Justin Gillis/The New York Times
PREHISTORIC SHORELINES Researchers explored ancient rock formations on South Africa's coast. They are looking for critical clues from records of past climate change to help predict sea level rise in a warming world.
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.
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