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The marketing behind Greenpeace New Zealand's Sign On campaign is impressive. The logic behind it is more questionable.

While the case for co-ordinated international action on climate change is clear and compelling, it is far from obvious that New Zealand needs to reduce its greenhouse gas emissions by 40 per cent by 2020.

Climate change is a long-term, gradually evolving problem. The impacts that arise from a build-up of greenhouse gases in the atmosphere are primarily due to the accumulation of carbon dioxide over time, rather than annual emissions.

This doesn't mean we can go on emitting greenhouse gases indefinitely - the case for us to reverse recent growth in emissions and move to a lower carbon economy is undeniable.

But as colleagues and I argued in the journal Nature recently, what really matters is the total amount of carbon dioxide released over the next few hundred years, not the details of the timing. The climate doesn't care exactly when we burn carbon, only how much eventually gets burnt.

As my colleague Myles Allen put it: "Mother Nature doesn't care about dates. To avoid dangerous climate change we will have to limit the total amount of carbon we inject into the atmosphere, not just the emission rate in any given year."

Let's say New Zealand emits some fixed amount of CO2 between now and 2030. The climate isn't sensitive to how we distribute this total across time - only what the total is.

We could have five years of increased emissions followed by steep cuts, or we could make gradual reductions. It doesn't matter to the climate. This is because emissions in any given year - be it 2020 or 2050 - are largely irrelevant, except insofar as they contribute to the overall total.

But it does matter to our society and economy, since the costs of rapid near-term cuts are different from those of a gradual and sustained reduction in emissions.

The main constraint on the details of our emissions reductions can't and shouldn't come from physical science; it comes from the realm of socio-economic possibility. What cuts can we afford, when?

Focusing on near-term emissions targets without considering how they might be achieved or what the costs might be is cavalier. Choosing an arbitrary but onerous target such as a 40 per cent reduction in emissions by 2020 without a clear, realistic strategy for getting there is a lot like the owner of a premiership soccer team declaring that his team will beat its rivals 5-0 without having consulted the coach or players about how this might be done.

It is unlikely to lead to success, and in the long run simply damages his credibility.

Instead of grand promises based on fairly arbitrary numbers we need a sensible, long-term national plan for emissions cuts.

It should take into account the costs of emissions reduction and balance these against the reputational hit of being a laggard in the climate game, which may be substantial in New Zealand's case.

New Zealand's reliance on agriculture does muddy the picture somewhat - some 35 per cent of our emissions are in the form of methane, a short-lived but high-impact gas. But even here it isn't clear quite what is to be gained by substantial near-term cuts, other than a very small reduction in the overall rate of climate change.

Furthermore, there is a strategic issue about how much New Zealand should try to lead in cutting agricultural emissions. The EU has pretty much given a free pass to its own agricultural sector in its emissions trading scheme.

So why should New Zealand - small, isolated and poorer than many of our trading partners - lead? This is not to say we should never lead in any sector, of course, but we should only do so if we think it in our strategic interests to do so.

Rather than set economically precarious top-down goals we should plan for emissions reduction by comparing our economic sectors against those of our competitors and trading partners, and work out where we might do well by leading, and where we ought to wait.

Otherwise we risk asking the impossible from productive sectors of the economy that already have to overcome the burdens of distance, size and foreign protectionism.

The language of project management is often mocked, often reasonably, for being short on content and long on jargon. But a handy litmus test for New Zealand's climate policy might be to evaluate proposals against the "Smart" criteria - is a proposal specific, measurable, achievable, realistic and timely?

Greenpeace New Zealand's Sign On proposal delivers in terms of being specific, measurable, and timely but, like many non-governmental organisations, it struggles in terms of being achievable and realistic.

New Zealand has had an unfortunate proclivity for extreme positions in the climate change debate. A small and vocal, but scientifically ignorant, sceptical lobby have tended to exchange fire with a slightly better informed, but occasionally hysterical, green movement. It would be nice to see us move beyond this.

The case for global emissions reductions is strong. But how this burden falls internationally and across generations is not obvious or agreed. We ought of course to play our part, but what that entails ought to be a matter of negotiation both internationally and, especially, within New Zealand. Arbitrary targets such as that advocated by Greenpeace are a bad place to start.

What New Zealand really needs is a long-term, considered plan to move away from fossil fuels, as painlessly as possible, over the next half-century or so. There are no particular reasons, economic, scientific or moral, why we need to be 40 per cent of the way there by 2020.

* New Zealand scientist Dr Dave Frame is deputy director of the Smith School for Enterprise and the Environment at Oxford, and visiting lecturer in the Department of Physics at the University of Oxford. He has a background in physics, philosophy, economics and policy.