The gargantuan chunks of ice breaking off the Sermeq Kujalleq glacier and thundering into an Arctic fiord are a spectacular sight. But to Greenlanders the spectacle is also deeply worrisome.

The frequency and size of the icefalls are a powerful reminder that the frozen sheet covering the world's largest island is thinning -- a sign of global warming, scientists say.

"In the past, we could walk on the ice in the fiord between the icebergs for a six-month period during the winter, drill holes and fish," said Joern Kristensen, a fisherman and one of the indigenous Inuit who make up most of Greenland's population of 56,000. "We can only do that for a month or two now. It has become more difficult to drive dog sleds because the ice between the icebergs isn't solid anymore."

In 2002-03, a six-mile-long stretch of the Sermeq Kujalleq glacier broke off and drifted out of the fiord near Ilulissat, Greenland's third-largest town, 155 miles north of the Arctic Circle.

Although Greenland, three times the size of Texas, is the prime example of warming, scientists say the effects of climate change are noticeable throughout the Arctic region, from the northward spread of spruce beetles in Canada to melting permafrost in Alaska and northern Russia.

Indigenous people, who for centuries have adapted their lives to the cold, fear that even small and gradual changes could have a profound impact.

"We can see a trend that the fall is getting longer and wetter," said Lars-Anders Baer, a political leader of Sweden's Sami, a formerly nomadic, reindeer-herding people. "If the climate gets warmer, it is probably bad for the reindeer. New species [of plants] come in and suffocate other plants that are the main food for the reindeer," he said.

Rising temperatures are also a concern in the Yamalo-Nenets region of western Siberia, according to Alexandr Navyukhov, 49. He and other ethnic Nenets live mostly off hunting, fishing and deer-breeding.

"We now have bream in our river, which we didn't have in the past because that fish is typical of warmer regions," he said. "On the one hand, it may look like good news, but bream are predatory fish that prey upon fish eggs, often of rare kinds of fish."

Melting permafrost has damaged hundreds of buildings, railway lines, airport runways and gas pipelines in Russia, according to the 2004 Arctic Climate Impact Assessment commissioned by the Arctic Council, an intergovernmental body.

Research also shows that populations of turbot, Atlantic cod and snow crab are no longer found in some parts of the Bering Sea, an important fishing zone between Alaska and Russia, and that flooding along the Lena River, one of Siberia's biggest waterways, has increased with rising temperatures.

In Greenland, Anthon Utuaq, a 68-year-old retired hunter, worries that a warmer climate will make it harder for his son to continue the family trade.

"Maybe it will be difficult for him to find the seals," Utuaq said, resting on a bench in the eastern coastal town of Kulusuk. "They will head north to colder places if it gets warmer."

Arctic sea ice has decreased by about 8 percent, or more than 380,000 square miles, over the past 30 years.

In Sisimiut, Greenland's second-largest town, lakes have doubled in size in the last decade.

"Greenland was perceived as this huge solid place that would never melt," said Robert Corell of the American Meteorological Society, a Boston-based scientific organization. "The evidence is now so strong that the scientific community is convinced that global warming is the cause."

How much of it is natural and how much is caused by humans burning fossil fuels is sharply debated. Greenland endured sharp climate shifts long before fossil fuels were an issue, and the island sustained Norse settlements for 400 years until the 15th century.

"We know that temperatures have gone up and it's partly caused by man. But let's hold our horses because it's not everywhere that the ice is melting," said Bjoern Lomborg, a Danish researcher who plays down the magnitude of the global-warming threat. "In the Antarctic, only 1 percent is melting."

What is clear is that the average ocean temperature off Greenland's west coast has risen in recent years -- from 38.3 degrees Fahrenheit to 40.6 -- and glaciers have begun to retreat, said Carl Egede Boeggild, a glaciologist with the Geological Survey of Denmark and Greenland, a government agency.

The Sermilik glacier in southern Greenland has retreated nearly seven miles, and the Sermeq Kujalleq glacier near Ilulissat is also shrinking, said Henrik Hoejmark Thomsen of the geological survey.

In 1967, satellite imagery measured it moving 4.3 miles a year. In 2003, the rate was 8.1 miles.

"What exactly happened, we don't know. But it appears to be the effect of climate change," Hoejmark Thomsen said.

In August, the National Science Foundation's Arctic System Science Program committee issued a report saying the rate at which ice is melting in the Arctic is increasing. Within a century, the report said, summertime oceans could be ice-free for the first time.

With warmer temperatures, some bacteria, plants and animals could disappear, while others will thrive. Polar bears and other animals that depend on sea ice to breed and forage are at risk, scientists say, and some species could face extinction in a few decades.

The thinning of the sea ice presents a danger to both humans and polar bears, said Peter Ewins, director of Arctic conservation for the World Wildlife Fund Canada.

"The polar bears need to be there to catch enough seals to see them through the summer in open, warm water systems," Ewins said. "Equally, the Inuit need to be out there on the ice catching seals and are less and less able to do that because the ice is more unstable, thinner.".

When NASA started taking satellite images of the Arctic region in the late 1970s and computer technology improved, scientists noted alarming patterns and theorized that the culprit was gases emitted by industries and internal combustion engines to create a "greenhouse effect" of trapping heat in the atmosphere.

Inuit leaders are trying to draw attention to the impact of climate change and pollution.

"When I was a child, the weather used to be more stable. It worries me to see and hear all this," Greenland's prime minister, Hans Enoksen, said in Ilulissat in August, on the sidelines of a meeting of environmental officials from 23 countries. The forum ended with statements of concern -- and no action.

The Kyoto Protocol that took effect in February aims to reduce greenhouse gas emissions. But the more than 140 nations that have signed the pact do not include the United States, which produces one-quarter of the gases.

The Bush administration says participation would severely damage the U.S. economy. Many scientists say that position undermines the whole planet, and they point to Greenland as the leading edge of what the globe could suffer.

"Greenland is the canary in a mine shaft alerting us," said Corell, the American meteorologist, standing on the edge of the Sermeq Kujalleq glacier. "In the U.S., global warming is a tomorrow issue. . . . For us working here, it hits you like a ton of bricks when you see it."

Icebergs drift in the bay in Kulusuk, eastern Greenland, in August as scientists met to assess the threat of global warming in the Arctic area.Ice floated in Greenland's Disko Bay, above the Arctic Circle, in August.Water drips from an iceberg in Kulusuk. Environmentalists cite shrinking glaciers as a sign of global warming.