WASHINGTON - The U.S. government will begin requiring big companies to monitor and report greenhouse gas emissions, officials said on Tuesday, a move that could make it easier for federal regulators to cut emissions if Congress does not pass a climate change bill.

The U.S. Environmental Protection Agency said its new reporting system will help it understand where greenhouse gas emissions originate and ultimately help reduce emissions.

This is a major step forward in our effort to address the greenhouse gases polluting our skies, said EPA Administrator Lisa Jackson. The American public, and industry itself, will finally gain critically important knowledge and with this information we can determine how best to reduce those emissions.

The EPA said its reporting system will cover 85 percent of total U.S. greenhouse gas emissions spewed by roughly 10,000 facilities. An oil refinery, power plant or other facility would have to report its polluting data if its carbon dioxide emissions totaled at least 25,000 tons a year.

A 25,000 ton annual carbon dioxide threshold is comparable to the emissions from 131 rail cars of coal consumed, 58,000 barrels of oil consumed, or the emissions from the annual energy use of about 2,200 homes.

The public has both a need and a right to know about the country's biggest emitters, said Mark MacLeod, director of special projects at Environmental Defense Fund. The transparency provided today will inform smart policy that targets the biggest sources of heat-trapping emissions.

The global warming pollutants covered under the EPA's reporting system include carbon dioxide, methane, nitrous oxide and hydrofluorocarbons.

The new reporting system takes effect in January 2010 and large emitters are required to file their annual emissions data in 2011. Vehicle and engine manufacturers outside of the light-duty sector will begin phasing in their emissions reporting with the 2011 car model year, the agency said.

The EPA has said it would prefer Congress to cut U.S. emissions, but the agency has taken action that will allow it do to so if necessary.

Democratic U.S. Senators plan to propose a bill this month to slash U.S. greenhouse gas emissions, a bill that is expected to face stiff opposition. The House of Representatives has already passed a climate change bill.

President Barack Obama, citing in part the EPA's action, told the United Nations that the United States has done more to promote clean energy and reduce carbon pollution in the last eight months than at any other time in our history.

(Reporting by Tom Doggett; Editing by David Gregorio)