The North Pole has been frozen for 100,000 years. But according to scientists, that won't be true by the end of this century. The top of the world is melting.

There's been a debate burning for years on the causes of global warming. But the scientists you're about to meet say the debate is over. New evidence shows man is contributing to the warming of the planet, pumping out greenhouse gases that trap solar heat.

Much of this new data was compiled by American scientist Bob Corell, who led a study called the "Arctic Climate Impact Assessment." It's an awkward name — but consider the findings: the seas are rising, hurricanes will be more powerful, like Katrina, and polar bears may be headed toward extinction.

What does the melting arctic look like? Correspondent Scott Pelley went north to see what Bob Corell calls a "global warning."

Towers of ice the height of 10-story buildings rise on the coast of Greenland. It's the biggest ice sheet in the Northern Hemisphere, measuring some 700,000 square miles. But temperatures in the arctic are rising twice as fast as the rest of the world, so a lot of Greenland's ice is running to the sea.

"The entire planet is out of balance," says Bob Corell, who is among the world's top authorities on climate change. He led 300 scientists from eight nations in the "Arctic Climate Impact Assessment."

Corell believes he has seen the future. "This is a bellwether, a barometer. Some people call it the canary in the mine. The warning that things are coming," he says. "In 10 years here in the arctic, we see what the rest of the planet will see in 25 or 35 years from now."

Over the last few decades, the North Pole has been dramatically reduced in size and Corell says the glaciers there have been receding for the last 50 years.

Back in 1987, President Reagan asked Corell to look into climate change. He's been at it ever since.

In Iceland, he showed 60 Minutes glaciers that were growing until the 1990s and are now melting. In fact, 98 percent of the world's mountain glaciers are melting.

Corell says all that water will push sea levels three feet higher all around the world in 100 years.

"You and I sit here, another foot. Your children, another foot. Your grandchildren, another foot. And it won't take long for sea level to inundate," says Corell.

"Sea level will be inundating the low lands of virtually every country of the world, ours included," Corell predicts.

To find the sights and sounds of the arctic melting, there are few places better than a fjord in Greenland, with a glacier just a short distance away.

Pelley stood on a huge block of ice that had split off from the glacier and had dropped into the sea — a big iceberg.

"This part of Greenland is melting faster than just about any other. To get a sense of the enormity of what's happening, consider this: The ice that is melting here is the equivalent of all the ice in the Alps," Pelley explained, standing atop the iceberg.

That's more than 105 million acres of melted ice in 15 years. Just four minutes after Pelley cleared off this berg, part of the ice caved in.

60 Minutes got a bird's-eye view of how unstable the ice is becoming on a flight with glaciologist Carl Boggild.

Boggild anchored 10 research stations to the ice. Every time he comes to visit, the ice and his stations have moved.

Flying over the ice, Pelley noticed lots of fissures and crevices breaking through the ice.

Asked what causes this, Boggild explained, "This is actually the ice flow, where you have so much tension in the ice that it cannot stick together. And it breaks and opens a crevice which goes about 150, 200 feet down."

The ice is also melting on the sides, Boggild says.

High overhead, Pelley remarked that one could hear the water running.

"It's like a small river," Boggild said.

A leading theory says those little rivers lubricate the bottom of the ice sheet, helping it move off the bedrock and out to sea.

And there may be no stopping it. arctic warming is accelerating. It's a chain reaction. As snow and ice melt they reveal dark land and water that absorb solar heat. That melts more snow and ice, and around it goes.

There's long been a debate about how much of this is earth's naturally changing climate and how much is man's doing. Paul Mayewski, at the University of Maine, says the answer to that question is frozen inside an ice core from Greenland.

With funding from the National Science Foundation, Mayewski has led 35 expeditions collecting deep ice cores from glaciers. The ice captures everything in the air, laying down a record covering half a million years.

"We can go to any section of the ice core, to tell, basically, what the greenhouse gas levels were; we can tell whether or not it was stormy, what the temperatures were like," Mayewski explains.

60 Minutes brought Mayewski back to Greenland, where he says his research has proven that the ice and the atmosphere have man's fingerprints all over them.

Mayewski says we haven't seen a temperature rise to this level going back at least 2,000 years, and arguably several thousand years.

As for carbon dioxide (CO 2) levels, Mayewski says, "we haven't seen CO 2 levels like this in hundreds of thousands of years, if not millions of years."

What does that tell him?

"It all points to something that has changed and something that has impacted the system which wasn't doing it more than 100 years ago. And we know exactly what it is. It's human activity," he says.

It's activity like burning fossil fuels, releasing carbon dioxide and other greenhouse gases. The U.S. is by far the largest polluter. Corell says there's so much greenhouse gas in the air already that more temperature rise is inevitable.

Even if we stopped using every car, truck, and power plant — stopping all greenhouse gas emissions — Mayewski says the planet would continue to warm anyway. "Would continue to warm for another, about another degree," he says.

That's enough to melt the Arctic — and if greenhouse gases continue to increase, the temperature will rise even more. The ice that's melting already is changing the weather by disrupting ocean currents.

Corell points to floods in the U.S., heat waves in Europe; and 60 Minutes wanted to know about this catastrophic 2005 hurricane season.

"The one thing I think we can say with a fairly high degree of confidence is the severity of the storms, how strong the storms, these cyclonic events like hurricanes and cyclones in the Pacific, are going to get — they're gonna be more severe. Now one thing that is in doubt is whether there'll be more of them," Corell explains.

"The oceans of the Northern Hemisphere are the warmest they've been on record. When they get up in that temperature, they spin off hurricanes. Well, if it goes up another degree, it's gonna spawn these with more intensity," Corell says.

The name "arctic" comes from ancient Greek meaning "land of the great bear."

But the warming climate is threatening this icon of the arctic, the polar bear. Flying above the sub-arctic region of Hudson Bay, Canadian scientist Nick Lunn is hunting polar bears in a 30-year study that tracks their health. It's the job of his assistant Evan Richardson to take them down with a tranquilizer dart.

Once tranquilized, Lunn carefully checks the bear with a pole, without getting too close.

The polar bear is the largest predator on land. Native people in the region say he'll even hunt humans, but not on the day Pelley joined Lunn: with the tranquilizer, the bear was awake but immobile.

The scientists knew this bear by his tattoo. His history is written chapter and verse in the "bear bible."

"This is the record book of all the bears that have been handled by us or Manitoba Conservation," Lunn explains.

The study began at the Wapusk National Park, because the bear population was thought to be the healthiest in the world.

Lunn's annual checkup records changes in fat, dimensions and an inventory of weapons. The polar bear uses its teeth to hunt primarily one thing — seal. That's where arctic warming comes in.

Polar bears can only hunt on the ice. Lunn says the ice is breaking up three weeks earlier than it did 30 years go. He's now finding female bears 55 pounds lighter — weaker mothers with fewer cubs.

Asked how the bear population has changed since he started his research, Lunn says, "When we first started doing this research, we've done inventories in the mid-80s, in the mid-90s. Both times we came out with an estimate of approximately 1,200 animals for what is known as the western Hudson Bay population. The numbers now suggest that the population has declined to below 1,000."

The bears are unlikely to survive as a species if there's a complete loss of ice in summer, which the arctic study projects will happen by the end of this century.

There are skeptics who question climate change projections like that, saying they’re no more reliable than your local weatherman. But Mayewski says arctic projections done decades ago are proving accurate.

"That said, the skeptics have brought up some very, very interesting issues over the last few years. And they've forced us to think more and more about the data that we collect. We can owe the skeptics a vote of thanks for making our science as precise as it is today," says Mayewski.

One big supporter of climate science research is the Bush administration, spending $5 billion a year. But Mr. Bush refuses to sign a treaty forcing cuts in greenhouse gases.

The White House also declined 60 Minutes' request for an interview. Corell, who first studied the issue for President Reagan, believes the climate change facts are in, even if President Bush does not.

"When you look at the American government, which is saying essentially, 'Wait a minute. We need to study this some more. We can't flip our energy use overnight. It would hurt the economy.' When you hear that, what do you think?" Pelley asked.

"Well, what I do then is, I try to tell them exactly what we know scientifically. The science is, I believe, unassailable," says Corell. "I'm not arguing their policy, that's their business, how they deal with policy. But my job is to say, scientifically, shorten that time scale so that if you don't push out the effects of climate change into the long, long distant future. Because even under the best of circumstances, this natural system of a climate will continue to warm the planet for literally hundreds of years, no matter what we do."