Greenpeace is questioning why the Government is tabling options for its Zero Carbon Bill that could treat agricultural greenhouse gas emissions more softly.

Farming groups are meanwhile heartened to see the Government considering approaching shorter-lived gases differently to carbon dioxide, which lingered in the atmosphere for hundreds of years.

A discussion document on what will be the Government's climate policy centrepiece has set out a range of options for the bill.

They included making the 2050 target apply only to carbon dioxide, ramping down CO2 but stabilising short-lived gases like methane or nitrous oxide, or requiring that all emissions were reduced to net zero by the middle of the century.

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All of the options would have big implications for households, whose incomes would likely not grow as quickly, and for industries, which faced having to transform and innovate to meet ambitious new requirements.

Responding to the proposals, Greenpeace voiced its concern that agriculture - which, under current measures, accounted for nearly half of New Zealand's greenhouse gas inventory - could avoid being forced to greatly curb emissions.

"As the single biggest emitting industry in the country, it would be a travesty to exclude dairy and agricultural gases like methane, which make up half of New Zealand's total emissions," Greenpeace climate campaigner Kate Simcock said.

Greenpeace also questioned whether New Zealand could meet its climate commitments by buying carbon credits on the international market instead of through reducing our own emissions at home - and whether a fixed target should be legislated or left to a new Climate Change Commission to decide.

Simcock argued that without including agricultural emissions, and banning international carbon credits from being purchased instead of actively reducing New Zealand's emissions, the Zero Carbon Act would be a much weaker climate commitment than what has been promised.

"We've told the world we're going to be leaders on climate change," she said.

"That means we have to be all in and as ambitious as we can.

"If we were competing in the Olympics, we'd be aiming for gold, not bronze."

Federated Farmers climate change spokesperson Andrew Hoggard said the option of "stabilising" shorter-lived gases was "valid and practical".

Hoggard noted that carbon dioxide from transport had risen by 82 per cent since 1990, while methane produced by agricultural livestock increased by four to five per cent.

While research was underway, New Zealand hadn't yet produced a "silver bullet" science solution capable of stripping out much of the sectors' greenhouse gas emissions, which mainly came in the form of methane burped from ruminant sheep and cattle.

"So while this work is extremely positive we can in no way bank on the fact that farmers will have available to them options to reduce their methane and nitrous oxide emissions in a dramatic fashion."

Hoggard went on to point out the per kilogram carbon footprint of New Zealand meat and milk production was "far lower" than most overseas producers.

"If we cut production here, we run the risk that those competing producers step up to fill the gap, and more greenhouse gases are put into the atmosphere."

Hoggard's comments were echoed by Beef and Lamb New Zealand chief executive Sam McIvor.

"We know that the various greenhouse gases have different lifespans and roles in climate change, so we're keen to bring the latest science to the table so that the Zero Carbon Bill reflects the latest scientific thought and provides a workable framework for the sheep and beef sector."

A new study co-authored by a leading New Zealand climate scientist argued for a new approach to methane, finding that current methods of accounting for it was exaggerating its long-term effects on the climate.

A six-week consultation process on the proposals opened today. People can read them and have their say here.