The Zero Carbon Bill the Government is introducing is, on the face of it, good law.
It reflects the gravity of the challenge by setting a mid-century goal for greenhouse gas emissions, consistent with what would be required globally to limit warming to 1.5 degrees Celsius.
And it sets up a mechanism for translating that goal into a series of quantified medium-term targets against which policymakers can be held accountable and which should provide the essential balance of certainty and flexibility that businesses and investors need.
Whether the legislation achieves its purpose will depend on whether the National Party supports it, because the framework it establishes has to survive changes of government.
Whether National supports it will most likely depend on whether they think it is too hard on farmers.
Agricultural emissions make up 48 per cent of the national total. Farmers cannot – and many do not – expect a free ride to last forever.
But pastoral farming also generates 43 per cent of the country's export income. Therein lies the dilemma.
The legislation sets a target of net zero emissions by mid-century for the long-lived greenhouse gases, especially carbon dioxide and nitrous oxide, but carves out for separate treatment biological emissions of methane, the potent but short-lived gas mainly produced by belching sheep and cattle.
The bill would set a target of reducing biological methane emissions by 10 per cent by 2030 and by what the Government is calling a "provisional" range of between a quarter and a half by 2050.
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To calibrate the scale of ambition here, the emissions intensity of pastoral farming has been improving by about 1 per cent a year since monitoring started in the 1990s
Some will argue that any emissions price — and a rising price is almost certainly what would be required to meet the target — which is applied to farming is an impost their international competitors do not face.
Some will argue that the international carbon accounting rules embody too high an "exchange rate" in CO2-equivalence for methane. There is no right answer to that contention. It entirely depends on what time period you think is relevant.
But to calibrate the scale of ambition here, the emissions intensity (emissions per kilogram of meat or litre of milk produced) of pastoral farming has been improving by about 1 per cent a year since monitoring started in the 1990s.
That is the result of productivity gains farmers have achieved, even without the incentive of a price on emissions, essentially by reducing how much feed is overhead, needed to sustain the animal, and how much ends up in the product. That rate of progress cannot continue indefinitely, of course.
But, by the middle of the century, we can also expect to see changes in land use and shifts in consumer preferences (fake meat, anyone?).
Nor should we discount the potential for advances from research underway into animal genetics, pasture species or intervention in the complex microbiology of the rumen.
As Climate Change Minister James Shaw said, if the Americans had waited until the technology needed for the Apollo programme to succeed to be available before setting the goal of a moon landing, it would never have happened.