The Government has begun the process of public consultation on the most important legislation to come before Parliament in this term: the Zero Carbon Bill.

In many ways it is akin to the Reserve Bank Act, which established a durable and, crucially, depoliticised framework for monetary policy nearly 30 years ago.

Parliament decided that New Zealand was to become and remain a low-inflation economy.

It enacted a statutory objective — price stability then, an emissions target for 2050 now.

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It set up a mechanism for quantified medium-term targets consistent with that goal — the policy target agreements then, carbon budgets now.

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It gave an independent expert body — the Reserve Bank then, a Climate Change Commission now — the task of monitoring whether we are on track to achieve those targets.

If not, a policy response is required. That is the point where the analogy breaks down, because the policies needed to meet the climate targets would apply across a broad front and have to be decided by an elected Government.

In the discussion document, the Government proposes that three carbon budgets of five years each be in place at any given time.

"This would mean we have a minimum 'look-ahead' timeframe of between 10 and 15 years. We think this is a good balance between improving predictability and remaining flexible to changes in the future." The predictability is vital to encouraging climate-friendly investment decisions by business and households.

It might also help depoliticise the budget-setting process, it suggests, because the Government of the day would not be able to set or influence the budget for its own political term.

A key decision among those on which the Government seeks feedback is how to define the statutory objective of "carbon zero".

It gives three options for the 2050 target, in increasing order of stringency:

• A target for carbon dioxide emissions only: reducing CO2 emissions (net of sequestration in forests) to zero by 2050

• Net zero emissions of long-lived gases (mainly CO2 and nitrous oxide) by 2050 and stabilised flows of the short-lived gas, methane

• Net zero emissions across all greenhouse gases by 2050

The best of those three is the middle one, and not just because it would increase the chances of the legislation getting essential cross-party support if it did not look like a death sentence for pastoral farming.

Methane, which comprises 44 per cent of New Zealand's emissions as measured according to the international rules and which comes largely from belching cattle and sheep, is a potent greenhouse gas. But it is one which breaks down within a few years to CO2 and water.

The international accounting rules attribute a global warming potential of 28 to methane. That is, they reckon a tonne of methane emitted today will have the same global warming effect over the next 100 years as 28 tonnes of CO2 emitted today.

But that effect is not evenly spread through the century. It is front-loaded. If instead of the arbitrary period of 100 years they had opted for, say, 20 years, the "exchange rate" with CO2 would be more like a gulp-inducing 85 to one.

Will the next 30 years be any more predictable than the last 30 were?

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If the aim is to stabilise the level or stock of greenhouse gases in the atmosphere, emissions of the long-lived gases, which just accumulate, have to go to zero.

But to stabilise the level of a short-lived gas it is sufficient to throttle back the flow, not reduce it to zero.

A crude analogy might be running a bath. If you don't want it to overflow you have to turn off the taps. Then if you want it not to go cold you can add fresh hot water at a rate that matches the cooling effect. Eventually that will also cause it to overflow, but the time scales are quite different.

The discussion document does not indicate what rate of methane emissions would count as "stabilised".

There may be a clue, however, in the economic modelling the Government commissioned from the NZ Institute of Economic Research.

One of the scenarios NZIER models is one where emissions are reduced by 75 per cent from 1990 levels by 2050. This, we are told, "approximates an outcome where long-lived gases have been reduced to net zero in 2050 and short-lived gases from agriculture have been reduced by 45 per cent from 1990 levels by 2050."

Depending on the climate target scenario and the assumptions the modellers make about additional innovation in transport, energy and agriculture triggered thereby, the NZIER modelling suggests that economic output will continue to grow but more slowly, so that by 2050 it will be in the range of 10 per cent to 22 per cent less than if no further action is taken on climate change.

But there are a couple of reasons not to be too alarmed by such a conclusion.

One is that the counterfactual of no action on climate change over the next 30 years is unreal. No country would get away with that, let alone one whose national brand is clean and green.

The other is that 2050 is a long way off. A lot can happen in a third of a century.

Any economists undertaking a similar exercise back in 1986 would have missed some pretty significant developments: the internet, the rise of China as an economic superpower and the emergence of climate change as an existential threat. Will the next 30 years be any more predictable than the last 30 were?

The discussion document acknowledges this: "while modelling gives us a reasonable view through to 2030, beyond that the picture becomes less certain. Looking back both at the changes in technology and shifts in our economy over the past three decades shows that we can expect huge changes between now and 2050. This means modelling out to 2050 is stretching the models used to their limits." Quite.

Getting to zero
• Govt aims to have Zero Carbon Act passed into law by mid-2019
• The act will set a 2050 target for NZ's emissions of climate-change gases
• It will also set up emissions "budgets", detailing how much can be emitted in shorter periods leading up to 2050
• A Climate Change Commission will provide expert advice
• Submissions are now open — including on what "carbon zero" should mean