Opposition leader Brendan Nelson said he was committed to introducing emissions trading by 2012, but if big polluters like China and India do not make commitments to reduce emissions it should be deliberately toothless.

"Our policy has not changed, and that policy is we would implement an emissions trading scheme, cap-and-trade, no later than 2012," he told Sky News.

"And obviously what you would do, if for example we haven't got the big emitters on board, what you do is the price of carbon is set so low and the (emissions) trajectory is so low as to be near meaningless."

The government has committed to introducing a "cap-and-trade" carbon trading scheme by 2010, but its plans have been criticised as not going far enough to cut the greenhouse gases blamed for global warming.

Nelson has been trailing badly in the polls to Prime Minister Kevin Rudd since he became opposition leader following former prime minister John Howard's landslide defeat last year.

On a per capita basis, Australia is one of the world's top emitters of greenhouse gases.

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