To find out how fast, and how much, polar ice might melt in the future, scientists are looking to ancient rocks for clues of what happened in the past.
Image courtesy David Lewis
The day of our departure didn’t begin well for Paul Hearty. A clasp had ripped off his favorite clogs. A screw in his glasses had fallen out. His hasty repair, a safety pin in a hinge of his shades, gave him an incongruous punk look. The expedition—a drive from Melbourne across Australia’s southern and western coasts, in search of rocks—had also suffered a series of small setbacks and delays, provoking Hearty’s impatience and ire.
When he’s agitated, Hearty’s brows bunch into furrows reminiscent of the crevasses of glaciers he once studied. His eyes are light bluish-gray and deep set. He wears Crocs and no socks in dry weather, and ankle-high leather boots with white socks in the rain. Around his neck he wears a nylon cord, soft from wear, with two small hand lenses. The larger lens, a 10x magnifier, has a case engraved with his initials, PJH.
Hearty believes that Earth’s sea level rose 20 meters, about 65 feet, during the earlier of these heat bouts—higher than almost anyone else has yet determined. If Hearty is right, global warming poses a greater threat to the world’s coastlines than anyone is prepared for.
For thirty years, Hearty has labored to prove that if Earth warms just a couple degrees, as many scientists forecast, the huge glaciers in Greenland and Antarctica could melt substantially in mere decades. He bases his predictions on his studies of two recent (geologically speaking) warm spells 125,000 and 400,000 years ago. Hearty believes that Earth’s sea level rose 20 meters, about 65 feet, during the earlier of these heat bouts—higher than almost anyone else has yet determined. If Hearty is right, global warming poses a greater threat to the world’s coastlines than anyone is prepared for.
Hearty had been invited to Australia to scout for marine fossils once domiciled on a shore but left high and dry millions of years ago when the sea receded. The invitation itself was something of a surprise. It had come from Maureen Raymo, a geochemist who had earned her scientific kudos analyzing gummy seafloor sediment. She’d rarely handled limestone and sandstone—hard rocks—like those she wanted to collect in Australia. She’d once owned a rock hammer, but had long since lost it.
Hearty, on the other hand, had blistered his palms for decades cracking open such rocks. He reads them. Miles from any current shoreline, he might see scrub-covered limestone and fossilized coral, but his mind’s eye sees surf and spray and dunes. In the field, Hearty stares at rocks, sometimes from several angles. He tastes them with the tip of his tongue. He strikes them with his hammer and listens to the pings.
Hearty and Raymo, then, were not likely colleagues. Hearty generally disdains geochemists, and Raymo harbored questions about Hearty’s 65-foot finding. They’d joined forces because he wanted the grant-funded work, and she needed his skills. Together they formed a team that promised to accomplish what had proved elusive to many other scientists: to determine the level of the sea about 3 million years ago, the period geologists call the Pliocene.
Hearty had studied warm periods more recent than the Pliocene, thinking they could be analogs to our near future. But while he studied, the cars and homes and factories of modern civilization filled the air with even more planet-warming carbon dioxide. The air now contains as much carbon dioxide as during the Pliocene, when the planet was warmer than at any time since.
Raymo’s reservations about the accuracy of Hearty’s 65-foot sea level calculation did not change her high regard for the man. She still admired his clairvoyant ability to envision that moment in the past when a million-year-old rock was formed. Raymo affectionately calls him the Rock Whisperer.
Can ancient rocks really tell us how fast and how much polar ice will melt? Hearty believes they can, unequivocally. Such predictions matter tremendously. The world’s glaciers and ice sheets hold about two percent of Earth’s water. It’s a small fraction, but if all of it—about 6 million cubic miles of water—melted and poured into the oceans, the sea would rise by more than 230 feet. Most of today’s largest cities would vanish. A map of this waterlogged world would be missing Florida, the eastern shore of Maryland, Bangladesh, and Denmark.
Most agree that total melting, if it occurs, will take centuries. But scientists differ greatly over how much melting could occur in the shorter term. They’re uncertain how quickly the world’s largest glaciers will react to additional warmth, and they can only guess when people will wake up to this crisis and stop burning so much fossil fuel. Hearty, Raymo and many others like them agree on one thing: the way that ice sheets respond to an increase of several degrees Fahrenheit will decide the fortunes of every nation on Earth.
It’s hard to predict how ice sheets will respond, in large part because they melt unpredictably. Frozen streams carry an ice sheet’s bulk from the interior to the perimeter, which can be hundreds of miles away. Sometimes the streams speed up, sometimes they taper off; scientists aren’t sure why, nor do they know the limit of an ice stream’s speed. They also have no idea how much ice these frozen rivers could cast into the sea.
Hearty explores these issues by reading “archives” of climate history—records embedded in tropical stones. He earned his doctorate in glacial geology from the University of Colorado in Boulder, but he decided never to take samples anywhere near an ice sheet. I once asked him why. “Polar bears,” he said.
In the normal course of Earth’s history, an ice age is due “any millennium now.” Today’s ice sheets in the Antarctic and Greenland are doing the opposite. They’re melting, and the sea is rising at an accelerating rate.
So he studies tropical and subtropical seashores. He began, in 1982, on the sweet-breezed Mediterranean seacoast. His work became a lifelong quest: a global survey of low-latitude coastlines. As he worked thousands of miles from the poles, the young scientist ruminated on troubling theories about the northern and southern ice sheets.
For about the last 3 million years, Earth has cycled in and out of ice ages. When it’s cold, millions of cubic miles of water freeze solid near Earth’s poles. When it’s warm, about two-thirds of the ice melts and drains into the oceans. Each cycle takes about a hundred thousand years to complete, but the cold and warm periods are unequal in length. Because our civilization arose during the current warm period, it is surprising to learn that ice ages predominate.
The most recent ice age ended about fourteen thousand years ago. With spurts forward and occasional pauses, the sea rose until several thousand years ago. Since then, sea levels have remained virtually unchanged. If a new ice age were to come, the sea would retreat, as it has many times. In the normal course of Earth’s history, an ice age is due “any millennium now,” as one glaciologist once told me. But today’s ice sheets in the Antarctic and Greenland are doing the opposite. They’re melting, and the sea is rising at an accelerating rate.
I joined Hearty and Raymo and two other colleagues on their field research in Australia. We shared honky-tonk outback hotel rooms and bars. Hearty was subject to sour moods and subjected us to his occasionally boorish manners. Despite the fact that he wasn’t the boss—Raymo had thought up the trip and finagled grants for expenses—he acted as if he should be. Day in and day out, the rest of the crew and I waited on edge for Hearty to decide when we’d break camp, take lunch, fill-up on gas. He earned a less affectionate out-of-earshot nickname: Prickly Pear Paul.
Prickly Paul sometimes declared which team member was the day’s “loose link”—the man still in the gas station restroom after the others had buckled up, or the woman buying a last-minute pasty, meat pies dispensed at every convenience store. Once I asked Hearty if I might join him for a chat in the van. Prickly Paul replied, with just a hint of a grin, “As long as you don’t talk.”
In the month I spent listening attentively from our vehicles’ back seats and watching across pitted roadhouse tables, I saw the human dimensions of scientists that an outsider rarely experiences in an office or lab interview. Raymo tended to hang her glasses over the neckline of her shirt and then lose them when they fell off. Michael O’Leary, Hearty’s congenial former student, exclaimed “Cool bananas!” when excited, and “We’re not here to fuck spiders,” when he was ready for work. They told geology jokes. Marci Robinson, a research geologist from the U.S. Geological Survey, told us about her geology-themed T-shirts, such as “Stop Plate Tectonics.” To express his agreement, Hearty once said, “Exactly my sediments.”
One night in Perth, desperate for a DVD to watch, we dug up The Wizard of Oz. We watched as the Wicked Witch imprisoned Dorothy in the mountaintop castle. Toto ran to the rescue. “Hey, that’s laminar planar bedding,” exclaimed Raymo, perking up and pointing to a cliff face. “Sandstone.” The geologists also noticed a shaft they called a “volcanic plug” just behind Munchkin Land.
Dorothy left for Oz from Kansas. Hearty grew up next door, in Nebraska. He, too, took off at a young age, though he left his house and pets behind. In his first Mediterranean expeditions, he honed a method pioneered by earlier glaciologists: identifying and dating “high stands,” the highest points on land reached by the sea at earlier times. Of course, the sea rises globally when polar glaciers shrink. But at any given spot on Earth, there are complications. Where the movable plates holding up the continents collide, Earth’s surface lifts up. Where one plate dives under another, land nearby sinks. A plain bulges because magma oozes from Earth’s interior. A river delta sinks because gravity compacts sediment deposited by water. Titanic forces like these buckle and shift Earth’s surface everywhere, leaving no spot forever fixed in place. Geologists trying to chart the ebb and flow of the oceans in geologic time are like children on a playground: bobbing on a seesaws, they attempt to estimate the highest point reached by a nearby playmate pumping a swing.
Each time Hearty measured the elevation of a wharf piling or other marker of earlier sea level, he penciled the observations in one of the yellow field notebooks he always kept at hand. He filled many such books with entries about the current elevations of once-coastal rocks. Hearty’s ambition back then was to document how high the sea level rose during Earth’s most recent warm spell, between 116,000 and 128,000 years ago. This phase, labeled Stage 5e by specialists, is thought to have been slightly warmer than the present day; many researchers consider it an analog for our warmer future.
Paul Hearty’s credo, “never trust anything you can’t hit with a rock hammer,” is a quixotic view of how researchers should assemble evidence to interpret Earth’s past. He abhors the notion that glaciologists would support their conclusions about sea level on a foundation less solid than bedrock. He’s especially troubled by some of the findings of ocean geochemists, who’ve made their own estimates of past sea level by studying the chemical signature of sediments. Perhaps he’s also a touch jealous, considering the hundreds of millions of dollars that federal agencies like the National Science Foundation and the National Oceanic and Atmospheric Administration shower on ocean geochemists.
In 1992, after completing his graduate studies and taking several short-term university posts, Hearty packed up his field gear and flew to the Bahamas. He contends that the Bahamas is one of the best places on Earth to trace the history of global sea level, because based on the geological evidence he’s interpreted, he believes that the islands don’t move much up or down through the millennia. If Hearty is right, the islands have stayed virtually immobile over geological time, while sea level has risen and lowered around them. At the high points, the sea left imprints that mark its presence, just as barnacles clinging to pilings mark high tide. Past global sea level, in this view, is the same as the elevation of these markings.
For nearly a decade, Hearty mapped outcrops, quarries and road cuts in the Bahamas and 900 miles northeast in Bermuda, looking for traces of past sea level high stands. Gradually, he collected evidence to chart the rise and fall of the ocean over the last half million years as the northern and southern ice sheets waxed and waned.
Researchers had discovered high stands–those highest points the sea reached earlier–in the Bahamas and Bermuda before. But Hearty says that no scientist had ever found as many ancient shorelines in one place as he discovered on the island called Eleuthera. There, Hearty stumbled upon deposits left during the six most recent transitions between periods of ice age and warmth—“a continuous record of glacial/interglacial cycles,” stacked like sliced meat and cheese in an Italian sub. It was an unbelievable archive of sea-level oscillations deep into the past. Hearty said it was “damned exciting.” He nicknamed the several-mile-long rock-strewn coastline “the Rosetta Stone.”
Two warm stages recorded in Eleuthera’s rocks keenly attracted Hearty’s interest: the most recent one, Stage 5e, and Stage 11, about 400,000 years ago. Researchers believe that Stages 5e and 11 were the hottest in the last half-million years, even though global temperatures averaged no more than about one degree Celsius hotter than today’s.
When Hearty made elevation measurements at the Rosetta Stone, in the Bahamas, and at a site in Bermuda, he discovered something surprising. The Stage 11 deposits stand about 65 feet above the current sea level. Since he considers these islands to be yardsticks of sea level, he concluded that during Stage 11, the global sea level was 65 feet higher than it is today. That’s about three times higher than the figure geochemists have calculated.
Hearty’s figure implies that all three major ice sheets lost huge amounts of ice during Stage 11. That could mean that even a small warming could coax into motion the East Antarctic Ice sheet, generally considered the most stable of the world’s three great ice sheets, and unleash rapid, overwhelming inundation.
Most experts on past sea levels doubt Hearty’s conclusions, though their reasons vary. Numerous scientists have questioned his evidence and the links in his chain of reasoning. “It is amazing how many attacked us and how much criticism we got,” Hearty says. He categorically rejects each critique, often attributing unprofessional motives to his challengers. Gary McMurtry, a scientist at the University of Hawaii, says a tsunami, not elevated sea level, washed marine fossils into clefts in a cliff face that Hearty found on Bermuda.
“There’s no evidence. Zero,” responds Hearty. “He gets funding from studying tsunamis,” Hearty adds. “And if there are no tsunamis, there’s no funding.” (McMurtry says, in return, that Hearty gets “a little religious about these things.”)
John Mylroie, a professor at Mississippi State University who has also studied high stands in the Bahamas, thinks fine channels Hearty found in sandstone at the Rosetta Stone were created not by gentle waves where the deposit was found, as Hearty believes, but by windblown spray blasted up from far below. ”Nonsense and nothing more than an unsupportable opinion,” retorts Hearty.
By 1998, Hearty had concluded that the Bahamas and Bermuda had offered up all of their secrets. He flew to Hawaii. He collected more rocks. He married. After two years he moved to Australia. He worked at a couple of universities on Australia’s east coast. He led graduate students on grueling field trips to Western Australia, where they dug up and cataloged more shells and corals, from more fossil beaches.
Despite his disdain for ocean geochemistry, Hearty was invited to collaborate in Australia with one of the field’s stars: Raymo. She’s coauthored a frequently cited marine geochemistry paper. She oversees a temple of geochemistry, one of the most important archives of ocean-sediment cores: the Lamont-Doherty Core Repository, in Palisades, New York. She’s advised the Integrated Ocean Drilling Program, an international consortium of seabed researchers. Raymo practices a kind of saintly forbearance around Hearty. She ignores slights, such as his habit of chanting in singsong at any mention of geochemistry’s achievements: “Money, money, money.” She invited Hearty to put his name on a multimillion-dollar application to the National Science Foundation.
He is, after all, the Rock Whisperer.
Hearty sometimes circles a geological feature for a half hour before fingering a pebble. He assembles a mental picture of the events that created and subsequently altered the rocks. He carries his rock hammer, usually holding it shaft-down by its dinged, hardened-steel head. Sometimes he twirls the tool while he walks. He idly tosses it end over end. He cries “Ouch!” if he fails to catch it, whether it strikes him or not. When he targets a stone to sample, he breaks a hunk off with a whack. He trims the piece with more blows.
“You’ll have to excuse me,” Hearty said when I asked if I could join him surveying an escarpment. I thought his comment meant he wanted a private moment to water some rocks. But he explained that he needed to gather his thoughts. I watched, seated comfortably, from a respectful distance. He threaded his way between knobs of ruddy rock. The ocean sparkled in the distance. A while later, Hearty beckoned—my cue, I figured naively, to join the master for a lesson. He directed me like a ballet choreographer. “Over there, by the tree.” He pointed his camera at me and snapped a shot. Click. “By the ledge… 10 feet farther.” Click. I realized that he wanted my body for scale.
Finished with snapping photos, Hearty allowed me to approach him and join his tour. He headed toward a knoll. An anomalous white patch on top had attracted his interest. “In our business, white is good,” he explained. “It represents limestone,” a marker of the sea. At length, we stepped onto the raised rock. But he found no limestone. “Just old bedrock,” he said. It was flat and smooth on top. He said waves and currents in shallow waters had probably worn away overlying rock, most likely limestone. Later, the water had subsided some, and the knoll emerged as a small offshore island. “Nice sea cliff,” he muttered.
Our expedition came to an end in Perth. Raymo boxed up some thirty pounds of rocks and shipped them back to the United States, airfreight. She couldn’t know yet if Hearty had correctly identified Pliocene-age deposits. Months later Michael O’leary joined Raymo in Boston and dated the samples in a university lab. The dating showed that some of the beaches they’d found were 3 million years old, as hoped.
Raymo and her rock wranglers had measured the elevations of each fossil beach above the current coastline. But she didn’t know yet how much of a change in the global level of the sea these measurements represented. She suspected that the rim of Australia might have bulged up or sunk down since the time when waves lapped the area where the team had discovered stranded beaches.
One day, Raymo dreamed up an idea for how the Rosetta Stone and other sites could be 65 feet above sea level today.
She wanted to figure out how much the land had shifted, to correct the elevation measurements the same way a navigator trims a ship’s rudder to compensate for cross currents. For that, Raymo needed Jerry Mitrovica, a geophysicist at Harvard. They decided first to crack a related, but simpler, problem, the one that had pitted Paul Hearty against almost all of his geology colleagues.
At the Rosetta Stone and in Bermuda, Hearty had found four-hundred thosand-year-old Stage 11 beaches that tower 65 feet above present sea level. But what did that signify? Did it mean, as Hearty believed, that global sea level had gone up by 65 feet during Stage 11? That would in turn mean that a dozen other research teams who’d calculated much lower Stage 11 sea level–including some of Raymo’s own geochemistry cohorts–must be wrong. Most important, it meant that the warmth of Stage 11, a temperature Earth could reach in coming decades, almost completely defrosted the Greenland and West Antarctic ice sheets and a major fraction of the Eastern Antarctica sheet. “It created huge, perplexing problems for everybody,” Raymo said.
One day, Raymo dreamed up an idea for how the Rosetta Stone and other sites could be 65 feet above sea level today even if the sea itself was not 65 feet higher during Stage 11. Two ideas no one had linked before–the bulging of land during Earth’s cold periods and the difference in length of its warm periods–could explain it. Scientists know that Stage 11 lasted longer than any warm stage since. What if the islands sit atop, or to one side, of the bulge—what geologists call a “peripheral bulge”—created by the massive ice sheet that covered half of North America during past ice ages? The elongation of Stage 11 would have given that bulge extra time to sink down after the world warmed and the ice retreated.
Raymo knew that continents and nearby offshore land behave kind of like a waterbed with sleepers who get up and lie down at regular intervals. When you climb into the bed, the center goes down and the edges rise up. When you leap out of the bed the opposite occurs. Ice sheets are like the sleepers, and the water in the bed is like the Earth’s land. Land pushed down by an ice sheet in a cold period slowly rebounds when the sheet retreats, and nearby areas that aren’t covered—the peripheral bulges—subside.
If you had put a stake in the sand at the ocean’s edge 380,000 years ago, and if nobody stole it to roast a pig or erect a tent, today it would be about 28 feet higher than it was back then.
To determine how much and how quickly such bulges rise and fall is a complicated matter. Mitrovica set to work. Mitrovica researches how movements deep below Earth’s crust show up on the surface. By the time he’d been lured to Harvard University from a post in Canada, he’d earned a reputation for path-breaking insights into esoteric mechanisms governing changes in sea level over long stretches of time.
The Harvard scientist contemplates the movement of Earth inner layers in a building next to the university’s Geological Museum. His second-story corner office is as Spartan as the museum’s neoclassical façade is ornate. Two years after his move to Harvard, cardboard boxes filled with reference books still crowd the floor.
Using formulas, called codes, that he’d calculated in his years of research, Mitrovica discovered that during Stage 11, the islands of the Bahamas and Bermuda had sunk roughly 28 feet lower than where they are today. To put it more concretely: If you had put a stake in the sand at the ocean’s edge 380,000 years ago at the end of Stage 11, and if nobody stole it to roast a pig or erect a tent, today it would be about 28 feet higher than it was back then—because the land there rose up so much during the last ice age, and it still has not settled all the way back down.
Correcting for this change in elevation at the Rosetta Stone and on Bermuda, Mitrovica determined that sea level during Stage 11 was roughly 31 feet above today’s sea level. This figure conforms generally to the conclusions of most other scientists—and suggests that Hearty was wrong about the elevation of Stage 11 sea level.
Mitrovica and Raymo’s findings are somewhat reassuring, but it’s all a matter of degree. Mitrovica says the 31 feet the sea rose during Stage 11 inundated vast areas. He’s also troubled by his earlier work, which indicates that the Greenland and West Antarctic ice sheets lost the majority of their ice during the Earth’s most recent warming, which was on average only one degree Celsius above today’s temperature and saw sea levels rise by between 21 and 30 feet. Mitrovica finds it worrying that a warm period so recent and so close to today’s temperature could have lost so much ice.
Scientists like Raymo, Hearty, and Mitrovica hope to uncloud our crystal ball a bit. With more accurate information, we may be able to make a more informed assessment of how much effort to place in trying to avoid disasters and in planning for the inevitable. In the meantime, water rises around us at an accelerating rate. As the prominent glaciologist Lonnie Thompson told me that we might not know we’ve passed the point of no return until “we see it in the rear-view mirror.”
Though Hearty would prefer to be known for proving that Stage 11 sea level was 65 feet above that of today’s—a conclusion he clings to—Raymo and Mitrovica’s paper offers a consolation prize. It substantiates Hearty’s contention that lapping waves, not high storm winds or a tsunami, created the channeled sand and other features he discovered on Bermuda and the Bahamas, proving many of his critics wrong. As Mitrovica put it to me, just before he and Raymo went public with the corrected figures for Hearty’s sites, “Nobody is going to be talking about mega-tsunamis after that paper is published.”
Daniel Grossman has been a print journalist and radio and web producer for twenty-five years, reporting from all seven continents. He holds a Ph.D. in political science and a B.S. in physics, both from MIT. He is a contributing editor of National Geographic’s News Watch and contributes frequently to PRI’s The World. Grossman has been awarded a Ted Scripps Fellowship in Environmental Journalism and an Alicia Patterson Foundation Fellowship. He is the coauthor of A Scientist’s Guide to Talking with the Media: Practical Advice from the Union of Concerned Scientists. This article is excerpted from his book Deep Water, published by TED books and funded in part by the Kendeda Foundation and the Putnam Foundation, in collaboration with the Pulitzer Center on Crisis Reporting,