If New Zealand wants to tweak its methane targets without increasing its contribution to climate change globally, steeper emissions reductions will be required in other areas, the Climate Change Commission says.
New Zealand's domestic 2050 target currently requires all greenhouse gases to reach net zero emissions, except for biogenic methane where only reductions from a 2017 baseline are needed.
As part of the National-ACT coalition agreement, the Government agreed to "review the methane science and targets in 2024 for consistency with no additional warming from agricultural methane emissions".
The findings of the review are expected by the end of the year.
Outgoing Climate Change Commission chairperson Rod Carr told Q+A that the phrase "no additional warming" was essentially a question about how New Zealand wanted to count methane's warming impact domestically.
From there, New Zealand needed to determine whether it would effectively mean its contribution to warming internationally would increase compared to what was implied by the current 2050 target, he said.
"It would depend on whether New Zealand was to go negative in long-life gas sequestration. More trees. More speculation about alternative technologies to withdraw carbon dioxide from the atmosphere. More ambitious reductions in gross emissions.
"To square the circle of having more methane is to do some or all of those things, which is to say… we need to go further than net zero," Carr said.
But relying more heavily on forestry removals was risky, he added.
"The gases that arrive from burning fossil fuels are removed by forests. But the emissions are permanent and the forests are, at best, temporary."
According to the commission's calculations, under one possible scenario, meeting the current 2050 target could still mean New Zealand contributes 0.0032°C to warming around the world in that year. This is mostly from methane emissions.
If everyone globally contributed to the same level of warming per capita as Aotearoa, the planet would blow past its effort to limit warming to 1.5°C, the Commission said.
“Changing the biogenic methane target from the current range to ‘no additional warming’ and keeping the net zero component (of non-methane greenhouse gases) of the current target as is would mean higher emissions and an increased amount of warming than the current target," the commission said in its report.
“We have not analysed in detail what biogenic methane emissions would result under a no additional warming approach, because such a technical analysis would hide the more fundamental question: should Aotearoa New Zealand cause more global warming than implied by the current 2050 target?
"Our review of the current target... has found no grounds that would justify an increase in the overall amount of global warming caused by Aotearoa New Zealand’s emissions.”
Internationally, methane's contribution to global warming has been expressed through GWP100 — Global Warming Potential calculated over 100 years. This estimates the impact of different greenhouse gases relative to carbon dioxide.
For example, over a century under this measure, one tonne of methane would have the same warming effect as 25 tonnes of carbon dioxide because of its increased ability to trap heat in the atmosphere.
Methane, however, has a shorter life span in the atmosphere than carbon dioxide.
It's partly for this reason groups like Federated Farmers say New Zealand should be able to "keep" a stable level of heating caused by methane.
Carr said: "The argument is if you've got a cloud of methane, and you're not adding to the cloud because the rate at which it decays is equal to the rate at which you're adding to it. 'Look, see. No additional warming.'
"But the planet is warmer than it would have been absent your new additional methane emissions."
Federated Farmers is among supporters of a more permissive measure called GWP*, or Global Warming Potential Star, which places heavier weight on methane's shorter life span when measuring its emissions impact relative to carbon dioxide.
ACT agriculture spokesperson Mark Cameron has suggested that if GWP* was used for international accounting instead of GWP100, "it could have a significant impact on New Zealand's [climate] liabilities".
Meanwhile, groups like Greenpeace have suggested the Government's review of "methane science" could mean a shift towards GWP*, which isn't used in international emissions calculations and has been criticised for allowing too much methane to continue to be emitted under the guise of "climate neutrality".
Carr said: "The judgement question for New Zealand is how, domestically, do we want to cut methane? Because we are not going to change how the world counts methane."
"Within a country, you can choose to measure what you want. You can choose to measure how you like.
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Q+A with Jack Tame is made with the support of NZ On Air.
