The poll of 1,500 adults by the Pew Research Center for the People & the Press found that only 57 percent believe there is strong scientific evidence the Earth has gotten hotter over the past few decades, and as a result, people are viewing the situation as less serious. That's down from 77 percent in 2006, and 71 percent in April 2008.
The steepest drop 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.
But while the evidence appears clear, only about a third, or 36 percent of the poll respondents feel that human activities - such as pollution from power plants, factories and automobiles - are behind a temperature increase. That's the first decline since 2006.
"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," said Andrew Kohut, the director of the research center, which conducted the poll from Sept. 30 to Oct. 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 full-court press by public relations firms and lobby groups trying to instill a sense of uncertainty and confusion in the public," he said.
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 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 Sen. James Inhofe, a Republican who opposes the Senate bill and has questioned global warming science. Republicans in general have grown even more steadfast in their opposition. A majority - 57 percent - now say there is no hard evidence of global warming, up from 42 percent last year, according to the poll.
Other results of the survey also suggest that it will be tough politically to enact a law limiting emissions of global warming pollution. 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 as they did last year.
Regional differences were also detected. People living in the Midwest and mountainous areas of the West are far less likely to view global warming as a serious problem and to support limits on greenhouse gases than those in the Northeast and on the West Coast. Both the House and Senate bills have been drafted by Democratic lawmakers from Massachusetts and California.
One of those lawmakers, Sen. Barbara Boxer, a Democrat, told reporters Thursday that she was happy with the results, given the interests and industry groups fighting the bill.
"Today, to get 57 percent saying that the climate is warming is good, because today everybody is grumpy about everything," Boxer said. "Science will win the day in America. Science always wins the day."
Earlier polls, from different organizations, have not detected a growing skepticism about the science behind global warming.
Since 1997, the percentage of Americans that believe the Earth is heating up has remained constant - at around 80 percent - in polling done by Jon Krosnick of Stanford University. Krosnick, who has been conducting surveys on attitudes about global warming since 1993 was surprised by the Pew results.
He described the decline in the Pew results as "implausible," saying there is nothing that could have caused it.