Debate over New Zealand's membership of the Kyoto Protocol, the international treaty to combat global warming, has been revived by new Government estimates of the likely cost.

Kyoto countries which exceed their agreed allocation of rights to emit greenhouse gases will have to buy additional rights, commonly called carbon credits, from other Kyoto countries which have more than they need.

New Zealand had been expected to have a surplus to sell, but if the revised estimates are right it will be a buyer.

Even at the relatively low price of $15 a tonne of carbon dioxide, the figure the Government used to set the initial level of the carbon tax due to come into effect in 2007, the revisions mean taxpayers will be $1 billion worse off between 2008 and 2012.

The National Party says it will review New Zealand's membership of Kyoto if it leads the next government.

"We have a reluctance for New Zealand to withdraw from agreements that have been signed because it is not good for our name internationally," said National's environment spokesman, Nick Smith. "But we now have to weigh up whether to withdraw, given the scale of this error."

National's policy has been to scrap the carbon tax but to remain in Kyoto for the first commitment period (2008 to 2012) and to be guided beyond that by what major trading partners such as the United States and Australia, the only OECD countries outside Kyoto, do.

"This new information shows the economy will take a hammering during that period so we will have to review that position," Dr Smith said.

National's leader Don Brash told Parliament on Tuesday that to justify participation in the protocol National would need to be convinced global warming was occurring, that it was due to human activity and that the sacrifices were commensurate to any potential gain.

On the first two points, at least, Dr Brash could look to a statement earlier this month signed by the US National Academy of Science, the Royal Society, and equivalent academies in the other six largest industrial countries plus China, India and Brazil. These bodies are the guardians of the scientific method.

Climate change is real, they said, and it is likely most of the warming in recent decades is due to human activities. "The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action".

Murray Ward, a veteran of international climate change negotiations and now a consultant, warns of a backlash and damage to New Zealand's clean, green image if it pulled out of Kyoto.