Americans seem to be cooling toward global warming.

Just 57 percent think there is solid evidence the world is getting warmer, down 20 points in just three years, a new poll says. And the share of people who believe pollution caused by humans is causing temperatures to rise has also taken a dip, even as the US and world forums gear up for possible action against climate change.

In a poll of 1,500 adults by the Pew Research Center for the People & the Press, released Thursday, the number of people saying there is strong scientific evidence that the Earth has gotten warmer over the past few decades is down from 71 percent in April of last year and from 77 percent when Pew started asking the question in 2006. The number of people who see the situation as a serious problem also has declined.

The steepest drop has occurred during the past year, as Congress and the Obama administration have taken steps to control heat-trapping emissions for the first time and international negotiations for a new treaty to slow global warming have been under way. At the same time, there has been mounting scientific evidence of climate change - from melting ice caps to the world's oceans hitting the highest monthly recorded temperatures this summer.

The poll was released a day after 18 scientific organizations wrote Congress to reaffirm the consensus behind global warming. A federal government report Thursday found that global warming is upsetting the Arctic's thermostat.

Only about a third, or 36 percent of the respondents, feel that human activities such as pollution from power plants, factories and automobiles are behind a temperature increase. That's down from 47 percent from 2006 through last year's poll.

"The priority that people give to pollution and environmental concerns and a whole host of other issues is down because of the economy and because of the focus on other things," suggested Andrew Kohut, the director of the research center, which conducted the poll from September 30 to October 4. "When the focus is on other things, people forget and see these issues as less grave."

Andrew Weaver, a professor of climate analysis at the University of Victoria in British Columbia, said politics could be drowning out scientific awareness.

"It's a combination of poor communication by scientists, a lousy summer in the Eastern United States, people mixing up weather and climate and a (major effort) by public relations firms and lobby groups trying to instill a sense of uncertainty and confusion in the public," he said.

Political breakdowns in the survey underscore how tough it could be to enact a law limiting pollution emissions blamed for warming. While three-quarters of Democrats believe the evidence of a warming planet is solid, and nearly half believe the problem is serious, far fewer conservative and moderate Democrats see the problem as grave. Fifty-seven percent of Republicans say there is no solid evidence of global warming, up from 31 percent in early 2007.

Though there are exceptions, the vast majority of scientists agree that global warming is occurring and that the primary cause is a buildup of greenhouse gases in the atmosphere from the burning of fossil fuels, such as oil and coal.

Jane Lubchenco, head the National Oceanic and Atmospheric Administration, told a business group meeting at the White House Thursday: "The science is pretty clear that the climate challenge before us is very real. We're already seeing impacts of climate change in our own backyards."

Despite misgivings about the science, half the respondents still say they support limits on greenhouse gases, even if they could lead to higher energy prices. And a majority - 56 percent - feel the United States should join other countries in setting standards to address global climate change.

But many of the supporters of reducing pollution have heard little to nothing about cap-and-trade, the main mechanism for reducing greenhouse gases favored by the White House and central to legislation passed by the House and a bill the Senate will take up next week.

Under cap-and-trade, a price is put on each ton of pollution, and businesses can buy and sell permits to meet emissions limits.

"Perhaps the most interesting finding in this poll ... is that the more Americans learn about cap-and-trade, the more they oppose cap-and-trade," said Republican Sen. James Inhofe of Oklahoma, who opposes the Senate bill and has questioned global warming science.

The poll's margin of error was plus or minus 3 percentage points.