by Susan Casey
Disappointed not to have finished her survey of the Ellett Line, Holliday wasn’t initially excited about the consolation prize: examining the data from Discovery’s wave recorder. “Lots of people said, ‘Oh, you must write something about those waves,’ ” Holliday said. “But I never really got round to it. And I’m not an expert on waves.” It was only in 2005, when another paper was published touting the ninety-one-foot waves that were measured in Hurricane Ivan, that Holliday’s competitive spirits were piqued: “I thought, ‘Hmmm, we’ve got bigger waves than that.’ ”
Yelland urged her on, interpreting the wind measurements and crunching the numbers. Even with countless hellacious ship days under her belt, she was floored by what Holliday and the others had endured.
“And we weren’t even in the windiest spot!” Holliday said, pointing to a graph on her paper that plotted wave heights and wind speeds. “So we probably weren’t in the place where the waves were at their highest.” She flinched behind her desk. “But they were high enough for me!”
Though the relationship between wind and waves has been well charted by science, neat formulas demonstrating that if the wind does x then the ocean will do y, one of the most intriguing aspects of Discovery’s ordeal was that the biggest waves did not accompany the strongest winds. Rather, the hundred-footers showed up more than a day after the most violent gusts had subsided, at a time when the scientists believed the worst was over. “The point is that all of these previously measured [giant] waves were under hurricane conditions, really extreme conditions,” Holliday said. “But our big waves weren’t.”
All of which begs the obvious question: What, then, did cause the “largest ever recorded” extreme waves?
Holliday and Yelland believed it was an effect known as “resonance,” an aspect of nonlinearity that is endlessly complex when scrawled across a whiteboard and kindergarten-simple when explained by the analogy of a kid pumping his legs on a swingset, dramatically boosting his height on each pass. Energy is continually being added to the system, more and more and more, in erratic bursts, until the swing can go no higher. Likewise in the North Atlantic, wind energy surged into the waves until they grew to enormous proportions. “The wind was actively forcing the growth of the waves for a very long time,” Yelland said. “So they kept building and building and building.”
“The wave model that we were looking at didn’t predict the waves that we actually encountered,” Holliday added. “It got the wind speed right, and it got the arrival time of the waves right, but they were much smaller than the actual waves we measured. So the implication—the concern—is that these big waves are out there, and if the models aren’t reproducing them, then the engineers who are using the models to design their ships or whatever, well, they might not be looking at the right limits.”
She paused to let this statement sink in. “I went to the library and looked through fifty years of synoptic weather charts,” she added. “The conditions we encountered were not that unusual. And so I think what we are trying to say is that these waves happen more often than we realize. We’re just not measuring them.”
During my time in Southampton I often felt as though I’d dropped into a parallel universe concerned entirely with waves and water; a place where everyone spent their days thinking about the ocean and yearned to uncover its most closely guarded secrets. In every corner of NOC, behind every stack of books, tucked into a warren of offices and conference rooms and libraries and labs, there was someone devoting his or her life to grappling with the mercurial sea. Rapid-climate-change specialists and tsunami experts crowded into the quayside cafeteria with submersible operators and marine biologists; wave physicists and sonar technicians nodded to storm modelers and ship captains in the halls. Brilliantly colored bathyspheric maps—depicting the slashes, trenches, and subduction zones rent into the seafloor, the lively underwater places where the earth is shifting and splitting and bashing into itself—lined the walls. “We know more about the surface of the moon than we do about the deep ocean,” the center’s press materials read. Its daily activities aimed to change that state of affairs.
“I tend to work on landslides and geohazards of the deep sea, from which tsunamis are spawned,” Dr. Russell Wynn told me by way of introduction. Though I had come to Southampton to learn about Holliday’s voyage, I also wanted to meet as many of the resident wave experts as I could. Wynn was a tall, lanky guy in his mid-thirties with sleek features and an intense presence. He sat in a spacious office with graciously high ceilings on the institution’s top floor. These days, he explained, tsunami science was hopping due to technological advances and increased interest after the horrific Indonesian waves of 2004. Suddenly funding had become available to determine threat levels elsewhere. “We’re coring all the way along the European Atlantic margin to Northwest Africa,” Wynn said, describing the process of drilling a probe into the seafloor and then examining the earth’s layers to discover what furious geological events had previously occurred there. From those findings it would be possible to deduce the odds of similar upheavals in the future.
This information matters more to western Europe than most of its inhabitants know. Though the Pacific is credited with all manner of killer-wave potential, the Atlantic has hosted its share of trouble. Prehistoric evidence depicts much giant-wave damage along the coast of Scotland. Sixty-seven tsunamis have struck Italy in the past two thousand years; in 1908 a powerful one in Messina killed eight thousand people. Devastating tsunamis have also hit such unlikely spots as the Virgin Islands (1867); Nova Scotia (1929); and even Monaco and Nice along the French Riviera (1979). In 1755 a tsunami leveled Lisbon, Portugal, killing sixty thousand people. The 8.8 earthquake that caused it was felt all the way to England; at its epicenter the waves it produced were more than fifty feet high. The tsunami had also surged north, busting into Irish harbors and British ports, and east, causing death and damage as far away as Puerto Rico.
Wynn had also logged much time studying past volcanic collapses in the Canary Islands, and he disagreed with Bill McGuire’s notion that the Cumbre Vieja volcano threatened the entire Atlantic basin with a megatsunami: “We’ve sort of locked horns a few times.” After having identified, cored, mapped, dated, and analyzed the seabed deposits around La Palma, Wynn believed the west flank of the island would crumble in bits and pieces, resulting in far smaller waves: “What the mud tells us is that these things are not as big a hazard as has been proposed.” He clicked on his laptop and pulled up a 3-D animation of La Palma’s western flank falling into the sea, rendered in pretty shades of green. It looked as though a set of giant jaws had bitten off half the island, spitting bungalow-size blocks of rock onto the ocean floor. Even at diminished size the resulting waves would make for dark days in the Canary Islands, but they wouldn’t make it across the Atlantic or even to British shores. But while Wynn did not buy into the severity of this particular scenario, he did second another one of McGuire’s concerns: that climate change will lead to increased tsunami risk worldwide.
“The sea level going up and down has a big impact on landslides,” he said, sitting at his desk, his hands clasped above a sheaf of papers and a book titled Surviving the Volcano. “And it can also have a big impact on the number of earthquakes you get.” Wynn’s voice was crisp and authoritative, and he spoke with the measured calm of someone who dealt in geologic time. There might be disasters, yes, but they would be handled. The content of what he was saying, however, left a different impression.
Geologists now knew, he told me, that when the last ice age ended about ten thousand years ago it had not gone quietly, but rather in a flurry of seismic fits. Millions of tons of melting ice drastically hiked sea levels, causing the entire ecosystem to go tilt. The ground trembled and shook; volcanoes that had been sleeping for aeons suddenly sparked to life. As the planet’s chemistry and equilibrium were knocked askew, the oceans seethed. It was not hard to see the parallels between that time of upheaval and our current situation, in whi
ch glaciers are dwindling at a startling clip. “Isostatic rebound,” McGuire had called it, a simple principle with awful implications.
“You start to load more water onto bits of the seabed that might not like being loaded,” Wynn explained, “and therefore they fail—that failure being in the form of an earthquake. I mean it doesn’t sound like much, but if you raise the sea level by a centimeter, and you extrapolate that centimeter of water across several hundred thousand square kilometers of sea bed, that is actually an enormous, enormous load. And at the moment, well, the sea level’s starting to go up quite quickly.”
“Quickly?” I asked. I’d heard scientists describe the oceans as rising “steadily,” and “inevitably,” but no one had put it in quite such immediate terms before.
“Yes,” Wynn said. “And it’s only likely to accelerate. So it could be that we may enter a phase of more instability of the seabed.”
If you were looking for the perfect set of circumstances to create memorable tsunamis, a fast-changing, unstable, undersea environment would be at the top of your checklist. The relationship is straightforward: when large batches of rock and sediment move around down there, mayhem is uncorked on the surface. Along with any extra earthquakes we can expect to roil the bottom, there is also another concern. “The land is eroding much faster than it did, say, a thousand years ago,” Wynn noted. “There’s more sediment being shed into the ocean now.” That silt, sand, soil, and other material then heap up underwater, a stockpile of extra ammunition for the next slide. “So the ocean’s stormier and the sea level starts to rise; there’s increased load on bits of the seabed and bits of the earth, and the possibility for future landslides is greater.” He sighed. “It doesn’t take a genius to work it out. Sooner or later these things are going to come back and bite us.”
For a demonstration of just how destructive landslide-induced tsunamis can be, scientists point to the Storegga Slide, a catastrophic slump in the North Atlantic that happened about 8,200 years ago. A section of Norway’s continental shelf the size of Kentucky gave way, plummeting down to the abyssal plain and creating a series of titanic waves that roared forth with a vengeance, swiping all signs of life from coastal Norway clear to Greenland (and reaching as far south as England). The Shetland Islands were especially hard-hit, by tsunami waves that likely measured over sixty-five feet high. Farther south, the waves drowned a Wales-size landmass that connected Britain to the Netherlands, Denmark, and Germany. (In other words, Britain wasn’t always an island.)
So what caused the underwater slide that produced the Storegga waves? Scientists aren’t entirely sure. An earthquake in the North Atlantic, perhaps. But there’s another, eerier possibility. It may have been due to a blowout of methane hydrates, gas deposits that are frozen into the seafloor. These gobbets of ice, which look like tiny snowballs (but generate a gas flame when lit), carpet the world’s oceans. In particular they are clustered on the continental slopes, ideal for landslides. Methane hydrates are hair-trigger sensitive to changes in pressure and temperature: one extra degree is enough to melt them. When they release, they not only collapse the seafloor around them, causing landslides, they can burp vast clouds of methane into the atmosphere—a greenhouse gas ten times more potent than carbon dioxide. As for how much methane is currently (and, for now, safely) frozen down there, the United States Geological Survey conservatively estimates that these submarine ice balls contain twice the carbon to be found in all known fossil fuels on earth.
It was Wynn’s job to spin present-day fact from these geological mysteries of yore, to apply rigorous science to the “what-if” disaster scenarios and pull solid probabilities out of fearful conjecture. “We’re the guys who say, well, in this particular area tsunamis are going to happen every hundred years, or whatever,” he explained. In the Canaries, for example, Wynn and his colleagues figured that major slides and tsunami events took place approximately every 100,000 years. (The last one took place 15,000 years ago.) There was a disclaimer on that return rate, however: “For geologists nothing’s certain. So I couldn’t put my hand on my heart and say that a big chunk of the Canaries will not fall into the sea tomorrow. You just don’t know.”
Another nearby area that had captured scientists’ attention was Spain’s southwest coast. “There’s quite a bit of seismicity around there,” Wynn said, “and it’s a heavily populated region.” After all, the 1755 quake that “clobbered Lisbon hard” had been a superheavyweight. The shaking—which lasted for almost ten minutes—spawned fifty- and sixty-foot tsunamis that wreaked havoc from Morocco to England. “Can we expect another one of those in the next ten years?” Wynn asked rhetorically. “Hundred years? And which parts of Europe have been affected by similar events in the past?” These questions were deemed urgent enough that Wynn was about to embark on a monthlong research cruise, one of several planned for the near future.
There must be something etched into human DNA that allows us to quickly forget such shattering events as the Lisbon tsunami, which upended the lives of millions across all of western Europe and northern Africa—“two hundred and fifty years ago,” Wynn pointed out, “that’s nothing.” Then there was Krakatoa’s supereruption and tsunami in 1883, so recent that its 140-foot waves coincided with the premier issue of Ladies’ Home Journal. But while our collective memory may be woefully short, the geological record is long. “By studying deep-sea deposits we can start looking at the past landslides in the area,” Wynn said, sweeping his hand across a map of the Atlantic margin. “We can tease out the earthquake record as well. That’s what we’re doing out there. We’re trying to unravel the history of these waves.”
“So where do you want to start?” Dr. Peter Challenor asked. “We’ve done quite a lot on extreme sea states. How much do you know about wave statistics?”
Challenor spoke quickly, punctuating his speech with sharp birdlike gestures. He seemed to take genuine pleasure in discussing his work. Even the greenish fluorescent lighting in his office couldn’t dampen his exuberant aura. His brown hair grew lavishly onto his face, happy curlicues of sideburn and mustache and beard. Across from him sat his colleague Dr. Christine Gommenginger, in a smart navy blue dress. Behind him, a blank whiteboard beckoned.
The two scientists specialized in remote sensing of the ocean, gleaning snapshots of its behavior from space. In particular, they examined the waves. Eight hundred miles up in the exosphere, the European Space Agency satellite known as Envisat zips around the planet fourteen times a day, shooting radar pulses down onto the sea surface. Using the information it (and other satellites) send back, Challenor and Gommenginger can chart wave heights anywhere in the world with ridiculous precision.
It wasn’t always this way. Before 1985, when a satellite called Geosat was launched, wave scientists had to rely on moored buoys and reports from ships for their data. Better than nothing, perhaps, but given that the buoys were clustered near coastlines and the ships could survey only pinches of ocean real estate, what was going on out there was really anybody’s guess. The latest in a series of increasingly sophisticated satellites, Envisat is the largest earth-observation spacecraft ever built. Resembling something dreamed up for a James Cameron movie, packed full of powerful instruments with impressive acronyms like GOMOS (Global Ozone Monitoring by Occultation of Stars), DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), and ASAR (Advanced Synthetic Aperture Radar), there’s not much it can’t do or see. Thickness of the Arctic sea ice? Surface temperature in the Somali Current? Size of the waves off Peru? No problem.
“We tend to concentrate on significant wave height,” Challenor told me. Rather than pinpointing individual waves, this number, the average height of the top 33 percent of the waves, paints an overall picture of surface roughness. Over time scientists can use it to determine a key statistic for any given patch of ocean: the size of the “hundred-year wave.” Theoretically, only one wave larger than this value (on average) should show up every century. “We produce this
mainly for people who are building structures and want them to survive big waves,” Challenor said. That included oil rigs, of course, along with coastal construction and a relatively new concern: wave farms. “Like wind farms,” Gommenginger explained. “Wave energy is coming.”
This low-impact form of alternative energy seems brilliant on paper, but in the past wave farms have not fared well. The devices meant to float at sea and capture the waves’ power have been destroyed in short order by … the waves. “They’ve all been smashed up in storms,” Challenor said, shaking his head. “That’s usually their fate around the second or third winter.” Heartier designs looked promising, but finding the ideal location was also part of the deal. “They want it rough,” Gommenginger said, “but not too rough.”
According to Challenor the waviest places of all were “the North Atlantic in winter. Or the Southern Ocean anytime.” There ships could expect to encounter thirty-foot seas on a good day. That was not to say, however, that monster waves couldn’t make surprise appearances in other locales, at other times. (And by the way, building to withstand the hundred-year wave wouldn’t help anyone who happened to meet up with the thousand-year wave.) “The way the radar system works, the very big ones are difficult to measure,” he said. When behemoth waves appeared in the satellite data, the space agencies considered these readings to be errors, and they were automatically deleted. “They give you missing value code instead, which is really annoying. We shout at them for that.”
