For the second year in a row a large amount of Arctic sea ice did not refreeze during the winter as it normally does, a team of scientists reports.

This trend may indicate an overall shrinking of Arctic ice cover due to rapid global climate change.

Mark Serreze is a senior research scientist at the National Snow and Ice Data Center based at the University of Colorado, Boulder, who monitors Arctic sea ice.

"Some calculations say that by 2070 we will have no sea ice left," he said.

"It's always dangerous to make predictions, but we are right on schedule" for this to occur.

The ice that floats on top of the Arctic Ocean typically melts a bit in the spring and builds up again in the winter.

Animals such as polar bears, seals, and walruses make their homes on the ice, and people living in the region rely on the ice pack for fishing and travel. (Read how ice melt may be killing polar bears.)

But this year and last year the winters were too warm for the ice to re-form normally, the scientists say.

"It's getting so warm in the Arctic now that the ice is not growing back in winter the way it used to," Serreze said.

Ice-Loss Loop

The Arctic region made the news last September when the same team of researchers reported that a very warm winter followed by a warm summer had resulted in less ice on the ocean than had ever been recorded.

If this spring and summer are also warmer than average, "come this September we could be in really bad shape," Serreze said.

Serreze has tracked the Arctic ice cover for 20 years by analyzing tens of thousands of detailed images taken from NASA satellites orbiting the Earth.

He studies the ice's extent—the area of ocean covered by at least 15 percent of ice. The extent is traditionally smallest in September.

Starting in the 1990s he began to notice an overall decrease in this already low September ice cover.

The best explanation for loss of Arctic ice cover is global warming, Serreze says.

Some climatologists believe the current global warming trend is part of a long-term natural process.

But most others, including Serreze, point to human activity as the main reason for the warming.

Samples of ancient ice show that carbon dioxide levels in the atmosphere today are the highest they have been in 600,000 years, Serreze said.

"We're really starting to see the effect of [human-generated] greenhouse gases kick in," he said.

What's more, the scientist believes the warming observed this winter may be evidence of a feedback loop: Water retains heat, and with less ice-cover more of the Arctic Ocean is exposed to summer sun. Less ice therefore means warmer waters, which may in turn be speeding up further ice reduction, he said.

"Wrong Foot"

Scientists are reporting changes throughout the globe due to warming, such as melting glaciers, longer summers in Alaska, and animal populations moving northward.

Chris Field, director of the Department of Global Ecology at the Carnegie Institution in Washington, D.C., recently spoke with National Geographic News about climate change.

"The bottom line is that parts of the Arctic, including Alaska, have warmed quite dramatically over the last several decades," he said.

Serreze says that soon the Arctic will begin its summer melting cycle, and it will start with much less ice than it had 20 or even 5 years ago.

"The spring ice cover is starting off on the wrong foot," he said. "Come summer it will be very easy to melt what is still left."