Over the past several decades, temperatures in the Arctic have increased at twice the rate of average global temperatures. Meanwhile, sea and land ice in the Arctic and parts of Antarctica have been shrinking at a surprising and ever-growing rate, and permafrost is thawing across the Arctic.

These and other changes pose an obvious threat to the polar regions. But, climate scientists say, they also signal changes to come around the globe.

"The Arctic is the canary in the coal mine," said geographer Mark Serreze, who studies Arctic climate change at the National Snow and Ice Data Center at the University of Colorado, Boulder. "We had always thought that as the effects of greenhouse gases take hold, the Arctic is where we would see the first signs and where the impacts would be most pronounced."

In addition, scientists say, polar climate change doesn't merely presage global climate change -- it also will contribute to it, because warming at the poles drive global changes in sea level and weather patterns.

"The climate system is very much linked in a global sense," said Serreze. "We can't separate changes at the poles from the rest of the planet."

Arctic thaw
The Arctic and Antarctica are very different climate systems, and right now scientists are seeing the largest climate changes in the Arctic.

The Arctic -- the North Pole -- is a mostly ice-covered sea surrounded by land, parts of which belong to Canada, the United States, Russia, Iceland, Greenland, Norway, Finland and Sweden.

The Arctic is particularly vulnerable to climate change because of that ice-covered sea. The thin layer of ice -- it's only about one to three meters thick -- has always acted as an insulator and reflector, reflecting the sun's rays and insulating the cold ocean below from the warmer atmosphere.

But if the Earth warms and the ice melts just a little, it can no longer insulate and reflect as well. This amplifies the small change, causing even more warming.

"It's a feedback loop that leads to more change," said Robert Bindschadler, a researcher at NASA's Hydrospheric and Biospheric Sciences Laboratory.

Sea-ice cover fluctuates over the course of the year -- it's at a minimum in the late summer. But scientists have watched as average sea-ice cover levels have shrunk over the past several decades, and they expect that shrinking to accelerate in the coming decades.

One recent climate model study, for example, predicted that Arctic sea ice would almost entirely disappear in the summertime by the year 2040.

"We have already witnessed major losses in sea ice, but our research suggests that the decrease over the next few decades could be far more dramatic than anything that has happened so far," said study author Marika Holland in a press statement.

Even more conservative models mostly predict that summer sea ice will disappear by the end of the 21st century. And this disappearance will affect not only the Arctic, but the rest of the globe, scientists say.

"[Sea ice] allows the air-conditioner of the global climate system to work," said Bindschadler.

Changes in the ocean temperature in the Arctic could change ocean circulation patterns and precipitation patterns around the world. Recent studies have suggested that one possible impact of losing sea ice cover is an extended drought in the American west, another points to precipitation changes in Europe, according to Serreze.

But, Serreze points out, it's difficult for scientists to predict these local changes with any certainty.

"Models can tell us a lot about how the globe will warm, but much of the uncertainty lies in the regional impacts," he said. Still, he added, "different climate models give different results, but the common thread is that the Arctic matters."

Meanwhile, land ice in the Arctic is also melting -- particularly large ice sheets on the coast of Greenland. That melting has accelerated the rate of ice returning to the ocean by 100 percent in the past five years, according to Bindschadler.

"As a community we've been astonished at how fast the margin of Greenland has been changing," he said. "When I was in grad school, it was thought that these ice sheets changed over time scales of thousands of years. When I was working there 20 years ago, it was over hundreds of years. Now, in the most recent observations, it's been changing by tens of percents each year."

When land ice melts, it contributes to global climate change in a different way -- by causing the global sea level to rise. Some studies have suggested that if the Greenland ice sheet continues to melt at its current rate, it could contribute to a global sea level rise of up to 19 feet in the next few centuries.

Antarctic changes
The effects of climate change in most of Antarctica (the South Pole), meanwhile, have not been as dramatic as in the Arctic. There are a few reasons for this. First, Antarctica -- a continent surrounded by ocean -- is more isolated from the rest of the planet than the Arctic is, so its climate patterns are not as easy to disturb.

"There's a huge ocean around Antarctica extending for many kilometers, which tends to stabilize climate patterns on the continent," said Ted Scambos, a geologist at the National Snow and Ice Data Center. It's much harder, he said, for large masses of cold or warm water to enter.

Also, Antarctica is simply colder than the Arctic. In the Arctic, temperatures can hover around the freezing point for part of the year, so a few degrees of temperature rise can cause sea and land ice to begin to melt. But in much of Antarctica, where temperatures might average -40 degrees Celsius, a few degrees of warming wouldn't have the same effect.

Finally, researchers think that the hole in the ozone layer above Antarctica is counteracting the effects of global warming. Ozone absorbs energy, so removing it cools that layer of the atmosphere.

Still, many scientists say Antarctica may not remain immune from the effects of climate change forever. "Antarctica is really the sleeping giant," said Serreze.

One part of Antarctica, in fact, is already changing. The Antarctic Peninsula, which sticks out into the ocean and is exposed to winds carrying warmer air, is among the most quickly warming places on Earth -- and an ice sheet the size of Rhode Island has already separated from the continent there.

"We thought these ice shelves would be stable for many centuries in a warming climate," said Scambos. "But what we're finding is that a tiny amount of warming in the ocean, when rapidly circulated, can cause these shelves to disintegrate within just a few years."

Understanding what will happen in Antarctica is key to understanding what will happen to global sea levels in the next few centuries, because Antarctica holds 90 percent of the world's ice.

"The main way that Antarctica will affect the rest of the world is going to be in relation to its impact on sea level rise," Scambos said.

-- By Lea Winerman, Online NewsHour