is below the CMIP6 model mean for the Amundsen region (fig. S10D), Weddell region (fig. S10E), and the Antarctic Peninsula (fig. S10F). On the contrary, the LR-CESM is warming for these three regions, in particular, for the Weddell region and Antarctic Peninsula during the last 30 years of the simulation. The model mean of the five higher-resolution CMIP6 models (HR-CMIP6) tends to stay below the CMIP6 mean for the Amundsen region, Weddell region, and Antarctic Peninsula, similar to the HR-CESM. For the other two regions, the HR-CMIP6 remains close to the CMIP6 mean.

The temperature anomalies are converted to GMSLR equivalent by using response functions from ice-sheet models, which are subject to basal melt (7). The largest GMSLR as a result of basal melt from the five Southern Ocean regions is the Weddell region and this region also displays the largest spread among the CMIP6 models (Fig. 4). For each region, the GMSLR as a result of basal melt for the HR-CESM and LR-CESM is within the CMIP6 ensemble. However, the HR-CESM is below the CMIP6 mean for each region, which results in a total GMSLR contribution outside the CMIP6 ensemble band at the end of the simulation.

The contribution as a result of basal melt to GMSLR (fig. S5, A and B, red curve) is partly compensated by an increase in snowfall over the AIS (fig. S5, A and B, blue curve). For the HR-CESM (LR-CESM), the AISs warm by a factor of 1.1 (1.3) with respect to the GMST anomaly and snowfall increases at 6.6%C−1 (6.2 %C−1) by Antarctic warming. These rates over the AISs are similar to the ones reported in the literature (29) (e.g., 1.1 ± 0.2 and 5.1 ± 1.5%C−1).

Recent observations (12) indicate that West Antarctica contributes 7.3 mm (55% of total AIS) to GMSLR between 1979 and 2017. Of this 7.3 mm, 6 mm (80%) originates from the Pine Island, Thwaites, and Crosson glaciers. These three glaciers are situated in the Amundsen region. For the HR-CESM and LR-CESM simulations (Fig. 4D), the GMSLR contribution of the Amundsen region is 5.3 and 4.4 mm in 101 years, respectively, which is considerably less compared to observations. In addition, the GMSLR contribution at the end of the simulation from the Weddell region is a factor 1.5, 18, and 3.4 higher compared to that of the Amundsen region for the HR-CESM, LR-CESM, and CMIP6 model mean, respectively. This demonstrates that either the GMSLR contribution by the Amundsen region is underestimated and/or that the GMSLR contribution by the Weddell region is overestimated by the different models.

The discrepancy between observations and models can be explained as follows. The temperature anomaly of the Amundsen region is not always representative for the temperature anomaly near the ice shelves where the basal melt occurs. The time-mean (model years 2071–2100) temperature anomaly of the Amundsen region is 0.08C and 0.15C for the HR-CESM and LR-CESM, respectively (fig. S10D). Over the same period, there are both positive (up to 1.1C) and negative (down to −1.0C) temperature anomalies in the Amundsen region for the HR-CESM (fig. S2C). For the LR-CESM, the temperature anomaly pattern in the Amundsen region is more homogeneous, and we find only positive temperature anomalies varying between 0.09C and 0.34C (fig. S2D).

In the ideal case, one would like to consider the subsurface temperature anomaly for each individual glaciers (e.g., the Pine Island, Thwaites, and Crosson glaciers). However, to resolve the ocean circulation near ice shelves, an even higher horizontal resolution is required [i.e., 200 m; (30)], which is about a factor 15 higher compared to that of the HR-CESM. Even if the temperature anomalies of each glacier is known, the corresponding response function is required and these are not available.

The relatively large GMSLR contribution of the Weddell region in the LR-CESM is due to model deficiencies (Fig. 3B). In all the low-resolution CESM simulations, strong positive temperature anomalies occur near the Weddell region (compare fig. S2D with fig. S9). These positive temperature anomalies in the Weddell region disappear in the HR-CESM (Fig. 3A).

Changes in the Southern Ocean circulation

To understand the difference between the temperature responses of HR-CESM and LR-CESM for the Weddell region and Antarctic Peninsula, the barotropic streamfunction (BSF), which measures changes in the vertically averaged circulation in the Southern Ocean, see Materials and Methods, is analyzed. The time-mean BSF fields (model years 2000–2029) of both simulations are shown in Fig. 5 (A and B). The large-scale pattern of the Antarctic circumpolar current (ACC) is represented in both simulations, as well as the Weddell Gyre and the Ross Gyre. The observed volume transport through Drake Passage is about 135 ± 10 sverdrup (Sv) (31, 32) (1 Sv ≡ 106 m3 s−1), while the modeled volume transport for the HR-CESM control with a time mean of 133 Sv (126 to 141 Sv, 101 years) is close to observations. The LR-CESM control has a much larger volume transport compared to observations with a time-mean value of 160 Sv (152 to 164 Sv, 101 years). It is well known that ocean eddies strongly affect the momentum balances in the Southern Ocean (33) determining the ACC strength.

The difference between the time-mean BSF fields over the years 2071–2100 and the years 2000–2029 is shown in Fig. 5 (C and D) for both HR-CESM and LR-CESM, respectively. The LR-CESM displays an increase in the BSF values between latitudes of 80S to 60S and decreasing BSF values between latitudes of 60S to 40S, increasing the meridional BSF gradient and consequently increasing the ACC strength. For the HR-CESM, the increase in the meridional BSF gradient between these latitudes bands is small. However, there is an increase (more negative) in the meridional BSF gradient close to the Antarctic continent for the HR-CESM, which leads to an increase in the Antarctic Coastal Current (fig. S11, A and C). This current is poorly resolved (fig. S11) in the LR-CESM. The total westward volume transport of the Antarctic Coastal Current (near the Weddell Gyre) in the HR-CESM increases by about 10 Sv (compared to the HR-CESM control) in the last 20 years of the simulation (fig. S11E). This increase in westward transport is related to an intensification of the Weddell Gyre. The wind-stress curl is similar for the HR-CESM between the earlier and later period of the simulation (Fig. 5) and hence is not responsible for this intensification.

The modeled Antarctic sea-ice extent in the HR-CESM control and LR-CESM control reasonably matches with that of observations (1993–2018; see Materials and Methods) but is lower compared to observations (fig. S12, A and B). For example, in the HR-CESM control, LR-CESM control, and observations, the annual maximum sea-ice extent is ranging between 14.8 million and 18.3 million km2 (101 years), 13.9 million and 15.7 million km2 (101 years), and 18.6 million and 20.9 million km2 (26 years), respectively. In the HR-CESM, the sea ice is rapidly decreasing in the last 30 years of the simulation, especially in the Weddell Sea (fig. S12, A, C, and E). Less Antarctic sea ice results in more vorticity input by the wind and, from this, an intensification of the Weddell Gyre. The temporal intensification of the Weddell Gyre in the HR-CESM control (fig. S11E) is associated with Maud Rise polynya and Weddell polynya formation. These Maud Rise polynyas also form in the HR-CESM (around 3E, 65S in fig. S12C). There is no substantial sea-ice loss in the Weddell Sea and no polynya formation (23) for the LR-CESM (fig. S12, B, D, and F) and, hence, the Weddell Gyre strength does not increase (fig. S11, B, D, and F).

An increase in the Weddell Gyre strength will thermally isolate the Weddell region and the Antarctic Peninsula, similar to the ACC thermally isolating Antarctica from the lower latitude Southern Ocean. This explains the absence of subsurface warming for these two regions in the HR-CESM. For the LR-CESM, only the large-scale ocean circulation (i.e., ACC) is affected under climate change, but not the Weddell Gyre and Antarctic Coastal Current. Relatively, warm water from the Weddell Gyre is in this model advected toward the Weddell region and Antarctic Peninsula, leading to relatively large temperature changes there.

DISCUSSION

For the two different versions of the CESM (HR-CESM and LR-CESM), the overall responses to the increase in CO2 [GMST, contributions by glaciers to the GMSLR (34), thermo(steric) effects (21), and surface mass balance changes of the GrIS (35)] are quite similar and compare also well to 31 CMIP6 models analyzed. The projected temperature change and snowfall anomaly over the AIS for the HR-CESM and LR-CESM are also similar to the ones reported in Gregory and Huybrechts (29).

However, the Antarctic basal melt (7) strongly deviates between the HR-CESM and LR-CESM. The HR-CESM and LR-CESM simulations provide GMSLR projections of 5.4 ± 0.3 cm (95% confidence level) and 15 ± 0.8 cm (95% confidence level) through basal melt in 2100, respectively, which gives a factor 2.8 difference. The LR-CESM GMSLR projection of basal melt is within CMIP6 projections, but the HR-CESM projects quite lower GMSLR values with respect to CMIP6 ones. These differences in basal melt are related to the different horizontal resolutions in the ocean component of the models.

The Southern Ocean is a rather complex region where the large-scale ocean circulation, mesoscale ocean eddies, sea-ice formation, and atmospheric processes all play an important role in the response under global warming. Mesoscale ocean eddies are highly relevant for the redistribution and transport of heat and salt (20, 22, 36, 37) and are critical for the correct momentum balance for the large-scale circulation. Explicitly resolving ocean eddies in the HR-CESM does not only lead to a better representation of the present-day subsurface temperature distribution surrounding Antarctica (compared to LR-CESM) but also to a different response under global warming. For the HR-CESM, we find changes on both the large scale (e.g., in the ACC, sea-ice fields) and the regional scale (Weddell and Ross gyres and the Antarctic Coastal Current), while in the LR-CESM (and CMIP6 models), these occur only on the large scale.

Because of the extreme computational costs, there is unfortunately only one high-resolution simulation available for the analysis done here (HR-CESM control and HR-CESM). More of those simulations are required to provide a broader range of GMSLR projections, also under different climate change scenarios. However, the results here already indicate that sea-level projections based on low-resolution climate models should be interpreted with great care, in particular, regarding estimates of the effects Antarctic basal melt.

MATERIAL AND METHODS

Model output

The standard model output of the CESM simulations consists of monthly averaged oceanic fields of sea surface height above geoid [i.e., dynamic sea level (38)], horizontal velocity, temperature, and salinity. For the atmospheric component of the CESM, we analyzed the (near surface) air temperature and (solid) precipitation. Besides, we retained the Antarctic sea-ice fractions from the sea-ice component of the CESM. The HR-CESM and the LR-CESM are spun up by 200 and 500 years, respectively. All the CESM simulations lasted for 101 years. Most of the monthly averaged quantities are converted to yearly averages unless stated otherwise. Two additional LR-CESM simulations (not used) were branched off after a spin-up period of 200 and 1200 years, the latter is used in (21). The shorter spin-up period has an initial sea-ice distribution similar to the HR-CESM control. The drawback of this simulation is that the (subsurface) ocean fields are strongly drifting, and the trends are sometimes in the same order of the applied forcing. The HR-CESM control that has a spin-up period of also 200 years does not show these strong drifts in the ocean fields. A longer spin-up period of the LR-CESM (1200 years) displays hardly any drift in the ocean fields. However, in this LR-CESM control simulation, the sea ice is further equilibrated and the maximum sea-ice extent dropped from 12.8 million to 14.5 million km2 (model years 1200 to 1300). So, results from both additional simulations were not used here.

CMIP6 model output

We use results from the latest release of the CMIP6 and compare these to the output of our CESM simulations (Table 1). We analyzed the model output of the CMIP6 preindustrial control simulations and in which the atmospheric CO2 levels increase each year by 1%. We analyze the monthly averaged oceanic temperature (variable “theato”) fields and near surface air temperature (variable “tas”) fields of the first 101 model years, as is done for the CESM output.

Reanalysis (Mercator)

We retained model output from the Operational Mercator global ocean physical reanalysis product (http://marine.copernicus.eu/services-portfolio/access-to-products/). The ocean component of Mercator (i.e., NEMO) has a horizontal resolution of 1/12, 50 nonequidistant vertical levels, and covers the altimetry era (1993–2018). Mercator assimilates various observational datasets and the model is “steered” toward observations. We retained the monthly averaged temperature fields of the Mercator between 1993 and 2018.

Sea-ice observations

We obtained sea-ice measurements by the Scanning Multichannel Microwave Radiometer and Special Sensor Microwave Imager [http://nsidc.org/data/G02202, (39, 40)]. We retained the monthly averaged sea-ice fraction fields from the NASA Team algorithm with Goddard Quality Check. The sea-ice fraction fields are analyzed between 1993 and 2018, same as for the reanalysis.

Contributions by steric effects

The local steric contribution (ηS) is determined from postprocessing the model output (41). The contribution of both thermal effects and haline effects is determined as the full-depth integral over the specific volume anomaly (42)ηS=∫−H0ρ0−ρ(T,S,P)ρ0dz(1)

The temperature, salinity, and pressure dependency are taken into account while determining the density, and ρ0 = 1028 kg m−3. The steric contribution is expressed as an anomaly with respect to the initial value of the first model year. The globally averaged steric contribution, indicated by ηSg, is shown in Fig. 1, also known as the global mean thermosteric sea-level rise (38). We corrected for any drift in ηSg using the control simulations. After local ocean bottom pressure changes, the local thermosteric sea level eventually becomes ηSg.

Contributions by glaciers

To determine the contribution to GMSLR by the melt of glaciers, we followed the procedure outlined in Church et al. (34). First, the 2-m GMST is determined for the HR-CESM and LR-CESM. Second, we determined the GMST anomaly with respect to the control simulations (see below for determining the anomalies). The time integral of the GMST anomaly between 2000 – t [indicated by l(t)] is used to determine the GMSLR contribution by glaciersηglaciers(t)=fl(t)p(2)where f and p are constants, which are derived from four glacier models, and these values can be found in Church et al. (34) (their table 13.SM.2); negative values of l(t) are set to zero. For the uncertainty in the projections at time t, we take 20% of l(t) as the SD of a normal distribution. Third, we retained 2500 surrogate time series where a random number (independent of time) from the time dependent normal distribution is chosen. Last, we take the model mean over the four glaciers models, the percentile levels of ηglaciers are shown in fig. S3 (A and B).

Each of the 19 regions in the Randolph Glacier Inventory contribute to ηglaciers. Therefore, the contribution of each of the 19 regions to ηglaciers is scaled by its fraction of the global glacier volume. Here, we used the values in (43) of the modeled glacier volume in 2009 (their table 3), and we did not include the contribution of the glaciers in the Antarctic and sub-Antarctic region. The mass loss of each of the 19 glacier regions is multiplied by its fingerprint (due to GRD effects); the results are shown in fig. S3 (C and D). We used the 50% percentile level for ηglaciers in Fig. 1 and fig. S3 (C and D).

Contribution by the GrIS

Changes in the surface mass balance (ΔSMBGrIS) of the GrIS are governed by changes in snowfall and (surface) melt (35)ΔSMBGrIS=ΔSFGrIS−84.2ΔT600JJA−2.4(ΔT600JJA)2−1.6(ΔT600JJA)3(3)where ΔSFGrIS is the yearly snowfall anomaly over the GrIS and ΔT600JJA is the June to August temperature anomaly over the GrIS at 600 hPa. The anomalies are deviations from the control simulations (see below for determining the anomalies). The components of the surface mass balance are shown in fig. S4 (A and B). Negative values of ΔT600JJA are set to zero.

Ice dynamics and the positive melt-elevation feedback are not considered in relation (Eq. 3) because the GrIS topography is fixed (35). Following (34), the terms containing ΔT600JJA (melt terms) are multiplied by a factor E to adjust for the fixed GrIS topography. Here, E is a random chosen number from a uniform probability distribution in the range of 1.00 to 1.15 (time independent). Changes in ΔSMBGrIS are converted to GMSLR (factor of 361.8 Gt mm−1). The GMSLR by changes in the surface mass balance of the GrIS is shown in fig. S4 (C and D); here, we used 2500 surrogate time series for the percentile levels. The mass loss by the GrIS (using the 50% percentile) is multiplied by its fingerprint (due to GRD effects); the results are shown in Fig. 1 and fig. S4 (E and F).

Contribution by the AIS

Changes in the surface mass balance (ΔSMBAIS) of the AIS contribute to a negative GMSLR (29). We determined the snowfall anomaly (with respect to control simulation) over the AIS for the HR-CESM and LR-CESM. An increase in snowfall results in an increase in AIS dynamics (34) and the surface mass balance becomesΔSMBAIS=σΔSFAIS(4)where ΔSFAIS is the snowfall anomaly over the AIS and σ is a random number chosen from a uniform probability distribution in the range of 0.65 to 1 (time independent). We converted the ΔSMBAIS to GMSLR equivalent, and the 50% percentile from 2500 surrogates is shown in fig. S5 (blue curve).

Basal melt by increased oceanic temperatures on the continental shelfs surrounding Antarctica eventually leads to GMSLR (7). We followed the same procedure outlined in (7) and determined the vertically averaged temperature over the five Southern Ocean region (cf. Fig. 2A) for the CESM simulations (Fig. 2, B to F), and the vertical ranges for each region are provided in table 1 of Levermann et al. (7). Next, we determined the temperature anomaly (with respect to control simulation) for each region (indicated by ΔTO; fig. S10, B to F), and the basal melt Δm is defined asΔm(t)=βΔTO(t)(5)where β is the melt sensitivity parameter varying uniformly between 7 and 16 m yr−1∘C−1 (time independent).

Using the appropriate response function [indicate by R(t)] for each Southern Ocean region, the basal melt can be converted to GMSLRΣ(t)=∫0tm(t)R(t−τ)dτ(6)

The response functions are retained from 16 different ice-sheet models (7), and we used the last 101 years for each response function. For each ice-sheet model, we retained 2500 surrogates by varying β, and afterward, we determined the model mean of Σ(t) for the 16 ice-sheet models. The GMSLR as a consequence of basal melt (i.e., 50% percentile) is shown in fig. S5 (red curve), and the contribution for each region is shown in Fig. 4.

The mass gain or loss by the AIS (using the 50% percentile) is multiplied by its fingerprint (due to GRD effects). For the increased snowfall anomaly, we used the fingerprint of the entire AIS. GMSLR as a result of basal melt at the East Antarctic region, half of the Ross region, and half of the Weddell region is multiplied by the fingerprint of East Antarctica. We included half of the GMSLR contribution of the Ross region and Weddell region since both regions consist of an eastern region and western region (12). The GMSLR contributions of the remaining regions (i.e., Amundsen region, Antarctic Peninsula, half of the Ross region, and half of the Weddell region) are multiplied by the fingerprint of West Antarctica. The final fingerprints of the AIS are shown in fig. S5 (C and D).

Dynamic sea-level changes

The sea surface height above geoid (variable “SSH”) is part of the standard output of the CESM, referred to as the dynamic sea level (38). The globally averaged dynamic sea level is about zero since the CESM has a volume constraint for the ocean and does not contribute to GMSLR; we uniformly removed the global mean from the dynamic sea-level fields. Local dynamic sea-level changes are shown in fig. S6 for the HR-CESM and LR-CESM.

Determining anomalies (e.g., of temperature)

The (depth-averaged) temperature anomalies are determined with respect to the control simulations. However, some of the temperature fields display a (significant) drift in the control simulations (fig. S2, A and B). First, we determined the linear trend (αC) of a time series (yC) in the control simulation. This linear trend was subtracted from the relevant time series (y˜=y−αC) and the control time series (yC˜=yC−αC). Next, the time mean of the detrended control simulation (yC¯) was subtracted to retain the anomalies (y′=y∼−yC¯). This procedure was applied to all temperature time series, as well as for the snowfall time series.

Area-weighted correlation coefficient and RMS deviation

First, we determined the area-weighted temperature between 80S and 60S of two climate models, indicated by T¯1 and T¯2. Next, the area-weighted covariance is determined using the following expressioncov(T1,T2)=∑iAi(Ti1−T¯1)(Ti2−T¯2)(7)where Ai is the normalized area of a grid cell i with respect to the total area A. The area-weighted correlation coefficient, r, becomesr=cov(T1,T2)cov(T1,T1)cov(T2,T2)(8)

In a similar way, the area-weighted RMS deviation can be determined usingRMS=∑iAi(Ti1−Ti2)2(9)

For the HR-CESM and LR-CESM, we interpolated the temperature fields onto the Mercator grid and the CMIP6 native grids before determining r and RMS.

Barotropic streamfunction

The barotropic flow is defined as the full-depth integral of the horizontal velocityBF→=∫−H0ν→ dz(10)

Starting from Antarctica (with a value of 0 for the BSF), we integrate the zonal component of the barotropic flow (indicated by BFx) meridionally to determine the BSFBSF(x,y,t)=∫90∘SyBFx(x,y′,t)dy′(11)

For convenience, the average value of the BSF along the African coast line is subtracted from the entire BSF field.