Methane is Up. It’s Not from the Arctic. Where Then?

LA Times describes scientist’s general uneasiness about a rise in atmospheric methane. Problem is, no one knows where it’s coming from – and we’re pretty sure it’s not from the fabled Arctic “Methane Bomb”. (wrong isotopic signature)
In a postscript to my interview with Carolyn Ruppel of the US Geological Survey, Dr. Ruppel raised the issue of increased out put from wetlands as, with hydrological changes, increased rainfall in some areas, and sea level rise, wetland areas could increase their out put of methane – one of the primary suspects in the ongoing detective story.

LATimes:

This enigma involves methane, a potent greenhouse gas. Twenty years ago the level of methane in the atmosphere stopped increasing, giving humanity a bit of a break when it came to slowing climate change. But the concentration started rising again in 2007 — and it’s been picking up the pace over the last four years, according to new research.

Scientists haven’t figured out the cause, but they say one thing is clear: This surge could imperil the Paris climate accord. That’s because many scenarios for meeting its goal of keeping global warming “well below 2 degrees Celsius” assumed that methane would be falling by now, buying time to tackle the long-term challenge of reducing carbon dioxide emissions.

“I don’t want to run around and cry wolf all the time, but it is something that is very, very worrying,” said Euan Nisbet, an Earth scientist at Royal Holloway, University of London, and lead author of a recent study reporting that the growth of atmospheric methane is accelerating.

Methane is produced when dead stuff breaks down without much oxygen around. In nature, it seeps out of waterlogged wetlands, peat bogs and sediments. Forest fires produce some too.

These days, however, human activities churn out about half of all methane emissions. Leaks from fossil fuel operations are a big source, as is agriculture — particularly raising cattle, which produce methane in their guts. Even the heaps of waste that rot in landfills produce the gas.
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For 10,000 years, the concentration of methane in Earth’s atmosphere hovered below 750 parts per billion, or ppb. It began rising in the 19th century and continued to climb until the mid-1990s. Along the way, it caused up to one-third of the warming the planet has experienced since the onset of the Industrial Revolution.

Scientists thought that methane levels might have reached a new equilibrium when they plateaued around 1,775 ppb, and that efforts to cut emissions could soon reverse the historical trend.

“The hope was that methane would be starting on its trajectory downwards now,” said Matt Rigby, an atmospheric scientist at the University of Bristol in England. “But we’ve seen quite the opposite: It’s been growing steadily for over a decade.”

That growth accelerated in 2014, pushing methane levels up beyond 1,850 ppb. Experts have no idea why.

“It’s just such a confusing picture,” Rigby said. “Everyone’s puzzled. We’re just puzzled.”

Scientists have come up with various explanations. Could it be growing emissions from fossil fuels or agriculture? An uptick in methane production in wetlands? Changes in the rate at which methane reacts with other chemicals in the atmosphere?

Nisbet and his team examined whether any of these hypotheses synced up with the changing chemical signature of methane in the atmosphere.

Some molecules of methane weigh more than others, because some atoms of carbon and hydrogen are heavier than others. And lately, the average weight of methane in the atmosphere has been getting lighter.

That seems to implicate biological sources such as wetlands and livestock, which tend to produce light methane. Daniel Jacob, an atmospheric chemist at Harvard who was not involved in Nisbet’s study, said that explanation squares with his own research. His results suggest most of the additional methane comes from the tropics, which are home to vast wetlands and a large proportion of the world’s cattle.

Estimates of emissions from coal mines and oil and gas wells suggest that fossil fuel contributions are rising too, but those sources usually release heavier molecules of methane, which would seem to conflict with the atmospheric observations.

Some researchers have proposed a way to resolve this discrepancy. Fires create an even heavier version of methane, and agricultural burning — particularly in developing countries — appears to have decreased over the last decade. A drop in this source of ultra-heavy methane would make atmospheric methane lighter on the whole, potentially masking an increase in emissions from fossil fuels.

Finally, reactions that break down methane eliminate more of the lighter molecules than the heavier ones. If that process has slowed down — causing methane to build up in the atmosphere — it would leave more light gas behind, possibly helping explain the overall trend.

Nisbet and his colleagues concluded they can’t rule out any of these explanations yet. “They might all be happening,” he said.

One possibility is conspicuously missing from the list. Scientists have long feared that thawing Arctic sediments and soils could release huge amounts of methane, but so far there’s no evidence of that, said Ed Dlugokencky, an atmospheric chemist at the National Oceanic and Atmospheric Administration who worked on the study, which will be published in the journal Global Biogeochemical Cycles.

Nisbet said he fears the rising methane levels could be a sign of a dangerous cycle: Climate change may cause wetlands to expand and allow the environment to support more livestock, leading to even more methane emissions.

“It clearly seems as if the warming is feeding the warming,” he said. “It’s almost as if the planet changed gears.”