TASIILAQ, Greenland — With a tense pilot gripping the stick, the helicopter hovered above the water, a red speck of machinery lost in a wilderness of rock and ice.
To the right, a great fjord stretched toward the sea, choked with icebergs. To the left loomed one of the immense glaciers that bring ice from the top of the Greenland ice sheet and dump it into the ocean.
Hanging out the sides of the craft, two scientists sent a measuring device plunging into the water, between ice floes. Near the bottom, it reported a temperature of 40 degrees. It was the latest in a string of troubling measurements showing that the water was warm enough to melt glaciers rapidly from below.
"That's the highest we've seen this far up the fjord," said one of the scientists, Fiammetta Straneo.
The temperature reading was a new scrap of information in the effort to answer one of the most urgent - and most widely debated - questions facing humanity: How fast is the world's ice going to melt?
Scientists long believed that the collapse of the gigantic ice sheets in Greenland and Antarctica would take thousands of years, with sea level possibly rising as little as 7 inches in this century, about the same amount as in the 20th century.
A rise of 3 feet by 2100?
As a result of recent calculations that take the changesinto account, many scientists now say that sea level is likely to rise perhaps 3 feet by 2100 - an increase that, should it come to pass, would pose a threat to coastal regions the world over.
And the calculations suggest that the rise could conceivably exceed 6 feet, which would put thousands of square miles of the American coastline under water and would probably displace tens of millions of people in Asia.
The scientists say that a rise of even 3 feet would inundate low-lying lands in many countries, rendering some areas uninhabitable.
It would cause coastal flooding of the sort that now happens once or twice a century to occur every few years. It would cause much faster erosion of beaches, barrier islands and marshes. It would contaminate fresh water supplies with salt.
N.C. would be affected
In the United States, parts of the East Coast and Gulf Coast would be hit hard. In New York, coastal flooding could become routine. About 15 percent of the urbanized land in the Miami region could be inundated. The ocean could encroach more than a mile inland in parts of North Carolina.
Abroad, some of the world's great cities - London, Cairo, Bangkok, Venice and Shanghai among them - would be critically endangered by a 3-foot rise in the sea.
Climate scientists readily admit that the 3-foot estimate could be wrong. Their understanding of the changes going on in the world's land ice is still primitive.
But, they say, it could just as easily be an underestimate as an overestimate. One of the deans of American coastal studies, Orrin H. Pilkey of Duke University, is advising coastal communities to plan for a rise of at least 5 feet by 2100.
Skeptics say it's natural
Global warming skeptics, on the other hand, contend that any changes occurring in the ice sheets are probably due to natural climate variability, not to greenhouse gases released by humans.
Climate scientists note that while the science of studying ice may be progressing slowly, the world's emissions of heat-trapping gases are not. They worry that the way things are going, extensive melting of land ice may become inevitable before political leaders find a way to limit the gases - and before scientists even realize such a point of no return has been passed.
"The past clearly shows that sea-level rise is getting faster and faster the warmer it gets," German climate researcher named Stefan Rahmstorf said. "Why should that process stop? If it gets warmer, ice will melt faster."