There was a polar bear sighting at Harvard last week.

At Pforzheimer House on Thursday (Oct. 2), global warming expert James J. McCarthy delivered a crisp summary of how fast ice is melting in the Arctic — and why we should care. The audience of 80 took in his companion slide show, including images of ice-stranded polar bears.

Ursus maritimus, the largest land predator in the world, is “the signature animal in this whole discussion,” said McCarthy, who is master of Pforzheimer House and Harvard’s Alexander Agassiz Professor of Biological Oceanography. Vanishing ice shortens the hunting season for these cold-weather bears, he said, and reduces access to the ringed seals they require for lipid reserves.

McCarthy, an expert on plankton productivity, is a former co-chair of the working group on impacts for the Intergovernmental Panel on Climate Change. He was also one of the lead authors of the Arctic Climate Impact Assessment, a 2005 document that outlined the likely consequences of sustained warming in the Arctic for animals, people, and ecosystems.

To dramatize the change in ice cover, McCarthy showed a picture of a Russian icebreaker during the mid-1990s, when Arctic ice was still relatively thick (up to 13 feet). The 7,500-horsepower nuclear-powered ship “was just crawling through,” he said, “with some hesitation and a lot of noise.”

By 2000, McCarthy saw that vast stretches of white ice had been replaced by blue water. Ice cover in many places had thinned to as little as 3 feet.

On a trip to the Canadian Arctic this summer, McCarthy was aboard an icebreaker, peering over the side to watch thinning ice crack open. Revealed were algae-rich brine channels shot through with sunlight — radiant columns of frigid water where one-celled plants thrive. (How rapid warming affects Arctic algae, the bedrock of the marine ecosystem, is being studied.)

The decline in Arctic ice cover is steady, said McCarthy, and sometimes dramatic. On average, it has been vanishing at the rate of about 1 percent per year. But the decline jumped to 20 percent last year, he said, “and no one predicted that.”

On Greenland, said McCarthy, exit glaciers that empty into the ocean are “retreating, speeding up, and thinning” — a rapid melting replicated since 1960 on “tens of thousands” of Alpine glaciers worldwide.

The implications of melting sea ice are troubling for the polar bears, sea birds, baleen whales, seals, and arctic foxes that make up the Arctic ecosystem.

Troubling for people, too. In villages McCarthy visited last summer in Greenland, temperatures hovered at 68 degrees, up from average highs of 53 degrees. The climbing mercury was speeding coastal erosion, altering the migration patterns of game, and making sea ice — the platform for most hunting — unpredictable.

The villagers also faced electrical storms and other sudden weather anomalies, said McCarthy, “with no words for ‘thunder’ or ‘lightning’ in their vocabulary.”

If all the ice in the Arctic and in Greenland were to melt, said McCarthy, oceans worldwide could rise by 7 meters (about 22 feet). By 2100, one study predicted, the global sea-level rise could range from 0.8 to 2 meters (31 to 78 inches).

At the lower figure every coastal city will be in trouble and island nations will disappear. The higher figure, McCarthy said, would spell “coastal catastrophe.”

McCarthy’s slides included projected views of U.S. coastlines by the year 2100. They showed a truncated Florida, Venice-like ocean-side cities, and a shortened, saturated Gulf Coast. “Here’s 1 meter,” said McCarthy of one map showing a 39-inch sea level rise. “Forget New Orleans. It’s just gone.”

Polar ice is not melting in Antarctica, the windiest and driest continent, where about 85 percent of the Earth’s fresh surface water is immobilized in ice. Frigid temperatures there, McCarthy said, are maintained by the highest average continental altitudes in the world.

By contrast, said McCarthy, the Arctic is at sea level, making it vulnerable to warming and “an early indicator of change in global climate.” As more ice melts, more dark open water appears. It’s more efficient than snow and ice at absorbing heat. That decreases the albedo (light-reflecting) effect that has kept the Arctic ice-bound and cold for millennia.

It’s possible to avoid catastrophic sea-level rises, said McCarthy, by adopting the “stabilization scenario” espoused by both presidential candidates: an 80 percent reduction of U.S. greenhouse gas emissions by 2050.

In the meantime, admirable localized efforts are under way, he said, including emissions-reductions goals espoused by 500 or more U.S. mayors, a dozen governors, and by universities, including Harvard, which this year pledged to reduce its greenhouse gas emissions by 30 percent by 2015 — “a very aggressive target,” said McCarthy.

Moreover, “an entire wedge” of other steps could reduce the world’s carbon footprint, he said. That includes renewable energy sources; energy efficiency and conservation; policy changes like carbon taxation and cap-and-trade systems; emerging technologies for carbon capture and storage; and nuclear power — “after a period of R&D we have not had,” said McCarthy.

Pforzheimer resident Karen McKinnon ’10, an Earth and planetary sciences concentrator from Boulder, Colo., who co-chairs the Harvard College Environmental Action Committee, took in the McCarthy lecture.

“There’s always more to know,” she said.