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 seven inches in this century, about the same amount as in the 20th century.

But researchers have recently been startled to see big changes unfold in both Greenland and Antarctica.

As a result of recent calculations that take the changes into account, many scientists now say that sea level is likely to rise perhaps three 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 six 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 three 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.

In the United States, parts of the East Coast and Gulf Coast would be hit hard. In New York, coastal flooding could become routine, with large parts of Queens and Brooklyn especially vulnerable. 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 three-foot rise in the sea.

Climate scientists readily admit that the three-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 five feet by 2100.

David Fuller's view Whether changes in the world's land ice are caused by natural climate variability or greenhouse gasses released by humans, the trend, should it continue, would have sobering consequences.

It would not be surprising if the cause was a combination of both natural and human factors. After all, the world's climate has varied considerably over millennia and we have been fortunate to live in such a stable period. Also, how could the ever rising quantity of greenhouse gasses released by mankind's enterprises over two centuries and counting not be a factor?

My guess is that long before we found ourselves wading in our low-lying costal cities, climatic changes would create havoc with our food supplies. Hopefully, the turbulent weather patterns responsible for reductions in global yields for many agricultural commodities this year were flukes, but I would not count on it.

We will find out over the next few years.

Meanwhile, the investment and speculative fashion for commodities obviously contributed to this year's prices rises. We know this from the speed with which prices have fallen back in the last few days. This confirms that peaks of at least near-term and quite possibly medium-term significance have been established in many commodities. This is good news in terms of global stability because it will reduce concern over commodity price inflation... for a while.

Note: For more on the many changes in short and possibly medium-term trends, please listen to the Audios. Eoin and I can always cover much more ground in an Audio than we can by writing.