How High Could the Tide Go?


Justin Gillis

BREDASDORP, South Africa — A scruffy crew of scientists barreled down a dirt road, their two-car caravan kicking up dust. After searching all day for ancient beaches miles inland from the modern shoreline, they were about to give up.

Suddenly, the lead car screeched to a halt. Paul J. Hearty, a geologist from North Carolina, leapt out and seized a white object on the side of the road: a fossilized seashell. He beamed. In minutes, the team had collected dozens more.

Using satellite gear, they determined they were seven miles inland and 64 feet above South Africa’s modern coastline.

For the leader of the team, Maureen E. Raymo of Columbia University, the find was an important clue as she tries to determine just how high the oceans might rise in a warmer world.

The question has taken on new urgency in the aftermath of Hurricane Sandy, which caused coastal flooding that scientists say was almost certainly worsened by the modest rise of sea level over the past century. That kind of storm tide, the experts say, could become routine along American coastlines by late in this century if the ocean rises as fast as they expect.

In previous research, scientists have determined that when the earth warms by only a couple of degrees Fahrenheit, enough polar ice melts, over time, to raise the global sea level by about 25 to 30 feet. But in the coming century, the earth is expected to warm more than that, perhaps four or five degrees, because of human emissions of greenhouse gases.

Experts say the emissions that may make a huge increase of sea level inevitable are expected to occur in just the next few decades. They fear that because the world’s coasts are so densely settled, the rising oceans will lead to a humanitarian crisis lasting many hundreds of years.

Scientists say it has been difficult to get people to understand or focus on the importance, for future generations, of today’s decisions about greenhouse gases. Their evidence that the gases represent a problem is based not just on computerized forecasts of the future, as is commonly believed, but on what they describe as a growing body of evidence about what occurred in the past.

To add to that body of knowledge, Dr. Raymo is studying geologic history going back several million years. The earth has warmed up many times, for purely natural reasons, and those episodes often featured huge shifts of climate, partial collapse of the polar ice sheets and substantial increases in sea level.

“I wish I could take people that question the significance of sea level rise out in the field with me,” Dr. Raymo said. “Because you just walk them up 30 or 40 feet in elevation above today’s sea level and show them a fossil beach, with shells the size of a fist eroding out, and they can look at it with their own eyes and say, ‘Wow, you didn’t just make that up.’ ”

Skeptics who play down the importance of global warming like to note that these past changes occurred with no human intervention. They argue that the climate is ever-changing, yet humans or their predecessors managed to prosper.

The geologic record does offer startling examples of the instability of the planet. Whale bones can be dug up in the Sahara. The summit of Mount Everest is a chunk of ancient seafloor.

But most climate scientists reject the idea that this history means human-induced climate change will be benign. They add that the fossil record indicates nothing quite like today’s rapid release of greenhouse gases and its parallel effect of raising the planet’s temperature, changes that are occurring in a geologic instant.

“Absolutely, unequivocally, nature has changed before,” said Richard B. Alley, a leading climate scientist at Pennsylvania State University. “But it looks like we’re going to do something bigger and faster than nature ever has.”

Clues From Fossils

In any given era, the earth’s climate responds to whatever factors are pushing it to change.

Scientists who study climate history, known as paleoclimatologists, focus much of their research on episodes when wobbles in the earth’s orbit caused it to cool down or warm up, causing sea level to rise or fall by hundreds of feet.

Carbon dioxide, the main greenhouse gas, appears to have played a crucial role. When changes in the orbit caused the earth to cool, scientists say, a large amount of carbon dioxide entered the ocean, reducing the heat-trapping properties of the atmosphere and thus amplifying the cooling. Conversely, when the shifts in sunlight led to initial warming, carbon dioxide emerged from the ocean and helped speed the end of the previous ice age.

Based on this record, scientists like Dr. Alley describe carbon dioxide as the master control knob of the earth’s climate. A large body of scientific evidence shows that the current increase in the gas is being caused by human activity, meaning that people are essentially twisting the earth’s thermostat hard to the right.

In most of the previous warm periods, some ice remained near the poles, in Greenland and Antarctica. Today, enough water is stored as ice in those regions to raise the level of the ocean roughly 220 feet, should all of it melt.

The fossil record suggests that temperatures slightly warmer than today would not be enough to melt the ice caps entirely. But an increase of even a few degrees Fahrenheit in the average global temperature does appear to cause severe damage. From the last time that happened, about 120,000 years ago, scientists have found more than a thousand elevated fossil beaches around the world.

Many scientists believe that, as a result of human-induced warming, temperatures are already entering the danger zone. They are seeing rapid changes in Greenland and western Antarctica.

“I can merely tell you that every time in recent earth history where we’ve had these kinds of temperatures for any protracted period of time, two polar ice sheets have catastrophically collapsed,” said Jerry X. Mitrovica, an earth physicist at Harvard who collaborates with Dr. Raymo.

Dr. Raymo works at Lamont-Doherty Earth Observatory, a unit of Columbia University just outside New York City. Like many of her colleagues, she is trying to run the movie of the earth’s history in reverse, finding an era with temperatures that mirror those expected before 2100.

She has zeroed in on the Pliocene epoch, roughly three million years ago. The level of carbon dioxide in the air then appears to have been about 400 parts per million — a level that will be reached again within the next few years, after two centuries of fossil fuel burning.

Previous efforts to estimate the maximum rise of the sea in the Pliocene did not take full account of some factors now known to be important.

In Search of Prehistoric Beaches

Two years ago, in hopes of pinning down a better answer, Dr. Raymo pitched an ambitious plan to the National Science Foundation, the federal agency that pays for much of the country’s scientific research. She proposed to pull together a worldwide network of expert collaborators: to find, date and measure Pliocene beaches on nearly every continent and then to work with experts in computer modeling to take careful account of all the factors known to alter sea level.

The N.S.F. awarded the group $4.2 million, with one anonymous scientific reviewer declaring that the plan would permit a “far more precise and quantitative prediction of future climate change.”

This summer, Dr. Raymo and her team drove hundreds of miles along South Africa’s southern and western coasts, scouting for prehistoric beaches.

To collect ancient seashells for laboratory testing, they hiked treacherous paths and descended into old quarries and diamond mines. At one point an Australian researcher, Michael J. O’Leary, and an Italian colleague, Alessio Rovere, climbed a steep cliff face to take measurements, clinging to shrubs as their feet kept slipping.

The team located suspected Pliocene beaches as low as 38 feet and as high as 111 feet above modern sea level. In similar work in Australia and on the East Coast of the United States, the researchers have found Pliocene beaches as low as 33 feet and as high as 295 feet above sea level.

Part of the explanation for such varying elevations, Dr. Raymo said, is that the land itself has almost certainly moved over the last three million years, unevenly — thus raising or lowering beach deposits after they had been laid down.

Scientists have come to realize this can happen anywhere in the world, even far from geological hot spots, a major factor complicating their interpretation of past sea level. “A lot of the big task we have is teasing apart this dance that the crust of the earth is doing with the level of the sea,” Dr. Raymo said.

Over the next few years, her team plans to gather new measurements from most continents, including North America, where the Pliocene ocean encroached as far as 90 miles inland. After several years of work, they hope to arrive at the magic number Dr. Raymo calls Pliomax, or the maximum global sea level rise during the Pliocene.

That figure may help to solve a vexing scientific problem.

A large body of evidence suggests that the ice sheets atop Greenland and the low-lying, western part of Antarctica are vulnerable to global warming. But together, they can supply no more than about 40 feet of sea level rise.

The previous estimates of Pliocene sea level, based on spotty evidence, range from 15 feet to 130 feet above today’s ocean, with 80 feet being a commonly cited figure. If Dr. Raymo’s work were to confirm such a high estimate, it would suggest that the ice sheet in eastern Antarctica — by far the biggest chunk of ice in the world, containing enough water to raise sea level by 180 feet — is also vulnerable to melting. And if it is, scientists do not fully understand why, because their computer forecasts — acknowledged to be imperfect — suggest most of it should remain stable even in a warmer world.

“Just the mere fact that we know the number will tell us right off the bat, is East Antarctica stable?” Dr. Raymo said. “Or is it a huge risk?”

Thus, if the project is successful, it may put an upper limit on how much the ocean is ultimately capable of rising if temperatures go up as much as expected this century.

But the Pliomax project will not be able to answer what might be an even bigger question: In a worst-case scenario, how fast could the rise happen?

Dr. Raymo and her team share an emerging scientific consensus that the increase in this century will probably be on the order of three feet, perhaps as much as six feet. That would almost certainly require millions of people to evacuate coastal regions.

Calculations by Climate Central, a research group, suggest that once the ocean has risen five feet, storm tides comparable to those of Hurricane Sandy could occur about every 15 years in New York City.

Scientists say that in the 22nd century, the problem would probably become far worse, and the rise would then continue for many centuries, perhaps thousands of years. Recent research suggests the likely rise could be 12 feet by the year 2300, inundating coastal regions around the world.

If the rise is slower than expected, society may have time to adjust, or to develop new technology to solve the problem of greenhouse emissions. But many scientists are plagued by a nagging fear that the opposite will occur — that their calculations will turn out to have been too conservative, and social stability will eventually be threatened by a rapid rise of the sea.

“At every point, as our knowledge increases,” Dr. Raymo said, “we’ve always discovered that the climate system is more sensitive than we thought it could be, not less.”