A third of Americans say global warming ranks as the world's single largest environmental problem, double the number who gave it top ranking last year, a nationwide poll shows.

In the new poll, conducted jointly by The Washington Post, ABC News and Stanford University, most of those surveyed said that climate change is real and that they want the federal government to do more about it. But the survey also shows there is little public agreement about the policies the United States should adopt to address it.

The findings come weeks after the Supreme Court ruled that the federal government has the right to regulate carbon dioxide, the largest contributor to human-caused warming. Congress is pressing to enact limits on all greenhouse-gas emissions linked to climate change, but it remains unclear how soon the House or the Senate could pass such legislation.

According to the poll, seven in 10 Americans want more federal action on global warming, and about half of those surveyed think the government should do "much more" than it is doing now.

By a 40-point margin, the public trusts congressional Democrats more than it trusts President Bush to handle global warming. More than nine in 10 Democrats in the poll said they trusted their party's leaders over Bush on the issue, as did 54 percent of independents and one in five Republicans.

Bush has maintained that he will rely on scientific developments and voluntary measures, rather than curbs on greenhouse-gas emissions, to tackle global warming.

Although many respondents say global warming is an issue that matters to them -- 52 percent say the issue is "extremely" or "very" important personally, double the percentage that said so a decade ago -- many base their views more on personal experience than on scientific findings.

Tom Sheppard, who lives in Dalton, Pa., described himself as "a typical Republican banker who doesn't have a lot of faith in the administration where this issue is concerned." He said he became concerned about climate change after seeing retreating glaciers in Alaska last year and reading about melting of the Greenland ice sheet.

However, there was no consensus among respondents about how -- or whether -- the government should regulate corporations or change the tax code to cut down on emissions.

One in five favors higher taxes on electricity to encourage conservation, and about a third support higher gasoline taxes. Sixty-two percent of those surveyed say the government should require power plants to reduce emissions of greenhouse gases. Forty-two percent think the government should require greater fuel efficiency for vehicles, something both the administration and Congress back, and 36 percent want to require manufacturers to produce more efficient air conditioners, refrigerators and other appliances.

Eighty-four percent think that average global temperatures have been rising over the past century, and more than half say weather has become more unstable where they live. Still, only four in 10 are "extremely" or "very" sure global warming is happening, and 56 percent continue to think there is "a lot" of disagreement among scientists about climate change.

This last finding may stem from Americans' skeptical attitudes toward scientists: A third of respondents trust what scientists say about the environment "completely" or "a lot," and a quarter say they trust such statements "a little" or "not at all."