One possible outcome of the general election is a Labour-Greens coalition, in which case one of the most interesting areas for negotiation would be climate policy.

The climate change policies the two parties have recently released overlap a lot, in ways that distinguish them from National and the status quo.

But they are also at odds over which is the better way to put a price on emissions that will influence behaviour in the economy.

Labour wants to restore the emissions trading scheme (ETS), as designed by David Parker and enacted by the fifth Labour Government in the last few weeks of its ninth year in power, then promptly gutted by the incoming National Government.

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But the Greens favour a tax on emissions, the proceeds of which would be used to plant trees on erosion-prone land, and the rest (most of it) recycled as an annual payment to everyone over the age of 18.

Both parties would legislate something close to the Zero Carbon Act drawn up by Generation Zero and endorsed by the Parliamentary Commissioner for the Environment and the youth wings of the National, Labour, Green and Maori parties.

It would set a legally binding target of zero net emissions of greenhouse gases from New Zealand by 2050, and require interim emission reduction targets en route to that objective, based on recommendations from an independent Climate Commission.

The status quo is a gazetted - not legislated - target of cutting net emissions to 50 per cent of 1990 levels by 2050, with the interim target under the Paris Accord of an 11 per cent reduction from 1990 by 2030.

Officials estimate that New Zealand will fall short of that 2030 target by more than 200 million tonnes over the 2020s and will consequently have to buy carbon credits from someone, somewhere in the rest of the world to cover the gap. At a carbon price of, say, $50 a tonne, that would cost the country over $1 billion a year.

New Zealand First bridles at that outflow of money, but it is essentially a sunk cost - the consequence of abject policy failure to do anything to rein in emissions growth.

Turning around the trajectory of emissions from its current rising path will be neither quick, nor cheap, and the option of reneging on the Paris commitment is not advocated by any of the major political parties, including New Zealand First.

Nearly half of national emissions arise from pastoral farming. The cost of that is entirely socialised at present, which is unfair and distorts land use decisions in an unhelpful way.

Labour's policy is to bring agriculture into the ETS during its first term, on essentially the same basis as other emissions-intensive, trade-exposed sectors. That is, with a free allocation of units to cover 90 per cent of emissions but an exposure to the carbon price for the other 10 per cent.

The default point of obligation would be the processor - the dairy factory or meat works - but with an ability to opt in at farm level where practicable.

"This would recognise those farmers who want to accurately measure their own carbon inputs and outputs including pasture, soil carbon, farm forestry, fertiliser and emissions," says Labour.

It is non-committal on one aspect of Generation Zero's version of a Zero Carbon Act, however, which is the "two baskets" approach, where a separate target is set for methane, as a short-lived greenhouse gas, saying only that it is open to a science-led discussion on the subject.

Labour also envisages that either the Climate Commission or the Government would provide guidance on future carbon prices, including a price floor and ceiling and any prospective changes to the free allocation for agriculture or other emissions-intensive, trade-exposed sectors.

The Greens, however, argue that while the ETS could be reformed and rendered effective, it will always be susceptible to being undermined again in the future.

Their preferred carbon tax would, they expect, be set at initial levels of $40 a tonne for carbon dioxide (compared with an ETS price of around $18 now), $6 a tonne (on a CO2 equivalent basis) for nitrous oxide and $3 a tonne for methane emissions from agriculture. Carbon sequestered in eligible forests would be worth $40 a tonne.

At those rates they reckon that a dairy farm of 420 cattle would face a bill of $4600 a year, or need to plant 6.8ha to offset the cost over 10 years. A dry stock farm of 340 cattle and 2700 sheep would face carbon bill of $6000 or need to plant 9ha.

On that basis, CO2 emitters would be providing over 90 per cent of the carbon tax.

The tax would not go into the consolidated fund, but into a "Kiwi Climate Fund". The lion's share of it, about $1 billion a year initially, would be paid out as a dividend of around $250 a year to anyone over 18.

The rest would mainly be used to reverse the net deforestation which has been happening since 2008, by encouraging the planting of 1.2 billion trees on 1.1 million ha of land at serious risk of erosion.

Labour also talks about encouraging increased forest cover. Partly, that would come from restoring forest owners' confidence in the ETS, sorely tried by their experience in recent years.

It would also review the permanent forest sinks initiative, which is intended to encourage the development of permanent native forests on land where the opportunity cost is low and there is potential for riparian planting to qualify for carbon credits.

Another area where Labour and the Greens differ is on whether to allow any further quest for fossil hydrocarbons.

Labour would. It talks about the need for a "just transition" to a low-carbon future, during which gas is preferable to coal as an industrial fuel and some gas will be required for back-up generation in dry years when inflows into the hydro lakes are low.

The Greens would ban drilling and fracking outright, on the grounds that the world has already found more hydrocarbons than it can burn without cooking the planet.

Climate control

Labour:

• Revive the emissions trading scheme, bringing agriculture into the system

• Target of net zero greenhouse gas emissions by 2050, with legally binding emissions-reduction goals

Greens:

• Tax emissions. Use the money to plant trees and pay an annual dividend to anyone over 18

• Legislated target of net zero greenhouse gas emissions by 2050