The proportion of Americans who say that the earth is getting warmer has decreased modestly since January 2007, mostly because of a decline among Republicans, according to a new survey by the Pew Research Center.

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That puts most Republicans at odds with their standard-bearer, President George W. Bush, and with GOP presidential contender Sen. John McCain. Both men said this week global warming is real and must be addressed.

Republicans are increasingly skeptical that there is solid evidence that the earth has been warming over the past few decades, the survey found. In January 2007, 62 percent said they believed the evidence, compared to 49 percent in the new Pew findings. Pew found that self-described conservative Republicans are more likely than party moderates or liberals to reject the science.

Overall, 71 percent of Americans say there is solid evidence of higher global temperatures, compared with 77 percent at the beginning of last year. Fewer than half in the survey -- 47 percent -- attribute the rising temperatures to human activity.

Age played a role in opinions, Pew said. Fifty-four percent of people under age 30 believe that the earth is warming mostly because of human activity, compared with 37 percent of those ages 65 and older.

Fifty-one percent of college graduates said that human activity is causing global warming, compared to 43 percent of those with a high-school-level education.

Republican leaders this week sided with the scientists.

On Wednesday, the Interior Department declared the polar bear a threatened species, saying it must be protected because of the decline in Arctic sea ice from global warming.

Interior Secretary Dirk Kempthorne cited dramatic declines in sea ice over the last three decades and projections of continued losses. These declines, he told a news conference, mean the polar bear is a species likely to be in danger of extinction in the near future.

Kempthorne also said, though, that it would be "inappropriate" to use the protection of the bear to reduce greenhouse gases, or to broadly address climate change.

On Tuesday, President George W. Bush said that global warming has been "more clearly defined as a problem. But what hasn't changed is the realistic notion that new technologies are going to be the solution, and the fundamental question is how do you grow the economy at the same time, and at the same time encourage new technologies.

"You can't have a solution to global warming unless China and India are part of any international pact. I'm a realistic guy. If the major emitters of greenhouse gases are not a part of a solution, then those who are part of a solution are acting in a way that's simply not going to -- it will affect their own economies, but it won't affect the overall global warming issue."

On Monday, presidential contender Sen. John McCain said that global warming is undeniable and the country must take steps to bring it under control while adhering to free-market principles.

"For all of the last century, the profit motive basically led in one direction toward machines, methods and industries that used oil and gas," said McCain.

"Enormous good came from that industrial growth, and we are all the beneficiaries of the national prosperity it built. But there were costs we weren't counting, and often hardly noticed. And these terrible costs have added up now, in the atmosphere, in the oceans and all across the natural world," McCain said.

In a report issued in April 2007, the Intergovernmental Panel on Climate Change said that, "Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level."

The panel was established in 1988 through a resolution of the U.N. General Assembly. Made up of thousands of scientists, as well as governmental agencies, is concluded with near certainty that human activity has played a role in the rising temperatures through the effect of greenhouse gasses.

The IPCC and former U.S. Vice President Al Gore were awarded of the Nobel Peace Prize in December 2007 "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change".