New Zealand's bold new zero carbon legislation could fail unless it properly reflects how different greenhouse gases vary in impact, two leading climate researchers say.

The Government is still to pick from three options for its proposed Zero Carbon Bill - 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.

Victoria University climate change researchers Professor Dave Frame and Dr Adrian Macey argued only the second option would make for an effective Zero Carbon Act that was consistent with pledges already made under the Paris Agreement.

The agreement, which New Zealand ratified in 2016, committed countries to "holding the increase in the global average temperature to well below 2C above pre-industrial levels".


Frame and Macey suggested the best way of New Zealand reaching a zero-carbon point was by contributing no further warming, having struck a balance between emissions and sinks to absorb them.

But that couldn't happen, they said, without first making an accurate assessment of precisely where that balance lay, and then ensuring resources were properly allocated.

They argued there was a risk resources could be "seriously mis-aligned" if the Government continued to rely on a measure which they said failed to accurately account for methane emissions.

Called the Global Warming Potential 100, or GWP100, the measure was used to calculate how much heat a particular greenhouse gas trapped in the atmosphere over a period of 100 years.

Frame said it significantly underestimated the warming created by a permanent increase of one tonne of methane each year.

At the same time, it overestimated the level of warming associated with stable or falling methane emissions.

"We need to get emissions of CO2 to net zero to stop the warming - this simply isn't the case with methane emissions," Frame explained.

"As long as methane emissions are very nearly stable - or gently declining at about 0.4 per cent per annum - then there is essentially no warming associated with methane."


That result, he added, didn't turn on the level of methane emissions, either.

"Declining methane emissions cool the climate; whereas declining CO2 or N2O emissions continue to warm the climate," he said.

"That's a significant difference, and one that is not captured by GWP100.

"We need to get emissions of CO2 to net zero to stop the warming - this simply isn't the case with methane emissions," Professor Dave Frame says. Photo / File

"Its inaccuracies are especially apparent under strong mitigation policy: much of the recent work on metrics has been stimulated by people thinking about how to achieve the more stringent aspirational intention of the Paris Agreement to keep warming below 1.5C.

"The Zero Carbon Act is an opportunity to compare gases in a way that is better matched with the temperature targets we signed up to in Paris."

Frame and Macey concluded that the second option would offer the Government the best way to link up climate action with its goals, while solving the contentious debate surrounding agricultural methane emissions and providing a "solid basis" for domestic policy choices.

Climate Change Minister James Shaw couldn't comment on how or whether methane might be included in the Zero Carbon Act, as final policy decisions wouldn't be made until later this year.

"Regardless of what target we choose, shifting to low-emissions will require change across all parts of our economy," he said.

"There's no doubt this will be challenging for some communities and regions and they will need to be supported as part of a just transition." The Government wanted to make the transition planned, gradual and carefully phased in, so it was fair, inclusive and left no one behind, he said.

In other climate policy, Shaw yesterday released a discussion document outlining potential reforms to New Zealand's current linchpin legislation, the Emissions Trading Scheme (ETS).

The Government was considering capping the number of units in the system, and possibly raising the current upper limit of $25 per tonne of CO2, but has delayed deciding on whether agriculture should be pulled into the scheme until next year.

The farming sector opposed being included in the ETS and also backed a Zero Carbon Act that treated different gases separately, as Frame and Macey advocated.

Many others, including Greenpeace, Forest & Bird and some other researchers, have argued that all gases should be crunched to zero under the act. Climate Analytics, which ran the influential Climate Action Tracker, also favoured the third option, considering it the only one that would satisfy the Paris commitments.

Its co-founder Dr Bill Hare acknowledged there was no perfect way of comparing greenhouse gases, but felt to move away from the GWP100 system would be "immensely disruptive" as countries had already built their Paris pledges around it.

"[Attempting] to change this would be a massive waste of time in my judgment, and instead we need to focus on the next global stocktake due by 2023 from the scientific community to evaluate how this is going and work out what strong recommendations need to come from the scientific community, if any, to modify the system."