Scientists widely say human activity, especially emissions of heat-trapping greenhouse gases from burning fossil fuels, is warming the Earth's climate, posing risks from rising sea levels, more floods, droughts and expanding deserts.

Massachusetts also brought the case that led the Supreme Court to rule on April 2 that US environmental officials have the power to regulate greenhouse gas emissions -- a stinging defeat for the Bush administration.

"We intend to continue to press the federal government to live up to its statutory responsibilities to address excess emissions of greenhouse gases and other air pollutants," Massachusetts Attorney General Martha Coakley said.

In a statement, Coakley said she disagreed with the Department of Energy's assertion that it lacked statutory authority to move the new standards forward.

As a result, she said, tougher rules would be delayed until 2010 due to the Department of Energy's interpretation of the 2005 Energy Policy Act. "The Commonwealth (of Massachusetts) disagrees with the new interpretation," she said.

The proposed new standard would have affected products that account for a significant share of US energy consumption and are commonly used in offices, schools and hotels, she said.

Tightening standards for air conditioning products alone could save about 2.913 quadrillion BTUs (a basic unit of thermal energy) over 27 years, eliminating the need for several major power plants, she said, quoting federal studies.