NASA via Reuters, file

The Sheldon Glacier with Mount Barre in the background, is seen from Ryder Bay near Rothera Research Station, Adelaide Island, Antarctica, in this NASA handout photo.

OSLO, Norway - Global warming is expanding the extent of sea ice around Antarctica in winter in a paradoxical shift caused by cold plumes of summer melt water that re-freeze fast when temperatures drop, according to a study unveiled Sunday.

An increasing summer thaw of ice on the edges of Antarctica, twinned with less than expected snowfall on the frozen continent, is also adding slightly to sea level rise in a threat to low-lying areas around the world, it said.

Climate scientists have been struggling to explain why sea ice around Antarctica has been growing, reaching a record extent in the winter of 2010, when ice on the Arctic Ocean at the other end of the planet shrank to a record low in 2012.

Sinead Farrell / NASA

Ice floes are shown at the foot of an iceberg in Antarctica's Amundsen Sea in October 2010.

"Sea ice around Antarctica is increasing despite the warming global climate," said Richard Bintanja, lead author of the study at the Royal Netherlands Meteorological Institute.

"This is caused by melting of the ice sheets from below," he told Reuters of the findings in the journal Nature Geoscience.

Ice is made of fresh water and, when ice shelves on the fringes of Antarctica thaw in summer because of upwellings of warming sea water, the meltwater forms a cool layer that floats on the denser, warmer salty sea water below, the study said.

In winter, the melt water readily turns to ice because it freezes at zero degrees Celsius, above sea water at -2C (28.4F).

At a winter maximum in September, ice on the sea around Antarctica covers about 19 million sq kms (7.3 million sq miles), bigger than Antarctica's land area. It then melts away into the ocean as summer approaches.

Among other scientists, Paul Holland of the British Antarctic Survey stuck to his findings last year that a shift in winds linked to climate change was blowing a layer of melt water further out to sea and adding to winter ice.

"The possibility remains that the real increase is the sum of wind-driven and melt water-driven effects, of course. That would be my best guess, with the melt water effect being the smaller of the two," he said.

Bintanja's study also said the cool melt water layer may limit the amount of water sucked from the oceans that falls as snow on Antarctica. Cold air can hold less moisture than warm.

"Cool sea surface temperatures around Antarctica could offset projected snowfall increases in Antarctica, with implications for estimates of future sea-level rise," it said.

The U.N. panel of climate scientists has estimated that sea levels will rise by between 18 and 59 cm (7-24 inches) this century, more if thaws of Antarctica and Greenland accelerate.

The panel's main scenarios assume that Antarctica alone will make sea levels fall by between 2 and 14 cms this century because more snowfall will extract water from the sea.

But Sunday's study said that Antarctica was losing about 250 billion tonnes of ice a year - equivalent to 0.07 millimetre(0.003 inch) of sea level rise a year, Bintanja said. "Antarctic mass loss seems to be accelerating," it said.

Another study in Nature Geoscience said Antarctica's snowfall had been over-estimated by between 11 and 36.5 billion tonnes a year because of fierce winds blasting many regions.

Strong winds created conditions to "sublimate" snow, or make it pass from a frozen state to a gas without first becoming liquid, a U.S.-led team wrote. 

NBC News Correspondent Kerry Sanders recently returned from Antarctica where he reported on a series of stories about how climate change is affecting the breathtaking seventh continent.

He responded to reader's questions about Antarctica earlier today.

Click on the box below to replay the interesting chat.  

You also can click on the map below for more dispatches from across Antarctica. 

More on this series:

• Day 1: Greeted by dirt, not ice
• Day 2: Climate change decimates food supply for penguins
• Day 3: Watching Mother Nature in action
• Day 4: How to sleep outdoors in Antarctica
• Finale: Trips to the seventh continent are not just for scientists

NBC News Correspondent Kerry Sanders and Producer Nery Ynclan recently returned from Antarctica. Below, Ynclan chronicles the journey; you also can click on the map above for more dispatches from across the breathtaking seventh continent.

ANTARCTIC PENINSULA -- Visiting Antarctica is like visiting another planet, where the aliens are friendly and greet you in tuxedos.

Seeing the seventh continent is a bucket-list must, and it is more accessible than ever before. About 35,000 people visit each year.

Anyone over 13 years old can go to Antarctica using most of the 40-plus companies that host polar expeditions.

Trips can cost anywhere between $4,000 and $50,000 for 11 to 20 days. Prices depend on how early you book -- two years ahead for the best deals -- and whether you bunk with strangers or want VIP accommodations on a private yacht.

But the sights and the meals are the same: incredible for everyone.

In the elements
It is called an expedition and not a cruise for a couple of reasons: The storied Drake Passage is seriously rough, and the weather decides where you are going. Motion-sickness medication is a necessity.

Unless you live in Argentina or New Zealand, getting there is a schlep. We traveled from Miami to Buenos Aires, and a day later flew another four hours to Ushuaia, Argentina’s southernmost spot, where we spent the afternoon in Tierra del Fuego National Park.

NBC News

Nery Ynclan in front of signs showing the distance of various international destinations at the Brown science station in Paradise Bay, Antarctica.

The next day, we boarded Quark Expeditions to Antarctica with a busload of people from all over the world for an adventure that 100 years ago seemed impossible.

Our destination: A rocky land mass about twice the size of the continental United States, frozen over by mountains of ice and snow dating back hundreds of thousands of years.

After a merciful 18 hours through the Drake Passage, we disembarked onto Zodiacs and headed for our first landing on Antarctica.

Slideshow: Antarctica: Journey to the bottom of the Earth

NBC News

See photos from NBC's Kerry Sanders' voyage to Antarctica.

Launch slideshow

‘Lucky to be here’
Perched on the Zodiac, Louise Lewen of Canada capsulized the excitement of seeing our first wild penguins: “They’re all here as if they’re coming to say, ‘Welcome to my home, welcome to my world.’”

We were surrounded by glaciers the size of skyscrapers, Gentoo and Chinstrap chicks chasing parents for food and a beach awash in giant chunks of ice – it was unreal.

“I feel so lucky to be here,” said Eva Mallis of New York.

A big part of these eco-travel trips is onboard history classes. We had some extra special guests: Falcon Scott and Jonathon Shackleton, descendants of two of the most famous polar explorers who traveled to Antarctica more than 100 years ago.

Chile's connection to Shackleton's adventure

The original Scott and Shackleton traveled together to Antarctica in 1901 in one of various turn-of-the-century attempts to reach the South Pole. Scott finally reached the pole in 1912, but died along with his men on the bitter trek back. Shackleton secured his place in the history books with the 1914 trip of the Endurance, the storied ship that became trapped in the ice, stranding the crew for nearly two years years and forcing them to eat seals and even their sled dogs. They were eventually rescued.

‘Sleeping on ice’
Another highlight was spending the night camping on the ice, splayed out like elephant seals. The critical choice was whether to sleep in a traditional tent or a bivy sack -- essentially a plastic-zippered body bag to shield your sleeping bag from the elements.

With limited bivy sacks, we arrived early to be first in line -- it was our way to be closer to Mother Nature. That night, we were hit with rain and snow. No bivy-wackers slept a wink (except Kerry – correspondents are perpetually exhausted and can sleep anywhere, even when surrounded by penguins and leopard seals.)

A warm shower on the ship got us all back in the frozen wilderness mood and the Zodiac rides and landings that followed were each as magical as the first. Whether we floated up to rocks covered in penguins or an iceberg covered in napping seals, or spotted a pod of mammoth whales bobbing off the bow, it’s as if we entered an episode of “Planet Earth.” Everyone was quiet, partly to not scare the animals, partly in sheer awe.

Particularly spectacular was watching penguins “fly” on Cuverville Beach. Penguins don’t fly through the air, but they fly through the water in teams, like synchronized swimmers.

Catching it on film? Not so easy, but what fun trying.

More on this series:

• Day 1: Greeted by dirt, not ice
• Day 2: Climate change decimates food supply for penguins
• Day 3: Watching Mother Nature in action
• Day 4: How to sleep outdoors in Antarctica
• Finale: Trips to the seventh continent are not just for scientists

NBC News Correspondent Kerry Sanders recently returned from Antarctica, where he chronicled the dramatic changes in the world's last wilderness. Below is his main report; you also can click on the map above for more dispatches from across the breathtaking seventh continent.

By Kerry Sanders, Correspondent, NBC News

ANTARCTIC PENINSULA — There are serious changes taking place here at the bottom of the world.

Increasingly, experts say, the ice is disappearing at a disturbing rate in the Antarctic Peninsula and that in turn impacts the future -- and perhaps the very existence — of at least half of the world’s 18 penguin species, who depend on ice and frigid waters that support krill, the penguin diet mainstay.

"When cheetahs or lions get hunted, or elephants decline, there’s a big uproar. And I think, because you see penguins in large numbers [in some places] people are ignoring the larger rate of their decline," said Oxford University penguinologist Tom Hart. "The general public doesn't realize the penguins are declining so fast."

But it’s not just the penguins we have to worry about, Hart says, it’s the health of the planet itself.

"The last wilderness on Earth is impacted by us now," he said, describing the region’s decline as a "grave indicator" of what’s to come.

Life’s cycle disrupted for Antarctica’s penguins
It’s the end of the breeding cycle for most penguins here as summer comes to a close. The Gentoos, Adelies and Chinstraps are nudging their newborns from the rocks of Antarctica’s peninsula toward the waters of the Southern Ocean.

Experts say about 50 percent of the eggs will produce a penguin chick that makes it to sea. And about half of those will survive the hungry predators below, as they plunge into the frigid waters for their first swim. Leopard seals are lurking -- and for the newborns, avoiding their mortal enemy is not easy. Many will die. Those that do survive are subject to climate change that is threatening their food supply.

Hart has spent nearly a decade studying the creatures that have captured the world’s imagination for centuries. Each year, for three to four months, he positions himself along the Antarctic coast to observe, measure and chart penguin colonies. Some colonies have been followed since polar explorer Sir Ernest Shackleton and his men headed here some 100 years ago.

"When you look at all penguins they are largely in trouble," said Hart. "We're so concerned because we're seeing massive changes to their populations. They’re probably not going to go extinct anytime soon, but the environment is changing very fast.

Chinstraps populations seem to have declined up to 50 percent in the last 30 years," he added.

Hart, like most experts, is cautious to speak in absolutes because the harsh environment here makes it difficult to get a clear picture of what’s happening.  Experts use time-lapse cameras and sit at computers, laboriously counting penguins one by one to compare colony sizes from year to year.

Krill decline quickly as sea ice disappears
Ice is the source of all life in Antarctica.  It may seem at odds to think that ice gives life, but when you connect the dots, it’s a straight line to a penguin’s belly.

Algae live on top of the ice and underneath it too, providing a grazing ground for the krill that amass beneath -- the way a raccoon chooses to hide in a garbage can. 

Krill mostly stay put under the frozen Southern Ocean.  But as the ice sheet disappears due to climate change, that habitat shrinks and moves further south. 

"The West Antarctic Peninsula has increased three degrees since 1951,” Hart said. "We’ve seen a large reduction in sea ice over the same period."

Although the climate has always undergone oscillations in temperature, Hart says the recent changes are happening much faster than normal.

Logically, less ice has resulted in less krill, say marine biologists.  And since krill is the main diet for penguins, seals and whales, less food has in turn meant fewer births.  That theory is widely accepted by scientists like French marine biologist Fabrice Genevois.

He says it’s mostly Americans, who have confused politics with science by questioning global climate change.

"We have all the information now, that's clear enough,” said Genevois. "There's no argument any more. You have to be either a liar or be crazy not to understand what we are doing to change the climate. We are responsible, that's for sure."

Add to that equation: Fishing. Less ice has opened areas to more fishing boats that in turn have targeted krill as a profitable catch.

There’s a 620,000 ton catch limit for krill in Antarctica, which is only about 1 percent of the total estimated mass in the region.

But it’s the location of the krill fisheries — all aggregated in the Antarctic Peninsula near the South Shetland Islands — that is the main cause of concern.

The boats increasingly drop their nets in the same waters where penguins search for food. The nets are not catching penguins indiscriminately but they are competing for the krill that the wildlife eats to survive.

Where do those captured krill end up? In part, they’re used as fish food at salmon farms, desirable because krill help color salmon “pink” which increases sales at the supermarket.

Click here and here for more on managing the krill catch.

Slideshow: Antarctica: Journey to the bottom of the Earth

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See photos from NBC's Kerry Sanders' voyage to Antarctica.

Launch slideshow

Canary in a coal mine
The entire population of Emperor penguins, Chinstraps and Adelies live in Antarctica — if the ice continues to retreat those species are at risk. Meanwhile, the potential for disease outbreaks increases.  

"As regions of Antarctica warm it has much more potential as a petri dish," said Hart, citing disease from the north, in particular avian disease, as being a main concern. 

The penguins, marine biologists say, are giving us a warning. 

"We don't need to necessarily fear change," said marine biologist Maria Clauss, who works with tour company Quark Expeditions. But the penguin’s decline "will change the world as we know it," she said. "And we should not kid ourselves."

Day 1: Greeted by dirt, not ice

Day 2: Climate change decimates food supply for penguins

Day 3: Watching Mother Nature in action

Day 4: How to sleep outdoors in Antarctica

Finale: Trips to the seventh continent are not just for scientists

Lynn M. Arnold / National Science Foundation via AP

A De Havilland Twin Otter like the one missing since Wednesday lands at the National Science Foundation's Amundsen-Scott South Pole Station in 2003.

Bad weather continued to stop rescuers from searching for a Canadian airplane that went missing in Antarctica with three people on board, officials in New Zealand rescue team said Friday. 

Though winds, which had been blowing at over 100 mph, had calmed to just over 20 mph by 5 p.m. Friday New Zealand time (11 p.m. ET Thursday), conditions would not allow sighting of the downed twin-engine airplane.

"Visibility is down to (1,300 feet) and the snow is almost horizontal," Kevin Branaghan, an official with Rescue Coordination Center New Zealand, said in a statement. "The weather is expected to improve slightly after 12-24 hours."

The plane, owned by Kenn Borek Air of Calgary, Alberta, was on its way from the U.S.-run Amundsen-Scott South Pole station to Italy’s Mario Zucchelli station while supporting an Italian research project, according to the National Science Foundation, which manages U.S. programs on the icy continent.

It took off at about 3 a.m. ET Wednesday and flew for an hour before its emergency locator beacon was detected in New Zealand, which is responsible for monitoring that section of Antarctica.

The beacon was tracked to a spot about 11,000 feet above sea level at the northern end of the Queen Alexandra Mountain range, some 400 miles from the aircraft’s departure point near the South Pole, rescue-team spokesman Michael Flyger said Thursday.

Hours of flyovers by aircraft from the United States, Canada and New Zealand proved fruitless because of cloud cover and blowing snow, he said.

'Extremely cold'
Kenn Borek Air said in a Thursday statement that weather had kept another of its planes from landing at a makeshift airbase 35 miles from the site of the locator beacon.

The company has otherwise released little information, saying it is "maintaining a respectful silence" until the fate of the plane is known.

If the plane has crashed, any survivors would have faced extreme conditions in the mountains, Rescue Coordination Center spokesman Flyger said Thursday.

"It’s a cold place to start with," he said. "The elevation is around 11,000 feet so ... combined with the wind and snow ... it’s going to be extremely cold."

Flyger noted that the crew was carrying heavy-duty, cold-weather gear and a five-day supply of water.

"We are still operating with the expectation that we will find them alive," his colleague Branaghan said Friday.

The search-and-rescue team's website, however, referred to searching for a "crash site."

Related:

100 mph winds halt search for missing plane

Plane with 3 on board missing near South Pole