Photo / Hugh Chittock/Antarctica New Zealand, CC BY-SA
By Dan Lowry, GNS Science and Holly Kyeore Han, NASA
As the climate warms and Antarctica’s glaciers and ice sheets melt, the resulting rise in sea level has the potential to displace hundreds of millions of people around the world by the end of this century.
A key uncertainty in how much and how fast the seas will rise lies in whether currently “stable” parts of the West Antarctic Ice Sheet can become “unstable”.
One such region is West Antarctica’s Siple Coast, where rivers of ice flow off the continent and drain into the ocean.
Journal of Geophysical Research, CC BY-SA
This ice flow is slowed down by the Ross Ice Shelf, a floating mass of ice nearly the size of Spain, which holds back the land-based ice. Compared to other ice shelves in West Antarctica, the Ross Ice Shelf has little melting at its base because the ocean below it is very cold.
Although this region has been stable during the past few decades, recent research suggest this was not always the case. Radiocarbon dating of sediments from beneath the ice sheet tells us that it retreated hundreds of kilometres some 7,000 years ago, and then advanced again to its present position within the last 2,000 years.
Figuring out why this happened can help us better predict how the ice sheet will change in the future. In our new research, we test two main hypotheses.
Testing scenarios
Scientists have considered two possible explanations for this past ice sheet retreat and advance. The first is related to Earth’s crust below the ice sheet.
As an ice sheet shrinks, the change in ice mass causes the Earth’s crust to slowly uplift in response. At the same time, and counterintuitively, the sea level drops near the ice because of a weakening of the gravitational attraction between the ice sheet and the ocean water.
As the ice sheet thinned and retreated since the last ice age, crustal uplift and the fall in sea level in the region may have re-grounded floating ice, causing ice sheet advance.
AGU, CC BY-SA
The other hypothesis is that the ice sheet behaviour may be due to changes in the ocean. When the surface of the ocean freezes, forming sea ice, it expels salt into the water layers below. This cold briny water is heavier and mixes deep into the ocean, including under the Ross Ice Shelf. This blocks warm ocean currents from melting the ice.
AGU, CC BY-SA
Seafloor sediments and ice cores tell us that this deep mixing was weaker in the past when the ice sheet was retreating. This means that warm ocean currents may have flowed underneath the ice shelf and melted the ice. Mixing increased when the ice sheet was advancing.
We test these two ideas with computer model simulations of ice sheet flow and Earth’s crustal and sea surface responses to changes in the ice sheet with varying ocean temperature.
Because the rate of crustal uplift depends on the viscosity (stickiness) of the underlying mantle, we ran simulations within ranges estimated for West Antarctica. A stickier mantle means slower crustal uplift as the ice sheet thins.
The simulations that best matched geological records had a stickier mantle and a warmer ocean as the ice sheet retreated. In these simulations, the ice sheet retreats more quickly as the ocean warms.
When the ocean cools, the simulated ice sheet readvances to its present-day position. This means that changes in ocean temperature best explain the past ice sheet behaviour, but the rate of crustal uplift also affects how sensitive the ice sheet is to the ocean.
Veronika Meduna, CC BY-SA
What this means for climate policy today
Much attention has been paid to recent studies that show glacial melting may be irreversible in some parts of West Antarctica, such as the Amundsen Sea embayment.
In the context of such studies, policy debates hinge on whether we should focus on adapting to rising seas rather than cutting greenhouse gas emissions. If the ice sheet is already melting, are we too late for mitigation?
Our study suggests it is premature to give up on mitigation.
Global climate models run under high-emissions scenarios show less sea ice formation and deep ocean mixing. This could lead to the same cold-to-warm ocean switch that caused extensive ice sheet retreat thousands of years ago.
For West Antarctica’s Siple Coast, it is better if we prevent this ocean warming from occurring in the first place, which is still possible if we choose a low-emissions future.
Dan Lowry, Ice Sheet & Climate Modeller, GNS Science and Holly Kyeore Han, NASA Postdoctoral Fellow, NASA
This article is republished from The Conversation under a Creative Commons license. Read the original article.
![New Zealand In Depth12 Dec 2024 'Giant batmobiles': In search of the manta rays of the Hauraki Gulf 8:49 am on 12 December 2024 Share this Share on Twitter Share on Facebook Share via email Share on Reddit Share on Linked In Kate Newton Kate Newton , Senior Journalist, In Depth @katenewtonnz Every summer, the world's largest ray species cruises the waters of the Hauraki Gulf. Researchers are racing to find out more about these charismatic creatures as their habitat continues to decline around them. The plane's shadow skims over the wide ripples of the ocean, its twin engines throttled back into a low burr, searching. To the left in the distance is the Tutukaka coast and Whangārei Heads, and to the right, the Hauraki Gulf fades to a hazy horizon. But our eyes are fixed on the surface of the water just a few hundred metres below us. Then, crackling through our headsets, comes the one word we've been waiting to hear: "Manta." Exclamations erupt over the comms and everyone's eyes now swivel to where Catherine Meyer, seated near the front of the 10-seater on the left, is gesturing out the window. "My nine o'clock." The plane banks round into a tight loop. There, unmistakable against the peacock-blue, is the black diamond of an oceanic manta ray, te whai rahi - the largest ray species on the planet. A photo of a manta ray photographed from a planeDespite reaching up to nine metres wingtip-to-wingtip, spotting a manta ray from above still requires a sharp eye. Photo: RNZ / Cole Eastham-Farrelly For the past half-hour, we've been following the 200-metre depth contour beyond the Poor Knights Island group, along a route determined earlier that morning at Claris airport on Great Barrier Island. There, we rendezvoused with Lydia Green - island resident and project director of Manta Watch NZ, a charitable trust she founded in 2017 and has run mostly solo ever since. Manta rays, which can measure up to nine metres wingtip-to-wingtip, are found worldwide. Until recently, though, there was only anecdotal evidence of them visiting and living in New Zealand waters. Once Green and a crew of researchers and volunteers started running boat surveys, and encouraging citizen scientist sightings from fishing boats and other vessels, verified sightings exploded. Since 2020, there have been more than 1200 manta ray sightings between North Cape and Wellington, and Manta Watch has satellite-tagged 20 individuals that have been tracked as far north as the tropical Pacific. What Green really wants to prove is that these manta rays are no passing visitors, but a native population that breeds here. Consistent, frequent data collection is crucial. Despite being globally endangered, the species is classed as 'data deficient' in New Zealand - denying it the greater protections that would come with being declared locally endangered. The manta ray may need those protections. The biodiversity-rich Hauraki Gulf has been in a state of decline for decades, and last-minute carve-outs to the Hauraki Gulf / Tīkapa Moana Marine Protection Bill, a piece of legislation years in the making, have dismayed marine researchers and environmentalists. Today's flight is the first aerial survey of the gulf's manta ray population - covering in just a few hours an expanse of ocean that would take days and days to survey by boat, and taking Green ever closer to cracking the manta code. An aerial view over Claris and Kaitoke on Great Barrier Island's east coastThe survey flight took off from Claris on Great Barrier Island's east coast. Photo: RNZ / Cole Eastham-Farrelly At Claris, we board an angular, blunt-winged Britten-Norman Islander - a Swiss Army knife of a plane with big windows, and perfect for the low-altitude, low-speed cruising necessary to spot rays from above. From take-off, it takes just a few minutes to clear the hills of Great Barrier, where tree fern crowns pop like green starbursts from the otherwise dark bush. Then we're out over the ocean again, and already Green has spotted something. There's a work-up on the surface, where fish gather to feed, and in turn attract birds and larger marine animals. There's no ray action, though, and we head further out into the gulf. At the controls is Island Aviation pilot Hobby Hafiz, who has also flown aerial surveys in this area for BBC's Blue Planet. Green sits up front next to him, quietly directing Hafiz towards anything of interest she spots on the surface. Lydia Green is wearing a red tshirt and sunglasses and rests her arm on the back of a seat while sitting onboard a small plane.Manta Watch NZ project director Lydia Green prepared the route for the flight based on known manta ray hotspots. Photo: RNZ / Cole Eastham-Farrelly Also on board are World Wildlife Fund New Zealand chief executive Kayla Kingdon-Bebb, and two volunteer spotters and marine science postgraduate students, Sanaa Nair and Catherine Meyer. For Nair, this is the best day of her short career so far. As soon as she discovered Manta Watch existed, she pestered Green via email for a year, before eventually tracking her to a conference and demanding to volunteer. "I had no idea [manta rays] were here until two years ago. It just blew my mind." Manta rays have the largest brain of any fish, have no stinger, and eat only krill - like whales, they are gentle giants of the ocean. From her first sighting on a boat, Nair was captivated by these "large, beautiful, charismatic, insanely intelligent" animals. Armed with clipboards, she and Meyer take a side each, resting their foreheads against the windows as they scan intently for signs of life below. Catherine Meyer holds a pen ready to take notes while looking out a plane window.Marine science postgraduate student Catherine Meyer was one of two dedicated spotters on the flight, noting down the time and details of each manta ray sighting. Photo: RNZ / Cole Eastham-Farrelly Before Meyer spots that first ray, Green already has a working theory that they'll congregate along the contour lines, where the depth of the ocean floor changes. Here, the upwelling of fresh ocean water into shallower areas brings with it an abundance of the zooplankton that manta rays love to feast on. And after that first sighting, they're suddenly everywhere. It's all Meyer and Nair can do to note down the details of each one: the time it's spotted, its colouring, what kind of behaviour it's exhibiting. "I can't write fast enough!" Meyer says, but it's not a complaint. The excitement has infected everyone on board, shuffling and turning in our cramped seats every time someone says that magic word, "Manta." Hafiz the pilot joins in on the action, his keen aviator's eyes making him a natural - the rays may be big, but they're still tricky to spot from afar. Manta rays aren't all we spot. There are pods of dolphins, a Bryde's whale, flocks of pelagic birds, and a fever of 15 devil rays - the other species Manta Watch NZ aims to document and protect. Sanaa Nair, wearing an aviation headset, looks out a plane windowManta ray spotter Sanaa Nair pestered Lydia Green for a year to let her volunteer. Photo: RNZ / Cole Eastham-Farrelly By the time we land in Whangārei for a refuelling stop, the team has found 17 manta rays - a mix of the more common 'chevron' rays, which are black with white shoulder markings, and melanistic rays, which are completely black: "like giant batmobiles", Green says. The route back to North Shore airport takes us through the inner Hauraki Gulf, where we spot one more cruising manta ray to add to the day's total. "It's like a huge, exciting game of hide and seek," Green says the day after. The residual "manta buzz" kept her awake most of the night. "They're such an elusive animal and the biggest thing for me was just having a good idea of where I thought they'd be, because it's the first time we've ever conducted a dedicated aerial survey." Confirming her suspicion that manta rays would be found on the depth contour lines was "massive", she says. "I was just really proud of the knowledge that we've been able to collectively gain." Sanaa Nair, Lydia Green and Catherine Meyer laugh together sitting outside a small airport terminal.Sanaa Nair, left, Lydia Green and Catherine Meyer share their excitement together while going through a final pre-flight briefing. Photo: RNZ / Cole Eastham-Farrelly The concentration of sightings in the outer Hauraki Gulf also supports another hypothesis: that the rays stay out in deeper water at this time of year, and then as summer progresses and blue oceanic water inundates the gulf, they follow their food sources further inshore. "It sets a really good foundation for how we're going to conduct all of our survey efforts moving forward this year," Green says. From now until autumn, Green's life will be dictated by wind and tide. As Manta Watch's only permanent staff member, she juggles fundraising, upskilling volunteers, and doing outreach education, but her overriding concern at this time of year is organising vessels and crew for the boat surveys that the trust hopes to conduct over the season. Born in the middle of the UK, hours inland, Green nonetheless had an early and deep connection to the sea. "I was always drawn to bodies of water and I had an aquarium. And basically as soon as I had autonomy over my body, I went to the coast and I never looked back." She completed a marine biology degree and then dedicated her life to following the ocean, continuing to learn from first-hand experience. Seventeen years ago, she fetched up in New Zealand, and when she and her partner moved to Great Barrier four years ago, manta rays became a full-time job. An aerial image of the east coast of Great Barrier Island.Much of Manta Watch NZ's work is based out of Great Barrier Island in the Hauraki Gulf, where project director Lydia Green lives. Photo: RNZ / Cole Eastham-Farrelly The trust's work has grown organically, helped along by Green's "manta husband" Mark Erdmann, a coral reef ecologist based at the University of Auckland and non-profit organisation Conservation International. "He owns a boat, which is very helpful. He enabled the project to really grow in terms of the satellite tagging project." At other times, Green's work maintains a DIY element. She owns a drone, and will often put it up from shore at Okupu, the island settlement where she lives. At the height of the season the rays come close enough in that she's been able to collect footage of them, adding to the growing tally of sightings. But what she really wants is to get as many people involved as possible. Manta Watch will launch its citizen science app this week, which will let anyone who spots a manta ray log a sighting with the time, date, GPS location and other details such as photographs. Over 70 percent of the trust's sightings come from the general public, but Green says there is a lot of to-and-fro at the moment to glean the details they need to verify a sighting, and get good-quality images where they exist. Flourish logoA Flourish data visualization "With the app, it just streamlines everything and hopefully cuts out a big chunk of admin for our project. "The really cool function is that you can upload everything offline because a lot of these areas don't have cell service … and then it will upload when you've got signal." But at the same time that Manta Watch is seeing sightings rocket as people become aware of them and go "manta crazy", the ray's habitat is also under threat. The Hauraki Gulf is in "a state of advanced decline", WWF's Kayla Kingdon-Bebb says. "We've seen that through successive State of the Gulf reports produced through the Hauraki Gulf Forum. We're very concerned to see the health and mauri of Tīkapa Moana restored. We know that oceanic manta rays are a sentinel species, so they can tell us a lot about the health of the ecosystems in which they reside." Negotiations and give-and-take over more than a decade between ecologists, locals, mana whenua, commercial and recreational fisheries interests led to the drafting of the Tīkapa Moana / Hauraki Gulf Marine Protection Bill, which was introduced under the previous government and supported by the current government. The Bill will create 19 new protected areas, tripling protected areas from 6 percent to 18 percent of the gulf. Some of those are 'high-protection' areas, where fishing and other extractive activity is forbidden. A manta ray with white markings on its black back cruises in bright blue waterManta rays are globally endangered but the are classified as 'data deficient' in New Zealand as very little was known about them here until recently. Photo: Manta Watch NZ / Mark Erdmann](https://aoteagbi.news/wp-content/uploads/2024/12/4KFBYNY_NZ_MB_0089_2022_02_22_8_1_Mark_Erdmann_png-600x338.webp)
