WASHINGTON -- Ditching its past cautious tone, the nation's top scientists urged the government Wednesday to take drastic action to raise the cost of using coal and oil to slow global warming.

The National Academy of Sciences specifically called for a carbon tax on fossil fuels or a cap-and-trade system for curbing greenhouse gas emissions, calling global warming an urgent threat.

The academy, which advises the government on scientific matters, said the nation needs to cut the pollution that causes global warming by about 57 percent to 83 percent by 2050. That's close to President Barack Obama's goal.

"We really need to get started right away. It's not opinion, it's what the science tells you," said Robert Fri, who headed one of the three panels producing separate climate reports.

Fri was acting Environmental Protection Agency chief under President Richard Nixon and until recently on the board of American Electric Power Co., a major producer of carbon dioxide.

"The country needs both a prompt and a sustained commitment to reducing greenhouse gas emissions," he said Wednesday.

In the past, the academy has called climate change a problem, but it has never recommended a specific policy. The impetus for its bolder stance now was a set of questions posed by Congress on climate change and how to deal with it.

The cap-and-trade idea, which is supported by the Obama administration, has been proposed for several years in Congress but never passed the Senate. It would set overall limits on carbon dioxide pollution, but would allow companies to pollute more by paying for it and buying pollution credits from cleaner companies.

Last year the House approved a cap-and-trade bill, but it stalled in the Senate as health care legislation took center stage. A new version, that doesn't use the cap-and-trade phrase but has similar characteristics, was introduced last week. Lawmakers have pledged a floor vote on the bill this summer.

The national academy is an elite independent organization chartered to give the federal government advice on science and technical matters. Being elected as an academy member is considered a major honor for a scientist.

In a series of three reports, the academy tried to illustrate the challenge ahead by describing tons of polluting gases as money in a budget. America is on an escalating trajectory to blow its budget. The budget allows for the use of 170 to 200 billion tons between now and 2050. In 2008, America spewed 7 billion tons of greenhouse gas.

"If we continue at the same rate we're going, we're going to use that up quickly, which is the case for urgency," said panel member Ed Rubin, an engineering and public policy professor at Carnegie Mellon University.

Panel members said it's unusual for the academy to be so blunt about what needs to be done. But Congress asked the academy in 2008 to answer four questions, starting with: "What short-term actions can be taken to respond effectively to climate change?"