Washington - US legislators this week are considering the first-ever mandatory limit on greenhouse-gas emissions in the United States, ignoring White House stiff opposition, instead looking ahead to a more favourable political climate after the November elections.

The Senate began debating a bill that would bring US targets on cutting climate-changing emissions roughly in line with the recommendations of international scientists, and would introduce a cap-and-trade system along the lines of what already exists in the European Union.

The initiative was sharply rejected Monday by President George W Bush and allies in the centre-right Republican Party, who have pushed for voluntary targets and resisted tougher emissions curbs on the grounds that it will cost jobs and harm an already-sluggish US economy.

Environmental groups have mostly welcomed the Climate Security Act as a solid first step, introducing an emissions-trading system that allows companies to buy and sell the pollution allowances allocated to them by the government.

The bill has some bipartisan support, though supporters acknowledge that climate legislation is still unlikely to be enacted until next year.

"We've done the best we can. Let's show the American public this institution can work," said John Warner, a senior Republican who co- sponsored the bill in the Senate. "Doing nothing is not an option."

Most proponents are instead looking eagerly toward a new White House team in 2009. All three major presidential candidates - Republican John McCain and centre-left Democrats Barack Obama and Hillary Clinton - have advocated stiffer measures to combat global warming and support a cap-and-trade system.

Meanwhile, the mere fact that a discussion among US politicians is taking place represents progress in the eyes of some activists. Senators introducing the bill on Monday called it a "historic debate" that could only add momentum to climate action.

"Today marks the end of the United States' long hiatus in dealing with global warming," said Phyllis Cuttino of the Pew Environment Group. "This debate is the first step toward regaining US leadership and momentum in forging solutions to global warming."

Climate advocates believe it is critical for the United States to adopt mandatory cuts before the international community meets at the end of 2009 in Copenhagen to negotiate a global treaty on combatting climate change.

"It would move us into a leadership role, because you can't lead from the rear, and that's what we've been trying to do in the past with no success whatsoever," said Dan Lashof, director of the Natural Resources Defence Council.

Opposition to climate legislation in the US remains strong, and if anything has grown louder over the last year amid a sharp downturn in the world's largest economy. The exact costs of a cap-and-trade programme have been the subject of sharp disagreement on both sides of the aisle.

Republican opponents argue that the bill will damage international competitiveness and raise already surging energy prices, citing reports that the costs of the legislation could be anywhere from 700 dollars to 3,000 dollars per year for the average consumer.

"It will without doubt sharply raise the cost of gasoline and electricity in America ... and will surely damage our economy," said Republican Senator Jeff Sessions. "It would be a calamity, I'm convinced, to impose this process on the American economy and the American people."

Supporters argue that the costs have been exaggerated and say the long-term price of inaction - ranging from greater storm damage to real estate lost to rising sea levels - is far higher. Proponents also point to tax relief included in the bill to offset energy costs.

The trading system would cover about 85 per cent of US emissions. It would seek to bring pollution back to 2005 levels by 2012, cut another 30 per cent by 2030 and 70 per cent in total by 2050.

A series of reports last year by a United Nations-backed panel of scientists said that global emissions needed to be slashed about 50 per cent by mid-century to minimize global warming.

The Bush administration has pushed incentives and subsidies for renewable energy technologies as the best means of reducing greenhouse gases blamed for global warming.

Climate activists argue that placing a price on those emissions will give the private sector the financial incentive it needs to plug money into research and greater efficiency.

Debate on the Climate Security Act is expected to continue throughout the week in the Senate. The lower House of Representatives is considering its own version, which may not come up for discussion until later in the summer. (dpa)