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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 chaired 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?" Panel members said that is what gave them freedom to say more about what needs to be done.
The three documents issued Wednesday come after a winter in which mainstream climate science took a beating because of leaked e-mails from a British university and errors revealed in the Intergovernmental Panel on Climate Change's report. And in December despite dozens of world leaders seeking a climate deal in Copenhagen, countries could not agree to renew and tighten mandatory curbs on greenhouse gas emissions.
All the criticism and inaction spurred the national academy to deliver a sharper message, said Pam Matson, dean of earth sciences at Stanford University, who chaired one of the three panels.
Change is occurring in warmer temperatures, melting ice caps, sea rise and many other areas that have been carefully studied, Matson said. Future changes will include water and food shortages in many areas, more heat waves and increased intense rainfall in some regions.
The reports agree with the international climate panel's assessment that climate change is already occurring "and poses significant risks for — and in many cases is already affecting — a broad range of human and natural systems."
White House science adviser John Holdren praised the group's work as "well-documented in their science ... and compelling in their conclusions about policy." He said he hoped members of Congress would read the reports or at least their summaries.
Also on Wednesday, the journal Nature published a study suggesting the world's oceans are absorbing more heat from global warming than previously thought. Scientists from the U.S. and Japan estimated the amount of energy swallowed by the oceans since 1993, a figure roughly equivalent to more than 2 billion Hiroshima-sized bombs.
"It's just a huge amount of energy," said John Lyman, an oceanographer for the National Oceanic and Atmospheric Administration's research institute in Seattle.
Oceans absorb about 90 percent of the heat from global warming and scientists have struggled for years to quantify it. Co-author Josh Willis, an oceanographer at NASA's Jet Propulsion Lab, said the amount calculated in the study translates to warming the top 2,300 feet of the world's oceans by about three-tenths of a degree.
Outside experts praised the study, saying ocean heat content is going to be one of the key indicators of climate change.