Clinging to a cliff in the Bahamas, high above the pounding surf, geologist Paul J. Hearty takes care to avoid any missteps as he surveys the craggy rock.
“These are 20-to-30-meter-high cliffs,” he says. “I’m suspended by my fingers and toenails, with the waves crashing below.”
Though the threat of a fall keeps Hearty on edge, the real drama of this spot lies exposed in the alternating stripes of stone running along the face of the cliff. Thin red bands of fossilized soil separate massive white sheets of limestone—strawberry icing spread between layers of coconut cake. For a geologist reading this code, the bands recount a distant time when Earth’s climate displayed its wild side.
The thick limestone layers are ancient beach deposits, formed during warm breaks between ice ages. In those cold times, dust from the Sahara laid down thin red sheets. Hearty has come to the cliffs searching for a distinctive limestone band from 400,000 years ago, a warm interlude known as stage 11.
Today, the former beach sits so far above the ocean that a fall from this height could kill. Back then, however, the seas washed over it. For the oceans to have swollen that much, significant portions of Earth’s polar ice must have melted, says Hearty, who works as a geologic consultant in Honolulu. If such a melting were to happen today, breakers would crest over much of the property in Miami, New York, and other cities near the sea.
For decades, coastal geologists such as Hearty have been finding scattered hints of greatly elevated sea levels during stage 11. Their voices, however, were drowned out by the majority of oceanographers, who viewed sea levels as unremarkable at that time. Swept away by the opposition, Hearty and his colleagues found themselves floating far from the main current of scientific consensus.
In the past few years, though, new evidence has surfaced to shore up support for this massive melting. “The geological evidence for higher sea level during stage 11 is beginning to mount up, so it’s got to be given a lot of credibility,” says Richard Z. Poore, an oceanographer with the U.S. Geological Survey in Reston, Va. “I think the evidence that sea level was significantly higher than today is pretty good.”
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The stage 11 question is critical because scientists are starting to view this time as a twin of our present climate. If we want to know what bobs ahead in the future—if polar ice will melt and sea level will rise catastrophically—this long-gone period could offer the clearest view.
The last ice age ended over 10,000 years ago, an apparent eternity to a society fueled by drive-through restaurants, microwave popcorn, and E-mail. To a geologist, however, today’s warmth is just a blip. For much of the past million years, Earth has shuddered through a series of ice ages, each lasting close to 100,000 years. Punctuating these chills are relatively brief interglacial periods like the current time.
In the 1950s, when oceanographers first discovered signs of this glacial cycle recorded in deep-sea sediments, they named the various epochs going backward, starting with the present interglacial as stage 1. Four separate glacial periods separate modern times from the epoch known as stage 11.
During ice ages, glaciers more than a kilometer high covered large swaths of North America and Europe. These temporary ice sheets drew so much water from the oceans that sea level dropped by 120 meters in the most frigid intervals. When climate warmed during interglacials, the North American and European ice sheets melted, sending sea levels back up to a height on par with today’s. At least, so goes the standard scientific story.
The new evidence of elevated sea levels is rewriting that well-worn tale. Along the cliff face on the island of Eleuthera, in the Bahamas, Hearty has found a distinctive herringbone pattern in the limestone. This arrangement of ridges would be familiar to any beachgoer. It’s made up of fossilized relicts of the sand ripples that swimmers can feel beneath their feet as they wade out from shore. The pattern means that these rocks, now 20 m above the sea, were once below the low-tide mark, Hearty and his colleagues reported in the April 1999 Geology.
His team has found similar evidence in Bermuda and, more recently, on the Hawaiian island of Oahu, he announced last December at a meeting of the American Geophysical Union (AGU) in San Francisco.
The deposits on Oahu now exist 26 to 30 m above sea level, significantly higher than the fossilized deposits in Bermuda and the Bahamas. The difference, says Hearty, is that the two Atlantic sites have remained geologically stable over the millennia, whereas the Oahu site has risen 8 m in 400,000 years, according to his calculations.
The problem of shifting coastlines complicates the job of geologists who are trying to unravel exactly how high the seas crested in stage 11. As the land rises or sinks, it skews the evidence of past sea levels.
Some scientists question whether even Bermuda and the Bahamas have provided a true record. “Given what we know about plate tectonics, it’s very unlikely that anywhere on the coast has been stable for 400,000 years,” says David Q. Bowen of Cardiff University in Wales.
At the recent meeting, Bowen reported on his studies of ancient beach deposits around southern Britain that now perch between 20 and 43 m above sea level. Bowen has estimated the uplift rate by looking for evidence of sands laid down during the last interglacial. Because this warm period happened relatively recently, about 120,000 years ago, scientists could independently determine its sea level.
Measuring against the deposits, Bowen calculated how quickly these sites have risen. This rate indicates that stage 11 sea levels reached around 15 m above the modern value. Though lower than Hearty’s figures, Bowen’s calculations still point to a substantial melting of polar ice at the time.
More support comes from sites in northern Alaska that preserve evidence of a 13-m rise in sea level, reports Julie Brigham-Grette of the University of Massachusetts at Amherst. “A lot of us who work on stage 11 have realized that we really have compelling evidence on a global basis, if we put it together,” she says.
A swarthy globe
The face of the globe would have looked far more swarthy during stage 11 if sea levels then surpassed the modern mark by 13 m or more.
Today’s porcelain-white island of Greenland would have lost its icy coating and, as vegetation sprouted across the rocky surface, turned a darker shade more true to its name. The melting of this northern ice could account for 6 m of the sea level shift.
To find the rest, one must head south, says Reed P. Scherer, a researcher at Northern Illinois University in Dekalb who studies the climate history of Antarctica. The most probable suspect in this case is West Antarctica—the half of the continent that reaches toward South America.
Ice on the other side of Antarctica rests high and dry on rock mostly above sea level, making it far more stable than the West Antarctic ice sheet, whose base lies below sea level. Some scientists have theorized that the arrangement in West Antarctica makes that region prone to collapsing when climate warms because the swelling oceans would undermine its base.
The theory itself lacked much support until Scherer started studying gravel that a drill team had collected from beneath the West Antarctic ice sheet. The marine sediments contained the shells of one-celled algae with a distinctive shape.
From records elsewhere, Scherer knew that the algal species found under West Antarctica had appeared only within the past 750,000 years. Scientists once thought that the West Antarctic ice had been stable for more than 6 million years, but Scherer’s evidence revealed its erratic nature. The presence of relatively modern algae indicates that much or all of West Antarctica must have melted sometime within the past million years, leaving open ocean in its place.
While he cannot pinpoint when West Antarctica collapsed, Scherer says that “stage 11 is the most likely candidate.”
Melting the western side of the continent, however, would only account for another 6 m of sea level rise. With Greenland’s contribution of 6 m, it would be enough to satisfy the lower estimates but not the full 20 m. To reach that upper figure, about 10 percent of the ice currently in East Antarctica would have had to disintegrate as well.
With so much freshwater flushing into the oceans at that time, the thawing of polar ice should have left its signature on the seawater’s chemistry. Oceanographers have searched for this imprint, but their results generally pour cold water on the idea of significant melting. “Our evidence does not strongly support a much higher sea level,” says Jerry F. McManus of the Woods Hole (Mass.) Oceanographic Institution.
To track ancient sea levels, McManus and others rely on the split nature of oxygen atoms in the ocean. When water evaporates from the seas, it preferentially carries away the lighter isotope of oxygen, which can become trapped in snow and glaciers. The seawater left behind shows an increased proportion of the heavier isotope. Tiny marine organisms preserve a record of the shift in their shells, which eventually fall to the seafloor.
If during stage 11, melting ice pumped up ocean levels by 20 m, it should have lowered the ratio of heavy to light oxygen in these shells, says McManus. The records from the North Atlantic, though, show no dramatic events at the time. The simplest explanation is that sea levels remained near today’s value, he says.
McManus admits, however, that something else may have happened at the time to hide the evidence of melting. If the North Atlantic cooled slightly, the temperature shift would have pushed the oxygen-isotope ratio in the opposite direction, counterbalancing the signal from polar melting.
That’s what Poore thinks happened. He has recently collected sediment cores from the Cariaco Basin off Venezuela, where the stage 11 story reads differently from the way it does in the North Atlantic. The shells of single-celled animals that lived in the basin’s waters during stage 11 contain significantly more of the lighter isotope than is seen in modern shells.
“The easiest way to explain this is to say that sea level was higher and global ice volume was less than today,” says Poore.
Scurrying for higher ground
As a species, people have tended to congregate close to the ocean, so a repeat of stage 11 would send much of the world scurrying for higher ground. A full quarter of the current U.S. population would find itself underwater if sea levels were to rise by 10 m, calculate Poore and his colleagues.
Scientists disagree on whether coastal residents should worry about the long-term value of their property. The present interglacial has lasted 10,000 years, only about a third of the length of stage 11. So, melting of the polar ice could lie many millennia in the future.
On the other hand, global temperatures are rising rapidly, and greenhouse gases threaten to accelerate the warming in the next few decades.
Poore takes a conservative approach. “There’s nothing to worry about next week. But it’s wrong to just think that it’s a problem 500, 600, or 700 years in the future. I think it could be problem much sooner than that.”
Hearty agrees, saying, “As the greenhouse effect tends to increase the temperature of the Earth over the next 50 to 100 years, we can probably expect that some of the Antarctic ice will melt and destabilize.” He notes that small blocks of Antarctic ice have already started to collapse from rising temperatures.
Bowen, who considers himself a skeptic regarding greenhouse warming, says that human activity has no bearing on the issue of polar melting. “My view is straightforward: The longer an interglacial lasts, the higher the sea level is eventually going to be,” he says.
“It’s like building a big snowman in the garden. If it’s a big enough snowman, it will last long after the weather has warmed up. It will keep trickling away slowly,” he explains. “For the present interglacial, sea level is going to trickle upwards until the climate switches and we start descending into the next ice age.”
Scientists once thought that polar ice would have little trouble lasting until the next big freeze. The current interglacial is nearing its conclusion, according to conventional wisdom. Once the world made it past a few centuries’ worth of greenhouse warming, the climate would start getting cold again.
This conclusion rested on the assumption that the current interglacial would persist only about 10,000 years, the length of the last interglacial, which occurred 120,000 years ago.
Recent studies, however, have demonstrated that the last interglacial does not make a good model for understanding the present. Instead, stage 11 has emerged as the better example.
Scientists base the comparison on features of Earth’s orbit. Every 400,000 years, the shape of the orbit varies from a squashed circle to a more nearly perfect one. This shape alters the amount of summer sunlight hitting the Northern Hemisphere—the factor believed to push Earth into and out of ice ages. Modern times resemble stage 11 in that the orbit is nearly circular, which tends to dampen the climatic influences of other orbital factors, says André L. Berger of the Catholic University of Louvain in Belgium, who presented his findings at the recent AGU meeting.
Berger uses Earth’s orbital variations to calculate how insolation—the amount of sunlight hitting the planet—changes with time. In simulations using a computerized climate model, Berger and his colleagues found that the orbital effects would keep Earth out of an ice age for 30,000 years, producing an exceptionally long interglacial matched only by stage 11.
For the next few thousand years, he says, the sunlight variations will be rather weak. “The insolation is not going to vary that much. It will almost remain constant. That means that the other factors are going to play a very important role. Among those other factors are greenhouse gases.”
This quirk in the timing of Earth’s orbital wiggles has given greenhouse gases unusual potency in terms of changing climate, he says. Even just a few thousand years ago, the shape of the orbit was different enough that the astronomical forces would have exerted a controlling influence on climate, leaving less room for carbon dioxide and other gases to exert power.
The result is the climatic equivalent of Murphy’s law: Humans have started exploiting fossil fuels and altering Earth’s atmosphere at precisely the moment when greenhouse gases could do the most damage to climate.
If societies had bloomed and used up all the coal, oil, and natural gas several millennia earlier, greenhouse warming might have come and gone quietly, without any possibility that melting polar ice caps would eventually flood what people have erected.