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 Hearty, a North Carolina geologist, 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 coastline.

For the leader of the team, Maureen 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 after 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 U.S. 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, enough polar ice melts, over time, to raise the global sea level by about 25 to 30 feet. But in the coming century, Earth is expected to warm more than that, perhaps 4 or 5 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, but on what they describe as a growing body of evidence about what occurred in the past. To add to that body of knowledge, Raymo is studying geologic history going back several million years.

"I wish I could take people that question the significance of sea level rise out in the field with me," 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.'"

'Something bigger and faster than nature ever has'

Skeptics who play down 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 sea floor. 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 Alley, a climate scientist at Pennsylvania State University. "But it looks like we're going to do something bigger and faster than nature ever has."

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 in the average global temperature appears 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. "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 Mitrovica, an earth physicist at Harvard who collaborates with Raymo.

Running the movie of the Earth's history in reverse

Like many of her colleagues, Raymo 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, about 3 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.

Two years ago, Raymo proposed pulling together a worldwide network of experts: to find, date and measure Pliocene beaches on nearly every continent where the ocean encroached as far as 90 miles inland and then to work with experts in computer modeling to take account of all the factors known to alter sea level. The federal National Science Foundation awarded the group $4.2 million.

Over the next few years, her team hopes to arrive at the magic number 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 Raymo's work were to confirm such a high estimate, it would suggest that the ice sheet in eastern Antarctica -- containing enough water to raise sea level by 180 feet -- is also vulnerable to melting. "Just the mere fact that we know the number will tell us right off the bat, is East Antarctica stable?" Raymo said. "Or is it a huge risk?"

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 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? Raymo and her team share an emerging scientific consensus that the increase in this century will probably be on the order of 3 feet, perhaps as much as 6 feet. That would almost certainly require millions of people to leave coastal regions.