"Unfeasible and unfair" — that pretty much sums up the reaction of pastoral farming sector groups to the Climate Change Commission's draft plan for reducing agricultural emissions out to 2035.
The latest national greenhouse gas inventory, released this week, tells us that enteric methane — belched out by ruminant animals and much the largest source of emissions from farms — made up 37 per cent of national emissions in 2019. That is too large a share to be left in the too-hard basket.
But the inventory also tells us that the increase in annual enteric methane emissions since 1990 has been only 5.5 per cent, when gross emissions from all sources have risen by 26 per cent over that period. Between 2018 and 2019, enteric methane emissions increased at only one-tenth of the pace of emissions generally.
This suggests they are not the most pressing problem; carbon dioxide from fossil fuel use is.
The commission's draft advice contends that methane emissions from agriculture can be reduced by 13 per cent by 2030 and 16 per cent by 2035 while maintaining current meat and milk production levels.
And it believes this can be achieved by the widespread adoption of current best practice; it does not rely on technological advances like methane inhibitors or genetically modified ryegrass.
Instead, it extrapolates forward the past rates of improvement in two trends.
One trend is productivity. That is, output of meat and milk per head of livestock.
The other is emissions intensity, or the "embedded" methane emissions per kilogram of meat or milksolids. That has been falling at a rate of about 1 per cent a year.
The commission's cheerful conclusion is that New Zealand should be able to maintain current levels of meat and dairy production over the next 14 years, even as livestock numbers fall by 15 per cent.
Federated Farmers, in its submission responding to the commission's draft report, accuses it of trying to avoid making difficult decisions by drawing on modelling that concludes that, at least in the short to medium term, you can "have your climate action cake and eat it too". But the modelling, Federated Farmers believes, was informed by overly optimistic data.
Dairy NZ argues that the increase in milksolids production per cow (about 1.4 per cent a year for the past 10 years) cannot be projected forward.
That productivity gain has reflected a couple of trends which have run their course.
One is the conversion to dairying of highly productive land in Southland and Canterbury, which produces greater than average milksolids per cow.
The other trend is greater use of supplementary feed, including imported palm kernel extract, to boost production in the shoulders of the pasture-growing season.
But with many regional councils' plans prohibiting further intensification and dairy conversions, the historical increase in milksolids per cow should not be projected forward, Dairy NZ says.
It also argues that the commission is being naive if it assumes that every option for reducing emissions can be implemented on every farm at once. "Some farmers are already making these changes while others have limited options."
The national inventory tells us that since 1990 there has been a gradual but sustained increase in methane emissions per head of all the major ruminant species — "emission factors" in the jargon — associated with the increased productivity.
"Increases in animal liveweight and milk yield per animal require increased feed intake per animal to meet energy demands," it says.
And there is — or has been so far — a linear relationship between dry matter intake and methane emissions.
The result has been higher emissions of methane per animal, and for that matter nitrous oxide, another greenhouse gas.
On the inventory's numbers, methane emission factors for dairy cattle and sheep have both increased at a compound annual growth rate of 0.78 per cent a year between 1990 and 2019, and by 0.54 per cent a year for beef cattle.
So there is a tension between these two trends: increasing emissions per head as livestock get heavier and need to eat more on the one hand, and falling emissions per kilogram of meat or milksolids on the other.
Whether the latter trend will in future dominate the former to the extent the Climate Change Commission expects is the question. The industry bodies doubt it, to put it mildly.
They also make the point that the commission's projected fall in enteric methane emissions by 2030 is one third larger than the 10 per cent target cut from 2017 levels enshrined in the Zero Carbon Act.
Dairy NZ regards 10 per cent as already a stretch target.
Beef + Lamb goes further and contends that the 10 per cent target is already unfair in that it was not based on the latest science on the warming impacts of emissions of methane, given that it is a potent but short-lived greenhouse gas.
Beef + Lamb argues that the commission is asking farmers to make sacrifices which will have a cooling impact on the planet so that other New Zealanders can continue to do things which will have a warming impact. And that is neither fair nor efficient.
This is based on an arcane but important debate about the relative merits of the measures known as GWP100 and GWP* when it comes to comparing the warming impact of a short-lived "flow" gas like methane with long-lived "stock" gases like carbon dioxide.
Oxford University researcher Michelle Cain explains it like this: "Consider a power station and a herd of cows. A power station emits CO2 by burning fossil fuels. This CO2 is taxed. When it shuts down permanently, it emits no more CO2, so is no longer taxed. However, the CO2 already emitted continues to affect the climate for hundreds, or potentially, thousands of years. So even after closing down, that power station still contributes to holding up global temperatures because of the CO2 that remains in the atmosphere.
"Now to the cows. A herd of cows emits methane, so the farmer is taxed for those emissions. If the herd remains the same size with the same methane emissions every year, it will maintain the same amount of additional methane in the atmosphere year on year. In terms of its contribution to warming, this is equivalent to the closed power station.
"The power station pushed up global temperatures when it was running in the past, just as the farmer's great-grandparent pushed up global temperatures when they were building up the herd of cattle. But neither a steady herd of cattle nor a defunct power station is pushing up global temperatures any more."
Federated Farmers quotes the commission's own evidence report back at it: "When GWP100 is used to look at mitigation scenarios over long timeframes, it does not provide robust estimates of actual temperature outcomes. It does not give good information for making decisions about trade-offs between reducing methane emissions vis-a-vis carbon dioxide emissions when considering trajectories for, or compliance with, temperature targets such as the 1.5C goals in the Climate Change Response Act."
It remains to be seen whether these arguments about feasibility and fairness will cut any ice with the commission when it releases its final advice. It will then be up to the Government to decide to what extent it follows that advice.
The risk it would run if the final policy changes are seen as unreasonably hard on pastoral farmers is that it will strengthen the forces of reaction on climate within the National Party caucus, among whom Judith Collins is perhaps the most conspicuous example.
That could undermine one of the key objectives of the Zero Carbon Act, which National supported, namely to remove climate policy from the arena of partisan politics, so the necessary investments can be made with confidence.