WASHINGTON — A Canadian scientist on Wednesday urged U.S. senators to be cautious about claims that polar bears are threatened by global warming, while an Alaskan colleague said he has no question that less ice off the state’s coastline is bad for the bears.

The Canadian scientist’s opinion drew a testy challenge from one senator and encouragement from another.

Lee Foote, an associate professor at the University of Alberta, said polar bear biology is too complex and, so far, too poorly studied to conclude that the bears will decline in the face of warmer temperatures.

“I liken this to trying to stack four bowling balls on top of each other, let them fall and make some predict which direction the top bowling ball will likely go,” Foote said. “It won’t be the same direction any two times.”

Brendan Kelly, who has studied seals and other marine mammals in Alaska for the past 30 years, acknowledged that animal populations react unpredictably to environmental changes. However, the effect of warming on polar bears, ringed seals and walruses is not one such mystery, he said.

“It’s clearly going to have an extremely negative impact on them,” he said.

The U.S. Fish and Wildlife Service recently decided to study whether to declare the polar bear a threatened species, based on concerns about the shrinking arctic ice pack. Bears hunt seals, their primary food, on the ice.

“It’s with good foresight that the Fish and Wildlife Service predicts that this kind of change in habitat will in fact threaten polar bears,” Kelly said.

The researchers testified before a subpanel of the Senate Environment and Public Works Committee created specifically to look at global warming’s effects on wildlife.

Foote acknowledged that the most studied population of polar bears, at Churchill on Canada’s Hudson Bay, has declined recently. It is the southernmost subgroup of bears, though, he said. The trend in other, more northern subgroups isn’t clear, he said.

It’s possible, he said, that while warming harms the southern bears, it helps more northern bears.

Imagine polar bear habitat as a circumpolar belt, he told senators. As the Earth warms, “the belt doesn’t necessarily get narrower, it may just shift upwards and back,” he said. “That’s an uncertainty at this point.”

Ringed seals could actually become more accessible to bears in places such as Davis Strait, which lies between Greenland and the large islands that lie off Canada’s northern shore, Foote said. The conversion of multi-year ice to annual ice in such areas “could become a net positive” for bears, he said.

In Alaska, though, the Beaufort Sea coast is a single line unbroken by major island bases to the north. When the ice pulls back, it leaves only open water.

Kelly noted that this geographic fact already appears to be affecting polar bear biology.

“They have very good information on denning sites from satellite tracking that’s gone on for several decades,” Kelly said of polar bear researchers. The majority of bears that live along Alaska’s northern coast used to den on the ice each year. Now the majority use land, because the ice is still so far off shore when it’s time to enter a den.

“This is a big response, a big change,” Kelly said. “We would be foolish not to extrapolate and be proactive rather than waiting until we can show with 95 percent confidence limit that the population has declined by `x’ amount.”

Foote’s said he urged caution about listing the polar bear as an endangered or threatened species because of that action’s potential effect upon Inuit communities in Canada. The villages earn substantial fees by setting aside a small amount of their polar bear hunting quota for outside, non-Native hunters.

Invoking the Endangered Species Act could devastate this small but important economy, he said.

In the United States, only Native people may hunt marine mammals.

Foote’s caution drew sympathy from Oklahoma Republican Sen. James Inhofe, the Senate’s most insistent critic of scientists’ assertions that the burning of fossil fuels is the primary cause of global warming. Inhofe asked Foote to share his observation that some who want the polar bear declared endangered have publicly stated that they hope to force reductions in fuel combustion, which produces carbon dioxide, a primary greenhouse gas.

Such statements “started a logical thought process that maybe there was some other agenda at work here,” Foote said.

California’s Democratic Sen. Barbara Boxer, chairwoman of the full environment committee, used her time to question Foote’s credibility.

In 2002, Boxer noted, Foote and dozens of other Canadian scientists signed a letter advocating a reduction in greenhouse gases and asserting that there was little disagreement in the scientific community about the causes of climate warming.

All the focus on polar bears caused Kelly, the Alaska scientist, to encourage senators to broaden their perspective. Kelly recently moved from the University of Alaska Southeast in Juneau, where he was dean of arts and sciences, to Washington, D.C., on a two-year assignment to direct the National Science Foundation’s arctic biology program.

“What’s really important in my view is the entire ecosystem,” Kelly said. Focusing on polar bears instead of the disappearing ice would be like watching the bison disappear and then “rolling up the plains behind them.”

“Having the sea ice go away is taking the whole ecosystem out, it’s not just taking the charismatic megafauna away,” he said.

This isn’t the first time the northern climate has changed, Kelly said, echoing a point also made by Inhofe.

“The current rates of climate change, however, are very steep. Witness that the the summer ice cover has decreased 26 percent during my career,” Kelly said. “If the biological and social environments change too rapidly, species and societies will not be able to keep pace.”