The government of New Zealand responded with some irritation to my column last week, which castigated a national strategy for meeting its Kyoto climate targets by allowing greenhouse gas emissions to rise by 22% from 1990 to 2007.

All was well, it said. The 600,000 hectares of forests that were planted in the 1990s would soak up all the excess CO2 – around 90m tonnes of it between 2008 and 2012. In fact, the country was likely to be ahead of its Kyoto target of stabilising emissions at 1990 levels.

But back home this policy is controversial, to say the least, with many experts accusing the government of a sleight of hand. They include the independent but prestigious Sustainability Council of New Zealand.

The central problem seems to be that when it comes to carbon, Middle Earth is a scientific minefield. And the Kyoto rules give the government considerable potential to pick and choose which carbon emissions and which carbon sinks from forests it declares for the purposes of meeting its targets.

There are, it turns out, two sets of carbon accounts.

The full statistics delivered to the UN Climate Change Convention show that the New Zealand landscape is, as the government says, absorbing more carbon today than it did in 1990. But only a bit more. Enough to cut its emissions growth from 22% to 185. That is nowhere near enough to bring New Zealand into Kyoto compliance.

But, as the spokesman for the climate change minister, Nick Smith, pointed out to me this week, those are not the only numbers. "The convention inventory includes a wider set of activities than under the Kyoto protocol." In a nutshell, the Kyoto protocol allows New Zealand to ignore what is happening across the wider landscape and simply report the growth of its 600,000 hectares of new forests, planted mostly during the 1990s.

That sounds dodgy, though within the Kyoto rules. Even so, if these "Kyoto forests" had been specifically planted as part of a genuine policy to cut the country's long-term contribution to global warming – we might still applaud.

Unfortunately it is not quite like that. Those forests are not long-term sinks; they are commercial plantations. As Smith's spokesman told me, they "are likely to be harvested in the 2020s". And, he added: "The government has no intention to ban the harvest." When they are harvested their carbon will return to the atmosphere.

The Sustainability Council of New Zealand attacked the government on this very point in a report on the country's climate policies published last week. It said: "The official Kyoto accounts ... have given a misleading impression of New Zealand's emissions position ... treating carbon absorption by forests as income rather than credit." Claiming the forests as a carbon sink today is cynically offloading the problem to the next generation, it said.

Sometime in the 2020s, New Zealand will become responsible for a massive surge in emissions from its forests – just at the time when global demands for ever-deeper cuts in emissions are likely to be going into overdrive.

The government's own civil servants seem to agree. The New Zealand Treasury recently called the carbon accumulating in the Kyoto forests a "contingent liability". It warned that negotiators should take this into account when agreeing future emissions targets – such as a Copenhagen deal on 2020 emissions.

There is a final problem for New Zealand's carbon credentials. The government's scientists have, in the past couple of years, been reassessing all their figures in a way remarkably beneficial to the government. Last April, they reported to ministers of the incoming government that emissions from deforestation were almost 10m tonnes a year less than previously supposed "due to new data showing smaller trees being felled". Meanwhile, they said, the Kyoto forests were absorbing a quarter more carbon than previously supposed "due to the trees not being thinned and being planted on better soils".

Very handy. But even Smith was moved to note the "volatility" of the numbers.

A number of scientists have been pointing out for some years that the Kyoto rules on forests were an Achilles heel in the protocol. "If [countries] plant sink forests and make inflated claims for them, they know it will be impossible to either prove or disprove those claims. It really is a cheat's charter," warned Michael Obersteiner of the forestry division of the International Institute for Applied Systems Analysis (IIASA), a thinktank based in Laxenburg, Austria, back in 2000.

It may not be cheating, but New Zealand seems determined to prove him right.

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