A photo of a polar bear walking on melting ice surrounded by water as the sun sets over the horizon. The background of the photo shows the article's headline and byline. The text reads: On Thin Ice. Melting ice shelves spell big trouble for sea levels. Climate Scientist David Holland is working at the ends of the earth to figure out just how much. By Audrea Lim

David and Denise Holland were closing down their camp near Greenland’s Helheim Glacier one night in 2018 when an awesome noise like an approaching train broke the stillness. Standing high on a cliff, with the midsummer sun still hovering high above the horizon, the couple looked down onto the vast expanse where ocean meets ice and saw a colossal chunk of the glacier breaking off. The mass was one-third the size of Manhattan—and, as it fell, was cracking into fragments, plunging into the frigid waters below. “First of all, you scream and shout, because it’s just so amazing to watch,” recalls David, director of NYU’s Environmental Fluid Dynamics Laboratory and NYU Abu Dhabi’s Center for Global Sea Level Change. The surround-sound thunder like gunshots and explosions “kept going and going and going,” he says. “It was just 20 minutes of pure adrenaline.” But in that same moment, another set of thoughts was also racing through his head: “Oh, my gosh, this is so complicated!” To predict future sea level rise, “we need to understand what we saw,” he reflects. “And we are so far from understanding.”


What the Hollands witnessed is known as a calving event, when ice shelves fracture and pieces fall into the ocean to become free-floating icebergs. David has been studying them since the ’90s. While they are a normal occurrence, recent accelerated calving has caused entire ice shelves to collapse in Greenland and Antarctica. When snow accumulates on land, lowering the global sea level, it’s called land ice and can be several miles thick. When land ice flows toward the ocean, thanks to gravity, it forms a marine ice sheet. This displaces ocean waters and raises the global sea level. The marine ice sheet thins as it spreads and goes afloat on the ocean, where it accumulates onto mile-thick ice shelves floating in the water but still anchored to the land ice, like a crust. Because ice shelves are in contact with water, they are more prone to melting, which can lead to a calving event. Ultimately, the ice shelves hold back the land ice and slow its progression into the ocean. When a piece of ice shelf calves off, it makes way for more land ice to flow into the ocean. That is why understanding calving is critical for grasping why the fate of the planet hangs on the issue of ice.

David grew up in St. John’s, Newfoundland and Labrador, where snowstorms were an opportunity to miss school, and he was fascinated by the Arctic explorers who braved the northern ice a century ago. That wonder stayed with him, and he studied sea ice for his PhD. Sea ice has received more media attention than the marine ice sheets he now studies. Sea ice is like an ice cube that melts in a glass of water, having no impact on the level of the water. Sea ice does affect the planet’s color, its whiteness reflecting sunlight into space, which helps keep the planet cool. The disappearance of sea ice is thought to be caused by global warming, and the replacement of white sea ice by blue ocean waters will push temperatures even higher. But a marine ice sheet’s impact punches above its weight, as it not only affects the planet’s color but also controls the global sea level.

David turned his attention to the ice shelves after joining NYU’s Courant Institute of Mathematical Sciences in 1998, approaching the subject of ice melt from a theoretical angle, and creating models of how calving might occur from warming ocean currents and shifting winds. It was “virtual reality,” as he refers to the models, intended to forecast different scenarios for the future based on changing conditions, like a TV weather forecast showing high- and low-pressure systems feeding a hurricane near Haiti, then pushing it toward the US coast. “You could play the math model just like Pixar movie animation, and you can see these different scenarios,” he explains. For a decade, he sat at his desk creating such climate models. He predicted how ocean currents would melt the ice and raise sea levels, “but with no way to say that was right, that it mattered.”

All of that changed with an accidental discovery. In the late ’90s, satellite images began showing parts of Greenland falling apart at an alarming rate. The melting Jakobshavn Glacier drains Greenland’s ice sheet—its land ice—to the Ilulissat Icefjord, a shelf hanging into the sea, and David’s models postulated that warm water had caused the calving. But scientists lacked evidence to support this, until the Hollands learned that Greenland’s fishermen had been attaching Post-it-size sensors onto their nets for years, thanks to research efforts by local fisheries. The fishermen shared the data with David. “We couldn’t believe it,” he recalls. “It was this incredible historical record that nobody had seen,” showing that the same period when the glacier was disintegrating corresponded exactly to when, according to the fishnet sensors, warm water suddenly arrived in Greenland. It turned out that part of the Gulf Stream, a warm ocean current flowing past Miami toward Europe, had looped around and crashed into Greenland. The reasons for this are still unclear, but the discovery showed that David’s model had legs, and it also fine-tuned his scientific understanding of how ice melt is connected to sea level rise and climate change. “We had done 15 years of theoretical work first, and then we saw all these events actually happening in nature, which was kind of weird,” he reflects. “We were kind of primed to understand it.”

A collage of three photos showing David and Denise Holland standing on a ship; a red helicopter being loaded off the back of a military transport aircraft on a field of snow; and a large iceberg in the water alongside a ship.

The Hollands’ polar expeditions require ships, helicopters, and military transport aircraft.

David pivoted to field research and to the questions that would continue firing off in his mind years later as he watched a huge piece of the Helheim Glacier collapse—an event so complicated that he wondered, “How are we ever going to produce a forecast?” Unlike the weather, which researchers have been measuring and studying for over a century, scientists do not understand the mechanisms that cause ice shelves to fracture and melt. “The weather and the flow of air is relatively simple science in comparison to the science of trying to understand something that breaks,” he explains. But above all, they needed more data: temperature sensors in the water, and sensitive radar detectors to measure movement and fracture in the ice. “Hurricanes are complicated, and we have mastered that to some degree, so maybe it’s just a few decades of research and we’ll get there,” he says.

For the past 15 years, Denise has been organizing and running their research expeditions to Greenland and Antarctica as the field manager, tackling the logistical problems that have all but prevented researchers from gathering ample data. The first problem: remoteness. The ice sheets sit at the top and bottom of the planet, unlike the weather stations that scientists have set up at every major airport and throughout the continents of the world. She ships their sophisticated robots and testing equipment from New York City to the polar regions on US Air Force planes, but from there they can’t just rent a truck—there aren’t even roads along most of the coast. She charters helicopters and icebreakers to navigate the local waters, and she plans meticulously. “If I forget one bolt or one screw,” she says, “the whole thing falls apart”—or they may have to pay someone thousands of dollars to ship it.

The second problem: the harshness of the climate. A few years ago, one of their icebreakers sank. The Icelandic captain was on his way to meet them in Greenland, his family accompanying him on board, when the sea ice crushed his boat “out in the middle of nowhere,” says David. A giant military rescue helicopter arrived, but its rotors created heavy waves and couldn’t complete the mission. Finally, a smaller helicopter lifted the captain and passengers off the boat before it sank. Helicopters have their own risks too. Many of the sites they visit can’t be accessed by boat, but helicopter engines sometimes fail mid-flight, for instance. Thankfully, the engines usually come in twos.

A photo of a seal on a rock next to the water while wearing a temporary sensor with an antenna on its back.

In Greenland’s icy waters, ringed seals with temporary sensors can reach unnavigable sites.

Denise happened upon an innovative workaround to this problem several years ago. At a logistics meeting in Greenland, a biologist mentioned the wealth of data they were gathering on the continent’s wildlife, including studies of seal habitat and behavior. “There is a great opportunity here to extend that data collection to include things that we want to know about the ocean in Greenland,” she told him. The Hollands were curious about the temperature, salinity, and depth of the waters where the seals traveled, in the waters just off the ice shelf. The biologist suggested that they attach tags to the seals’ necks—muffin-size sensors that fall off after a year, whenever the seals molt their fur. “So we just started a collaboration, and it’s been really useful, really interesting,” she says. They now tag six seals a year—three on the east coast, three on the west. But the logistical and scientific challenges remain abundant. Understanding why the ice shelves break is a mystery, akin to the earthquakes that scientists have yet to predict, and the Hollands are installing radar sensors close to the glaciers to measure their movements. But Greenland’s forceful winds “rip everything apart,” says David. “That’s been part of the slowness of understanding things.”

However, the “biggest threat to sea level in the world” resides on the opposite pole: the melting of the ice sheet in Antarctica, which is “bigger, badder, more difficult” to get to, and therefore much less understood. In the brief, sunny window of January, the Hollands sailed with a team from New Zealand to Antarctica, then helicoptered to where they set up camp by the Thwaites Glacier. It is disintegrating from the warm Antarctic Circumpolar Current. Drilling through the half-mile-thick ice shelf with hot water, they dropped a temperature sensor into the ocean below, which will record and transmit data for the next few years. “We’re drilling one hole, not a thousand holes, because you only have one month before you have to leave,” says David. A helicopter can only bring so many barrels of fuel, limiting the number of sites they can visit. “If you don’t leave by then, you’re never leaving, because nobody can get there,” he says. “The weather goes too bad.”

A photo of a red helicopter hovering over a small pool of water that's completely surrounded by ice.

Helicopter probe drops into the Ilulissat Icefjord measure ocean water temperature, salinity, and depth.

When they drilled a hole in the glacier in 2020, it was the first time scientists had recorded unusually warm water flowing underneath. Their discovery was made at the glacier’s grounding zone, which is key to its overall rate of retreat. Because the Thwaites Glacier is roughly the size of Florida, its collapse could have significant global impact. But so many unknowns puncture the overall scientific understanding about sea level rise that there’s no way to forecast exactly how, when, and by how much it will rise, says David. “The big picture is that the planet is warming,” he says, and “that humans have had an influence on that warming.” But we have a weak understanding of how humans have altered global wind patterns, how the winds are changing ocean currents, and how these currents are fracturing the polar ice shelves. David’s research has made him more realistic about climate change: it is happening, but scientists are far from understanding how it will play out.

Despite the challenges of carrying out this research, most outdoor adventurers can only dream of visiting these polar locales, which “are beyond beautiful,” says David. “Just the sense of being on an ice sheet, on a glacier, and there’s no one around for a thousand miles ...” Except for the wildlife. “Giant [polar bear] tracks always make us nervous. They’re always walking by, and you don’t want to meet them.” In Antarctica, the penguins are “supremely amazing,” always standing together in silence. “They stare at you like they’re saying, ‘What are you doing?’ ” he says with a chuckle. “And they always win the staring contest.” Recently, Denise was cooking bacon at a remote camp in Greenland when the pair spotted an arctic fox watching them from behind a rock, “like ‘You can’t see me!’ ” he recalls. Denise put out some bacon. (Though “you shouldn’t do that [more than a few times] because they become dependent,” David notes.) The fox accepted the gift and was deeply impressed. The next day, she brought her whole family—two little cubs. Research, for the Hollands, is wilderness camping in a place where “everything around you is ridiculously beautiful,” David says. “So doing it as a family unit is perfect.”

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Photos from top: Paul Souders/Getty Images; courtesy of Denise Holland