The United States formally notified the United Nations on Thursday that it embraced the Copenhagen Accord and would cut its carbon emissions 17 percent by 2020 from 2005 levels.

In a letter to the secretariat of the UN Framework Convention on Climate Change (UNFCCC), U.S. Special Envoy for Climate Change Todd Stern said robust action "will strengthen our economy, enhance our national security and protect our environment."

However, the U.S. move is conditional on other industrialized nations and large emerging economies following through with the deal reached in Copenhagen.

"We expect that all major economies will honor their agreement in Copenhagen to submit their mitigation targets or actions," Stern said.

The letter reaffirmed the promise U.S. President Barack Obama made to the UN Climate Change Conference held last month to cut U.S. emissions, saying that the 2020 commitment was a first step towards cutting America's global warming pollution by 42 percent in 2030, and by more than 80 percent by 2050.

Obama's promise was in line with current legislation in both chambers of U.S. Congress. The climate bill passed by the House of Representatives last June set a 17 percent reduction target for emissions by 2020 from 2005 levels, which is less than 4 percent below 1990 levels.

However, developing countries demand developed countries slash their emissions by at least 40 percent below 1990 levels by 2020. A Senate version aims for a 20 percent cut.

The U.S. submission came hours after Obama made his first State of the Union address, in which he urged a joint session of Congress to move ahead on climate legislation. However, the Senate's push to pass a climate bill could be harder in this congressional election year as public support has appeared to dip.

Public concern about global warming in the United States has dropped sharply since the autumn of 2008, according to the results of a national survey released Wednesday by researchers at Yale and George Mason universities.

The survey found only 50 percent of Americans now say they are "somewhat" or "very worried" about global warming, a 13-point decrease; the percentage of Americans who think global warming is happening has declined 14 points, to 57 percent; the percentage of Americans who think global warming is caused mostly by human activities dropped 10 points, to 47 percent.

In line with these shifting beliefs, there has been an increase in the number of Americans who think global warming will never harm people in the United States or elsewhere or other species.

In addition, Obama's Democratic Party suffered a big blow last week when Republican Senate candidate Scott Brown, who opposed restrictions on carbon emissions, defeated his Democratic rival in Massachusetts special Senate election. The election lost Democrats its 60-vote Senate super majority, which enables them to pass a bill without any Republican support.

The Copenhagen Accord, a legally non-binding agreement reached by last year's UN Climate Change Conference, has asked nations to offer formal notification of their plans on emissions by Jan. 31. However, Yvo de Boer, the executive secretary of the UNFCCC, said last week that it was a "soft" deadline, which countries could sign up whenever they chose.

"There is nothing deadly about it," he asserted in his Bonn office. "If you fail to meet it, you can still associate with the accord afterwards."