The US Congress has yet to pass a comprehensive plan on climate change but it is taking action on one front -- ordering an in-depth "carbon audit" of the tax code.

With the 192-nation climate summit in Copenhagen entering full swing, the House of Representatives approved funding to look at how the mammoth tax code offers Americans incentives to emit carbon dioxide blamed for global warming.

Representative Earl Blumenauer, a Democrat whose constituency of Portland, Oregon is considered one of most eco-friendly US cities, said the audit was a "small but significant" sign the United States is serious about climate change.

"The United States can no longer afford to waste more energy than any other country in the world, even if one had no fear of global warming," he told AFP.

"If we don't turn this around in the course of the next 10 years, it's going to get out of control, is the consensus of the scientific community," he said.

Blumenauer managed to have his proposal for an audit signed into law last year, even before President Barack Obama took over and reversed former president George W. Bush's resistance to action on climate change.

But no money was approved for the audit until Thursday, when the House voted as part of a wide-ranging spending bill to provide 1.5 million dollars to the National Academy of Sciences to carry it out. The Senate is expected to follow suit in the coming days.

Blumenauer, who is known in Washington for riding a bicycle to the Capitol, pointed to one policy he would like to revise -- the so-called "Hummer loophole" that provides tax breaks for gas-guzzling sports utility vehicles.

The loophole is the legacy of a 25-year law that was initially designed to support businesses when buying heavy vehicles.

But Blumenauer said he did not want to prejudge the audit's findings -- or to predict whether business interests would try to block any eventual changes to the tax code.

Instead, he hoped that any revisions would also meet other priorities for the United States -- reducing the complexity of the tax code and reining in the soaring public debt.

"My goal is to avoid our being involved with unnecessary conflict that is just a tug of war between competing interests. Instead I'm interested in seeing what the underlying framework is," he said.

"We have to be able to put in place a system that meets America's needs," he said.

Congress is going ahead with the audit before finalizing legislation that is much more eagerly watched overseas -- on creating a so-called "cap and trade" system to reduce carbon emissions nationwide.

The House in June narrowly approved a plan that would cut carbon emissions by 17 percent by 2020 from 2005 levels, but the Senate has postponed action until next year.

Three senators including a Republican, Lindsey Graham, on Thursday proposed a framework of what they hope the legislation will ultimately look like.

In a bid to win over reluctant Republicans, their framework also includes support for nuclear power and offshore oil drilling, policies deeply unpopular with many environmentalists.