A national survey carried out earlier this month found 57% of people say yes, there is solid evidence the earth is warming compared to 71% who said yes in an April 2008 survey. That 14 percentage point decline comes after surveys in 2006, 2007 and 2008 found a steady and widely held view that the climate was warming.

Of those who believe the earth’s temperature is rising today, fewer now blame human activity for climate change (36%) than did in the April survey (47%).

The marked shift in public opinion, which was measured among Democrats, Republicans and Independents, comes as lawmakers are weighing critical votes on plans to cap greenhouse gas emissions. There is ‘modest support’ for such an effort, the survey finds.

The results are based on a telephone survey of 1,500 American adults from 30 September to 4 October 2009, commissioned by the Pew Research Center for the People & the Press, a respected public opinion group.

The key mechanism to lower greenhouse gas emissions is a cap-and-trade scheme, contained in both the climate bill passed by the US House of Representatives earlier this year, and the bill under debate this month in the Senate.

“Despite the growing public scepticism about global warming, the survey finds more support than opposition for a policy to set limits on carbon emissions,” the Pew Center says. “Half of Americans favour setting limits on carbon emissions and making companies pay for their emissions, even if this may lead to higher energy prices; 39% oppose imposing limits on carbon emissions under these circumstances.”

But few Americans have heard much about cap-and-trade. A majority of respondents (55%) had heard nothing about cap and trade; 30% hard heard a little about the scheme; 14% had heard a lot.

Those who were relatively well informed on cap and trade opposed it by a 2:1 margin, but that may be in part due to the fact that more Republicans (20%) said they had heard a lot about the scheme than Democrats (8%).

On the subject of a global climate treaty, 56% of survey respondents say the US should join with other nations compared to 32% who say the US should set standards on its own.

The full survey with results broken down by party affiliation, age, education and geography, is available here.