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'Megastorms' that throw thousand-tonne boulders over clifftops may be on their way back thanks to global warming

The effort kicking off in Paris this week to hold the world’s nations to strict climate targets may be even more urgent than most people realize

Chris Mooney
Eleuthera, Bahamas
Monday 30 November 2015 14:37 GMT
One of the mysterious boulders on the island of Eleuthera in the Bahamas
One of the mysterious boulders on the island of Eleuthera in the Bahamas (Charles Ommanney/The Washington Post)

Standing atop a 60-foot cliff overlooking the Atlantic, James Hansen — the retired NASA scientist sometimes dubbed the “father of global warming” — examines two small rocks through a magnifying glass. Towering above him is the source of one of the shards: a huge boulder from a pair locals call “the Cow and the Bull,” the largest of which is estimated to weigh more than 1,000 tons.

The two giants have long been tourist attractions along this rocky coast. Perched not far from the edge of a steep cliff that plunges down into blue water, they raise an obvious question: How did they get up here?

Compounding the mystery, these two are among a series of giant boulders arranged in an almost perfect line across a narrow part of this 110-mile-long, wishbone-shaped island.

Hansen and Paul Hearty — a wiry, hammer-slinging geologist from the University of North Carolina at Wilmington who has joined him here as a guide — have a theory about these rocks. It’s so provocative — and, frankly, terrifying — that some critics wonder whether the man who helped spawn the whole debate about the dangers of climate change has finally gone too far.

(Washington Post)

The idea is that Earth’s climate went through a warming period just over 100,000 years ago that was similar in many ways to the warming now attributed to the actions of man. And the changes during that period were so catastrophic, they spawned massively powerful superstorms, causing violent ocean waves that simply lifted the boulders from below and deposited them atop this cliff.

If this is true, the effort kicking off in Paris this week to hold the world’s nations to strict climate targets may be even more urgent than most people realize.

Hearty, an expert on Bahamas geology, first published in 1997 the idea that Cow and Bull were hurled to their perch by the sea. Since then, Hansen has given the work much added attention by framing the boulders as Exhibit A for his dire view of climate change — which has drawn doubters in the scientific community. But as Hansen examines the rocks on a recent morning, Hearty explains some of the evidence. In particular, Hearty points out that the tiny grains that constitute the boulder rocks are more strongly cemented together and less likely to crumble than other rocks nearby, a sign that the boulders are older than what’s beneath them.

“Yeah,” Hansen says with a nod, rubbing the younger rock and watching it crumble a little. He sees the difference. It’s a key point the two use to argue that the placement of these boulders indicates a dramatic hurling of the rocks by the sea. Even on a calm day, the deep blue waters of the Atlantic slam against the cliffs below with audible force and huge plumes of spray. But could waves have lifted these massive stones?

While there is a suggestion in the scientific literature that the boulders were simply left behind after surrounding rocks eroded away, Hearty and another leading Bahamas geology expert, Pascal Kindler of the University of Geneva in Switzerland, agree that the boulders are older than the surface upon which they rest and, thus, probably were moved by the sea. Even the tourist placard near here takes their side, saying the ocean “lifted them atop the ridge.” But exactly how it could have done that is another matter.

Scientists have tended to attribute odd boulders such as these to tsunamis — there’s little doubt they have the power to move large rocks. One recent study found that in the Cape Verde islands, 73,000 years ago, a 300-foot-high mega-tsunami carried boulders as large as 700 tons atop a cliff almost as high as the Eiffel Tower.

But more recent studies have also attributed large boulder movements to storms. And now into the fray has stepped Hansen, who, in 1988 testimony before Congress, put the climate issue on the map by contending — correctly, as it turned out — that global warming had already begun. If he is also right about the boulders, Earth could be in for a rough ride.

And even if not, one thing is clear: Cow and Bull present a scientific mystery whose solution may serve as a reminder of just how violent and dynamic a planet we live on.

Eleuthera in the Eemian

Hansen says he first encountered Hearty’s extensive Bahamas research — on the boulders and much more — eight years ago.

The two began to collaborate, and the result, once Hansen pulled in 15 other specialists as co-authors, was a 121-page paper presenting a dire reinterpretation of much of modern climate science.

In it, the researchers contend that there was a catastrophic storm event here 118,000 years ago. This would have occurred at the turbulent close of a climatic period sometimes dubbed the Eemian, which was moderately warmer than our own but featured considerably higher seas — especially at its end, when oceans appear to have risen quickly and then fallen again.

That period was one where, in Hansen’s interpretation, “all hell breaks loose”: a collapse of polar ice, quickly rising seas, a shutdown of heat-transporting ocean circulation, and then superstorms spawned by a greater temperature contrast between warm tropics and cold poles.

All of this is contested. While Hearty’s many geological studies of the Bahamas and its boulders have all been peer reviewed and published, the broader study remains under public peer review by a “discussion” journal, Atmospheric Chemistry and Physics Discussions. It has been the most downloaded paper the journal has ever considered, receiving one positive peer review and one skeptical one — and many assorted challenges.

Meanwhile, as the scientific debate continues, Hansen and Hearty last weekend met up in Eleuthera, accompanied by Hansen’s wife, Anniek, and his 17-year-old granddaughter, Sophie Kivlehan. Along with 21 other young people, Sophie is plaintiff to a lawsuit against the U.S. government — with Hansen playing the dual role of their guardian and scientific expert — demanding protection of their generation’s fundamental rights in the face of climate change.

It’s public moves such as this, or getting arrested protesting the Keystone XL oil pipeline, that have led some to contend that Hansen, scientific luminary though he is, has also become an “activist,” a label that makes many scientists uncomfortable.

Yet Eleuthera itself, and the large body of evidence Hearty presented as they toured it together, certainly implies that something dramatic happened in this landscape more than a hundred millennia ago.

Eleuthera, in the northeastern Bahamas, is home to about 11,000 people. There’s some tourism on the island, but it is also a geologist’s dream — a testament to the natural process that, over vast periods, generated layer upon layer of island rock from the simple sands formed offshore.

As seas rose here during warm interglacial periods such as the Eemian, the shallow coastal waters formed particles called “ooids,” tiny, egg-shaped sand grains of calcium carbonate. When seas fell again, these ooids rapidly hardened into limestone rock, preserving a detailed geologic record in the process. “Hot wax,” Hearty likes to call it.

Ooids from the Eemian formed much of present day Eleuthera, Hearty explains, and left behind hardened dunes and seashores considerably higher than current ocean levels. They also formed the rocks at the base of Cow and Bull, but the boulders themselves, he says, appear consistent with a far older cliff face that is well beneath them.

One simple demonstration: Hearty hits the boulders with his hammer. They ring loudly at the blows; the rocks beneath them sound more muffled — a hint, he says, that the boulders have had more time to harden than the younger formation below.

Another dating technique, called “amino acid racemization,” has also suggested the rocks’ older age. And then there are their “bedding planes,” or angular alignment of sediments within the rock. “The rock is tilted from any natural inclination, which means it was transported,” Hearty says.

Although the world of Bahamas geology is relatively small and sometimes contentious, Hearty is not the only researcher who thinks these rocks were moved by the ocean. The University of Geneva’s Kindler, who has examined the boulders and published with Hearty in the past — but also, notably, criticized his paper with Hansen — agrees that the boulders were put in their present location by giant waves.

“These boulders are much more weathered than the underlying substrate. The underlying substrate consists of very well-preserved limestone,” Kindler says. “They are partly dissolved, so they are older than their substrate.” However, Kindler holds that a localized tsunami, caused by an undersea landslide that would have occurred as the seafloor off the coast collapsed, is the most likely cause of the waves that threw the rocks.

The 'rages'

Seeing the boulders in their setting also makes it clear that along this stretch of coast, the deep Atlantic flings a tremendous amount of energy.

Slightly to the north of here, a high bridge connecting two parts of Eleuthera shows signs of repeated battering by huge waves (locals call them “rages”) that have shoved concrete barriers entirely off the road. Circa 1885, Winslow Homer painted this spot showing a high rock arch that framed a windowlike view of the sea. But now the arch is gone, “destroyed by a raging hurricane,” the tourist sign says.

Then, in addition to Cow and Bull, Hearty has documented five other boulders of similar size. Two lie in a curious, nearly straight line from Cow and Bull across a roughly 500-foot-wide stretch of the island. First comes another 1,000-ton boulder that Hearty calls Maverick. Entirely across the island, meanwhile, lie a pair of large rocks dubbed the Twin Sisters, sitting pristinely in the shallow and calm green waters of Eleuthera’s protected side. And there are three other large boulders nearby of similar sizes and features.

So what could have caused this? In his original 1997 paper, Hearty proposed three possible explanations: a tsunami, superstorms and Kindler’s preferred “bank margin collapse.” A tsunami could have come from afar or from a fluke event such as a meteor strike in the Atlantic, but a sub-sea landslide right off Eleuthera’s coast could also have displaced a large volume of water that surged back and threw boulders.

Hearty agrees there probably was such a collapse but asserts there is no evidence that it happened suddenly, rather than slowly or in smaller stages. Moreover, he sees other evidence of superstorms across Eleuthera and other parts of the Bahamas.

Farther inland in areas not protected by large cliffs, he thinks, the storms created curious chevron ridges, large V-shaped walls of rock sometimes extending several miles and always pointing to the southwest.

At very high heights on the islands, meanwhile, Hearty suggests that the huge waves left sediment layers featuring telltale beach bubbles, or fenestrae, suggesting waves reaching as much as 80 feet above the current sea level.

All of these features taken together, Hearty contends, are best explained by gigantic storms. He likes to invoke the principle of “parsimony,” meaning that the simplest explanation tends to be the right one. “It’s really the parsimony between the trilogy of evidence that makes this story hold together,” Hearty says.

Kindler disagrees that giant waves made the chevrons, however; he thinks they were assembled by winds. Hearty is “a good scientist, a good observer,” Kindler says. “But, yeah, I just do not agree with his interpretation.”

He is not the only skeptic. Mississippi State University’s John Mylroie, who has also conducted extensive research in the Bahamas, previously published the idea that Cow and Bull are remnant towers whose surrounding rock has eroded (an idea that Kindler and Hearty both reject). Mylroie is also skeptical of the storm interpretation. “The boulders on Eleuthera, if the result of wave-water action, represent a very narrow footprint that apparently occurred only once, which is not what you would expect for an increase in number of storms or storm intensities,” Mylroie says.

Hansen, though — the man who has helped make Hearty’s ideas famous — seems quite confident in them.

“We looked at those criticisms,” he says. And “it became all the clearer that the interpretation that we’re making is right, and that the boulders are wave-deposited, and highly likely that they’re deposited by the same storms that are causing the other obvious features in the Bahamas.”

The debate comes not only at a turbulent time in the climate discussion but also when growing evidence suggests that even today’s storms can move large boulders, an effect previously attributed more exclusively to tsunamis.

On Ireland’s Aran Islands, geologist Rónadh Cox of Williams College found that in intense North Atlantic storms in the winter of 2013 and 2014, more than 2,000 boulders moved, based on GPS mapping. Many were pretty small, but the biggest was over 400 tons.

Cox has not studied the Bahamas, but from her own work infers that Hearty’s ideas are a “valid hypothesis and one that can’t just be thrown out.”

Hearty contends that on Eleuthera, smaller boulders have moved within the past 12,000 years and may still be moving today. Not far from the mega-boulders, he shows Hansen a rocky plain strewn with boulders closer to 100 tons than 1,000. Hearty thinks this is the most storms can do in the modern era, which is why he thinks Eemian storms must have been stronger.

There is also the Philippine island of Samar, struck in 2013 by the extraordinarily intense super-typhoon Haiyan. Max Engel of the University of Cologne in Germany and his colleagues used satellite imagery to document the transport of numerous onshore boulders after the storm, including one that weighed 180 tons.

Storms, Engel says, “can create a boulder pattern that is very similar to what we would expect for tsunamis.”

People skeptical that ocean waves can move boulders are not thinking about the physics right, said Robert Weiss, a geophysicist at Texas A&M University.

“The density of air is about a thousand times smaller than the density of water,” Weiss said. He added that a simple cubic meter of water, a cube that is a meter long on every side, already weighs a ton.

So when you start thinking about waves that are 25 or more feet high, traveling at speeds of over 10 mph, it’s not so odd to think of rocks moving. “That will cause significant damage. That will cause the movement of boulders,” Weiss said.

But for storms to fling boulders as big as those in the Bahamas, that would probably require an epic unleashing of energy.

Hansen looks at the clues assembled by geologists such as Hearty and thinks that is what is in store for the planet if warming trends continue. That suggests not just a decades-long problem of rising ocean levels but also the potential for massive destruction.

“Unfortunately, it’s pretty clear that we are right,” Hansen said. He clearly believes it — and what is so troubling is that even many scientists who are skeptical can remember when he was right before.

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