As climate change legislation crept forward on Capitol Hill, top scientists called Wednesday for the United States to move quickly to cut greenhouse gas emissions and put a price on carbon.

In a trio of studies that are part of the most comprehensive report to date on climate change by the National Academy of Sciences, experts urged the United States to set up a "greenhouse gas emissions budget" -- a set amount of greenhouse gases that can be emitted over a fixed period of time.

They warned that the country would have to throw all the resources it has and then some at global warming, starting now, if it hoped to meet the budget goals, which the scientists suggested should cap total US emissions between 2012 and 2050 at 170 billion to 200 billion tons of greenhouse gases.

The scientists said meeting even their lower level of cuts would require "a major departure from business-as-usual emission trends," which have seen emissions rise by about one percent a year in the US to a current level of around seven billion tons annually.

"At current emission rates, which are in the order of seven billion tons a year, we would use up the budget well before 2050," said Robert Fri, who chaired the committee of scientists that wrote the report on limiting climate change.

"Even if all available and emerging technologies -- energy efficiency, renewables, nuclear, carbon capture and storage for coal plants, and biofuels -- can be deployed to their fullest technical potential, we will still need new and additional reduction options to meet the budget," he said.

"If we're going to meet the budget, we really have to get started."

The scientists suggested that the best way to cut emissions would be through an "economy-wide carbon-pricing system, whether it's cap-and-trade or taxes or a hybrid of the two.

"The evidence is very strong that if one makes it worth the private sector's while to meet a public goal, in this case reducing the amount of carbon dioxide emitted into the atmosphere, you'll get a response that's very vigorous and broadly based," said Fri.

The upper emissions "budget" of 200 billion tons of greenhouse gases over 38 years, would mean reducing US emissions from 1990 levels by 50 percent.

Climate change legislation introduced in the Senate this month by Democratic Senator John Kerry and Independent Joe Lieberman seeks to reduce carbon emissions by 17 percent by 2020 and by 83 percent by 2050 -- but those cuts are based on emissions in 2005, or 15 years later than the benchmark year in the scientists' plan.

The Senate target is in line with a House of Representatives bill approved last year and President Barack Obama's position in international negotiations.

In a statement, Kerry called the scientists' report "yet another wake-up call on the threats of global climate change," but did not comment on the much tougher targets for emissions cuts that the the report set.

Two other reports released Wednesday reaffirmed US scientists' belief that climate change is occurring and caused largely by human activity, and stressed the need to adapt to impacts of global warming that cannot be avoided, such as rising sea levels, melting glaciers and more heat waves.

The reports were released as climate change legislation moved slowly forward on Capitol Hill, while the US and more than 190 other nations continue to hammer out the details of a successor treaty to the Kyoto Protocol, which set targets for emissions cuts for developed nations, based on 1990 levels.