Wetlands were thought to be the culprit, as they were less well understood than other methane producers. Photo: Aiden Bichen
A New Zealand study has found more methane is likely entering the atmosphere than previously thought, potentially making it harder to limit global heating.
The research uses 33 years of measurements taken in Wellington and Antarctica and shows levels of chemical "scrubbers" that allow the atmosphere to self-clean of methane have been rising.
The finding may get scientists one step closer to understanding trends in methane, which is the second biggest planet-heater after carbon dioxide emissions and responsible for about 40 percent of today's warming.
Earlier studies had suggested the atmosphere might be losing its ability to rid itself of methane.
The new study published in Nature Communications uses long-running measurements of OH or the hydroxyl radical, tiny chemical scavengers which remove 90 percent of methane emissions from the atmosphere as well as removing other air pollutants.
Analysis of air samples collected since the 1980s revealed the atmosphere's self-cleaning ability had been strengthening in the Southern Hemisphere since about 1997, the authors said.
Although it is positive for the climate that the atmosphere's self-cleansing capacity hasn't been dropping, the finding also adds to evidence that methane emissions to the air are rising even faster than thought.
However the researchers have said more studies were needed to confirm the finding, given how hard hydroxyl was to track.
Methane concentrations have risen faster in the last five years than any time in the instrument record, making it harder to control global warming.
"Methane would have contributed to global warming even more had it not been for this strengthening of atmospheric cleaning capacity," says the study's lead author, climate and atmospheric scientist Olaf Morgenstern.
"This means methane is being lost more quickly at the end of our study period than the beginning," he said.
"Except methane has nonetheless gone up in abundance in the atmosphere ... so the sources of methane must be higher," Morgenstern said.
Olaf Morgenstern was the study's lead author. Photo: Supplied / Niwa / Dave Hansford
The most likely culprit was wetlands, said Martin Manning, a climate science veteran, former Director of the New Zealand Climate Change Research Institute at Victoria University and one of the scientists behind the long-running hydroxyl record used in the study.
The other major sources of methane - oil and gas fields, and agriculture - were comparatively well understood, he said.
However, wetlands scientists had long been saying official global methane estimates might be too low for wetlands emissions.
"If you look at all the budgets that have been put together, the biggest uncertainty is wetlands," Manning said.
"We are starting to resolve that, now we know hydroxyl has gone up."
"The people who said [methane emissions] were higher, were right," he said.
Understanding methane is crucial to predicting global warming.
Previous studies using satellite instruments suggested tropical wetlands, particularly in Congo, Southeast Asia and the Amazon and southern Brazil might be behind the fast acceleration of emissions.
Rising methane makes it harder to meet global temperature goals.
Methane is 80 times more powerful than carbon dioxide at trapping heat over a timespan of 20 years.
Unlike CO2, however, methane washes out of the atmosphere after about a decade, so it has less of a long-term impact.
More than 150 countries including New Zealand have pledged to deliver 30 percent cuts by 2030, tackling leaky oil and gas infrastructure and, to a lesser extent, agriculture.
But scientists have not observed a slowdown, and satellite data showed oil and gas field emissions were being under-reported.
Morgenstern said New Zealand and the world owed a debt to researchers including Manning, who kept collecting air samples for decades.
"Environmental monitoring is about having the stamina to keep going even with measurements that are superficially not being using immediately used. We've seen this before in other contexts where people just kept going and the measurements turned out to be gold, essentially," he said.
The latest study used a carbon-14-containing carbon monoxide molecule called 14CO to track hydroxyl levels, which Morgenstern said was only possible because of work by Manning and others.
"We used 33 years of measurements, and that makes these series by far the longest and most consistent 14CO series in the world, both held by New Zealand in a collaboration between NIWA and GNS. That's cool," Morgenstern said.
"We are very indebted to Martin Manning ... he was in the first generation of scientists who are now at retirement age who started this."
"I think it' among the most valuable datasets New Zealand has given to the world."
The study took thousands of hours of work by NIWA scientists and researchers from Victoria University, GNS Science, and a collaborator from Finland.
Four of seven NIWA scientists who contributed to the research - including Morgenstern- have left the institute after losing their jobs in cost-cutting, and now work overseas.
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