Methane is 25 times more effective in trapping heat than carbon dioxide. But it decays in the atmosphere within about 12 and a half years, whereas carbon dioxide survives for thousands of years.
Submissions on the Zero Carbon Bill, which will establish a new 2050 emissions reduction target and also set up a Climate Change Commission, close on July 19.
The bill proposes three options for dealing with greenhouse gas emissions. Two of them propose a "split gas" approach that recognises the differences between gases.
This approach is supported by a number of commentators such as New Zealand's new Parliamentary Commissioner for the Environment, Simon Upton. He said forestry would be a suitable offset for continued methane emissions.
Victoria University's Professor Dave Frame and European scientists have also suggested putting methane in a different "basket" than carbon dioxide.
DairyNZ, Federated Farmers, Fonterra and other milk companies like the split gas approach. The agricultural sector is expected to support a proposal to reduce carbon dioxide to net zero, but stabilise methane - perhaps at a reduced level.
Methane may disappear rapidly from the atmosphere but it also causes global warming that continues for many decades, DairyNZ chief executive Dr Tim Mackle said.
"Stabilising emissions at current levels does not mean there is no contribution to any future warming."
Deciding what level methane should be reduced to requires additional work by Government and industry groups, he said.
Environment groups would prefer methane emissions to be reduced to net zero, like the other greenhouse gases. They point out that methane is a very potent greenhouse gas that causes ongoing damage, such as ocean warming and sea level rise.
At present farmers don't have to pay for methane or nitrous oxide emissions from their businesses, which means that taxpayers will end up paying if New Zealand cannot meet its pledge to reduce emissions.
Years of research have gone into finding ways to reduce methane emission from livestock farming. Small reductions can be made by changing what animals eat, breeding animals that produce less methane and suppressing the microbes in their guts that make the methane - but there has been no "silver bullet".
Former Parliamentary Commissioner for the Environment Jan Wright concluded that New Zealand farmers will have to find new ways to use rural land - especially using trees.
New Zealand is unusual in the world for being a developed country having nearly half its emissions coming from agriculture. The rest have carbon dioxide as the main greenhouse gas, from transport and energy use.
We produce only 0.15 per cent of the world's greenhouse gases.
But we are the fifth highest emitters per person in the world, after Australia, the United States, Canada and Luxembourg.
