Wildlife biologist Ian Bullock is a seasoned visitor to the Arctic, but even he was surprised by what he saw last month: a thin female polar bear, shadowed by her cub, trying to challenge a much bigger, stronger male for food.

It wasn't much of a challenge, but it showed just how desperate she was, Bullock told NBC News on returning from his 10th straight summer cruise to the Arctic.

That desperation, he feels, stems from the fact that the Arctic's summer sea ice — which polar bears using as floating stations from which to hunt seals — has been shrinking over the last few decades due to a warming Arctic, forcing polar bears into smaller areas and more intense competition. 

"She was the thinnest female with cub I have ever seen," he said. "She had a single cub which implies she has already lost one other cub this year.

"If she cannot feed, she cannot suckle her cub; with a hungry cub it is even harder for her to hunt effectively, so from what I saw her last cub is at risk and ultimately so is she," he added. "This is why she was challenging a big male with food. She was hungry enough to take a big risk." 

In a video filmed during the National Geographic Explorer cruise to the Arctic's Svalbard region, Bullock said it looks like that reduced ice is "really putting the bears under stress."

"The worst thing is when we've encountered bears, we've found them really packed in tight, in the last little areas of fast ice attached to land, or the last little patches of pack ice at sea," said Bullock, who served as a guide on the cruise ship. "And there they've been in competition."

Polar bears are listed as "vulnerable" and in decline by the International Union for Conservation of Nature, which estimates the population at no more than 25,000 across the Arctic.

The U.S., which has two Arctic regions where polar bears live, in 2008 listed its population as "threatened".

Last year, researchers cited three incidents where polar bears might even have resorted to cannibalism due to warming and reduced sea ice.

The diminished sea ice also got the attention of the National Geographic Explorer's skipper.

Captain Leif Skog told NBC News that he had e-mailed his boss, Sven Lindblad of Lindblad Expeditions, to describe "a shocking escalation of the reduction of sea ice."

One data graph he monitored daily, showing the total volume of Arctic sea ice, "could be called the death spiral of the Arctic sea ice," he said in his e-mail to Lindblad.

Because of the reduced sea ice, he added, the cruise was able to visit northeast Greenland "a month earlier than what was normal in the past."

"We expected to face some sea ice but everything was gone in the fjords upon our arrival," he added. "The sea water temperature in the fjords was also unbelievably high."

Another expert on the cruise called the outside temperature "surprisingly warm." 

"It was T-shirt weather," Paul Berkman, an environmental science professor at the University of California Santa Barbara, told NBC News. Berkman noted two other major Arctic developments over the summer:

• The amount of summer sea ice reached its lowest point in 30 years of records.
• Nearly the entire surface of Greenland's ice cap saw some melting in July, a phenomenon not seen in 150 years of ice records.

Berkman said the polar regions, and the Arctic in particular, show an "amplified response" to a warming climate ahead of other parts of the globe.

That response is twofold, he adds: Arctic temperatures have warmed 3-6 degrees F above the global average, and reduced ice removes huge amounts of reflective white from the sea and reveals a dark sea that absorbs heat.

The sea ice is like "a giant mirror on Earth's surface" he said. "Without summer Arctic sea ice, more heat from the sun is absorbed into the Earth system, which is a feedback that further accelerates warming of our climate."

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