New Zealand may be making a bigger contribution to global warming than scientists thought.
A Nasa study says climate scientists have underestimated by 20 to 40 per cent how much methane warms the planet - even though it is already believed to be 25 times more potent than carbon dioxide.
The study, led by Nasa's Goddard Institute for Space Studies in New York, said methane blocked the creation of aerosols that would otherwise cool the planet - a new finding not counted in current estimates of global warming.
New Zealand is almost unique in the developed world because of its large proportion of methane emissions. The gas is released by farm animals, as well as landfills, crops and coal mines.
New Zealand scientists reacted cautiously to the new study, saying more work was needed to back it up.
A New Zealand author of the latest report of the Intergovernmental Panel on Climate Change, Martin Manning, said at least one more study showing the same thing would be needed before the panel would change its stance.
It was a "very short paper on a very complex topic" but it would open up debate, he said.
Pastoral Greenhouse Gas Research Consortium manager Mark Aspin said New Zealand would be looking to the IPCC for guidance, "given our [emissions] profile it is something we want to keep an eye on".
The study found methane and another pollutant, carbon monoxide, soaked up an atmospheric "scrubber" called hydroxyl that would otherwise join other substances to make cooling aerosols. The sulphate aerosols elbowed aside by methane cooled the earth by scattering light and affecting the clouds.
Standard estimates covering the next 100 years say that each tonne of methane will have 25 times the global warming effect of a tonne of carbon dioxide.
But methane's effects are short-lived compared with CO2, so the shorter the term, the greater weight methane is given in the equation.
Sustainability Council executive director Simon Terry, a climate change analyst and lobbyist, said New Zealand would not necessarily have to increase its emissions cuts if the study was right.
But if methane's potency was officially increased, it could affect the "exchange rate" for people wanting to offset methane emissions with carbon dioxide sinks.
That meant more carbon dioxide would need to be cut or stored in forests and elsewhere to offset each unit of methane, he said.
The study found methane also had a helpful effect.
The cooling aerosols it blocked were linked to acid rain and breathing problems.
In an article published on the Goddard Institute's website, the study's lead author said climate treaties such as the Kyoto Protocol underestimated the effect of methane and carbon monoxide because they measured greenhouse gases once they reached the atmosphere.
The institute study was a better measure, he said, because it looked at greenhouse gas emitted at the earth's surface.