Keeping New Zealand's methane emissions to today's levels won't be enough to prevent further global warming, new modelling has shown.
Research released today by Parliamentary Commissioner for the Environment Simon Upton comes amid debate around how - or whether - the greenhouse gas should be covered by the Government's proposed Zero Carbon Act.
The three options on the table are forcing carbon dioxide (CO2) and nitrous oxide (N2O) down to zero by 2050; doing this while also stabilising shorter-lived gases like methane; or requiring all gases to be reduced to net zero by the mid-century deadline.
But the new modelling, led by leading agricultural greenhouse gas expert Dr Andy Reisinger, suggested that simply holding the line on current livestock emissions wouldn't avert more warming.
"It shows that holding New Zealand's methane emissions steady at current levels would not be enough to avoid additional global warming," Upton said.
The work indicated that, to ensure methane from livestock contributed no additional warming beyond current levels, emissions would need to be cut by at least 10 to 22 per cent below 2016 levels by 2050, with further reductions of between 20 and 27 per cent by 2100.
It also found that "holding livestock methane steady at 2016 levels would cause additional warming of 10 to 20 per cent above current levels."
The research didn't show what would need to happen to methane flows if New Zealand wanted to hold global warming to 1.5C above pre-industrial levels, which was the aim of the Paris Agreement."
Methane - accounting for nearly half of New Zealand's greenhouse gas inventory - was a relatively short-lived gas and most of it broke down within a few decades.
Livestock methane - mostly belched from sheep and cattle - accounted for 85 per cent of New Zealand's methane emissions.
The remainder came from waste landfills and wastewater treatment (11 per cent), and extraction and use of fossil fuels for energy generation and industrial processes (3 per cent).
Most of the warming caused by methane emissions occurred during the first few decades, though some warming lingered for centuries after the emissions themselves had disappeared.
One tonne of biological methane caused approximately 33 times more warming than a tonne of carbon dioxide over a 100-year period.
However, carbon dioxide caused sustained warming for thousands of years.
Part of Reisinger's work sought to estimate a trajectory for livestock methane that would generate no additional contribution to warming at any point from here on.
One trajectory - based on an assumption that emissions would peak in the next few years and then drop away significantly - wasn't considered a realistic one as it would require deep reductions globally in the immediate future.
A more gradual trajectory, meanwhile, would lead to some additional warming in the near term, requiring bigger emissions reductions later.
Through breeding more efficient animals and improvements in farm management, the emissions intensity of New Zealand agricultural production had improved by about 1 per cent per year over the last few decades.
If production were held constant, and efficiency gains of 1 per cent per year continued, total emissions of methane from livestock would reduce.
While this scenario wasn't modelled, it was expected the impact would still be likely to cause additional warming for many years to come, partly due to the legacy of past emissions.
Upton said he wasn't backing a specific climate target or approach for slashing livestock methane or other agricultural greenhouse gas emissions, but wanted to see an evidence-based debate on how best to approach it.
"I hope this new work will help promote debate on reducing methane emissions that is grounded firmly in science."
The farming sector wanted methane treated differently to other gases in the Zero Carbon Act, and also opposed agricultural emissions being pulled into the Emissions Trading Scheme.
Other groups - including Greenpeace and Forest & Bird - have strongly called for all gases to be cut to zero by 2050 under the act.
Victoria University climate change researchers Professor Dave Frame and Dr Adrian Macey have meanwhile argued only the second option of cutting CO2 and NO2 to nothing while stabilising shorter-lived gases would make for an act consistent with pledges already made under the Paris Agreement.
They suggested the best way of New Zealand reaching a zero-carbon point was by contributing no further warming, having found and struck a balance between emissions and sinks to absorb them.
But they argued there was a risk resources could be "seriously misaligned" if the Government continued to rely on a measure - Global Warming Potential 100, or GWP100 - which they said failed to accurately account for methane emissions.
Climate Change Minister James Shaw couldn't say whether it was likely methane would be included in the Zero Carbon Act, but added that shifting to low-emissions would require change right across the economy, regardless of what target was picked.