Frozen methane bubbles in Abraham Lake, Alberta, Canada
Scientists love a good mystery. But it’s more fun when the future of humanity isn’t at stake.
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.
Agriculture, including growing rice in flooded fields, is one of the biggest sources of human-caused methane emissions.
The atmosphere contains far less methane than carbon dioxide, which is the primary driver of climate change. But methane is so good at trapping heat that one ton of the gas causes 32 times as much warming as one ton of CO2 over the course of a century.
Molecule for molecule, methane “packs a bigger punch,” said Debra Wunch, an atmospheric physicist at the University of Toronto.
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.”
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