Parliament's environment watchdog has taken the unusual step of writing to three farming groups, accusing them of wrongly claiming they had new science on methane emissions.
The environment commissioner, former National Party Cabinet minister Simon Upton, also said the lobby groups had buried the most relevant conclusion of the research - a part which might have been seen as supporting stronger action on farm gases.
Two of the groups have responded, saying they think there might have been a misunderstanding.
Dairy NZ, Beef + Lamb and Federated Farmers made a joint submission to the Climate Change Commission on New Zealand's methane targets, which cover how much farms must reduce planet-heating gases from burping cows and sheep, as well as escaped landfill gases.
Methane is shorter-lived but more potent than carbon dioxide, and makes up an unusually high proportion of New Zealand's impact, because of the big dairy and meat export industries. The current targets are to reduce methane by 10 percent by 2030 (including steeper cuts to landfill gases) and 24-47 percent by 2050.
The farm groups used a scientific study they commissioned from Oxford and Cranfield universities to argue the targets for 2050 were too high. The study covered similar ground to previous work the Parliamentary commissioner released in 2018, with some differences in methods.
In a submission to the climate commission, the farm groups said the latest research "demonstrated a significant development in the scientific understanding of what is required for New Zealand to achieve no further warming from biogenic (animal or landfill) methane", implying the new understanding had come along since the law covering methane targets took effect in 2019.
Upton wrote that: "After reading the study you commissioned, I can find no support for this conclusion. This is a pity - by repeating work that has already been done, an opportunity to move the debate forward has been missed."
Dairy NZ and Beef + Lamb said they thought Upton had misunderstood. They said the primary purpose of the research they commissioned was to look at the heating impact of the 2050 targets and "in our view this research has not been completed before and provides new insights".
Upton also told the groups it was a shame they had put the most relevant finding - that the reduction of farming methane represents New Zealand's greatest opportunity to reduce its contribution to global warming - in an "appendix to an appendix".
He wrote that the recent study left New Zealand in exactly the same place it was in 2018.
"This is not "new science" and it leaves us exactly as I had previously stated the position to be - namely, that the sustained contribution to atmospheric warming that New Zealand makes through the emission of agricultural methane is a matter of choice, and choosing to maintain this warming at the current level is to claim a "right" to a certain level of warming from agriculture indefinitely."
He copied his letter to the Climate Commission, which was considering the methane question.
Farmer focus on 'no added warming'
National's policy was to "review methane targets for consistency with no additional warming from agriculture". But climate spokesman Simon Watts also told Newsroom that the starting point was that the party was committed to meeting methane targets.
The reference to 'no additional warming' refers to an idea (popular with farming lobby groups) that the heating created by New Zealand's farming methane should be stabilised, but shouldn't have to reduce below current heating levels.
Farm groups argue this is fair, to make methane's heating situation equal to that caused by carbon dioxide (from cars, factories and other places).
Heat from carbon dioxide physically cannot stop rising until carbon emissions get to net zero, decades from now. That is because carbon dioxide lasts a long time in the atmosphere, so the tap needs to be turned off completely to stop heating from rising.
Methane is shorter-lived, so its heating can reduce or stabilise within a decade if the flow of emissions turns down even a little bit. That was why New Zealand's climate targets allowed for partial cuts, rather than a complete stop to methane. It was also why farming groups sometimes argued they should get to 'keep' the amount of heating they are currently making with their methane, as they would if they had already produced carbon emissions.
Upton has argued there is no particular reason why farmers should get to 'keep' today's levels of heating, particularly given farming's climate impact is larger than it was in 1990. Methane has caused most of New Zealand's contribution to heating so far, partly because it acts more quickly than carbon dioxide, front-loading the impact before it tails off.
But even achieving 'no added heating' means some level of cuts to methane emissions.
The scientific review commissioned by Upton in 2018 found if methane was kept at 2016 levels, New Zealand farms would still keep increasing the amount of heating they were making year-to-year, because of the physical flow-on effects of stable emissions of methane.
To keep the level of heating stable, farmers would need to shave off at least 10-22 percent by 2050, but most of this - up to a 20 percent cut - would need to happen by 2030, if the world was on track to stay inside 2C heating. (Methane's impact is bigger if countries make steep cuts to carbon dioxide, which is the main heating driver globally).
The more recent scientific report commissioned by DairyNZ, Federated Farmers and Beef + Lamb found keeping the heating caused by farming's methane steady at 2020 levels would take a 15 percent-27 percent cut by 2050, based on the rest of the world taking climate action at a moderate pace (requiring cuts at the lower end of the range) or faster (the higher end). Keeping New Zealand farming's methane's contribution at 2017 heating levels (rather than 2020) would take deeper cuts, the researchers said.
The two studies used different climate models, different base years and slightly different assumptions.
Dairy NZ and Beef + Lamb said the latest study used open-sourced methodology, giving greater transparency.