New modelling suggests estimates for methane’s role in global warming are on the low side. Richard A Lovett reports.
Scientists seeking to refine our understanding of global climate change have found that methane is a significantly stronger greenhouse gas than previously suspected. They have also discovered that it has particularly strong climate-warming effects on certain parts of the globe, including Australia’s deserts.
The finding evolved, in part, from data collected by astronomers studying Jupiter and Saturn’s moon Titan. And it comes, ironically, just days after US President Donald Trump’s latest move to reduce regulations on methane emissions from U.S. oil and gas fields.
Like carbon dioxide, methane has long been known to warm the atmosphere by absorbing infrared emissions from sun-warmed surfaces — emissions that would otherwise carry heat back into space.
And while there is much more carbon dioxide in the atmosphere than methane, the latter is a much more potent heat trapper. It is so potent, in fact, that increases in methane may have accounted for more than one-fifth of the Earth’s climate change over the past century.
And that estimate might actually be too low, because it is based solely on methane’s ability to trap thermal, or “longwave”, infrared radiation. Thanks to studies of Jupiter and Titan, which have substantial amounts of methane in their atmospheres, scientists know that the gas also absorbs “shortwave” radiation, including visible light and near-infrared.
That means that in addition to trapping thermal emissions radiated upward from the Earth’s surface, methane also absorbs sunlight.
The first attempt to take this into account was made in late 2016 by a team led by Keith Shine, a climate scientist at the University of Reading, UK. In a study in the journal Geophysical Research Letters, he and colleagues concluded that by overlooking shortwave heating, traditional estimates of methane’s planet-warming potential were 15% too low.
But Shine readily admits that this study was incomplete. “We knew there were a number of limitations and simplifications in our work,” he says.
Now, in a study published in the journal Science Advances, a second team has taken on the same project in greater detail.
“All of our work to date on methane is based on laboratory measurements,” says William Collins, a climate scientist at Lawrence Berkeley National Laboratory and professor at the University of California, Berkeley, US. “And those only include the near-infrared.”
In an effort to fill in the gaps, he says, “we went to the astronomy community and borrowed measurements of Jupiter and Titan”.
As it turns out, those refinements didn’t make a big difference – most of methane’s shortwave absorption was indeed in the near-infrared. But it was work that needed to be done just to make sure nothing important was being overlooked.
Collins’s team then took the work a step further by looking for variations in the amount by which methane’s shortwave absorption affects climate at various spots around the world.
What they found was surprising: some parts of the globe, such as the Sahara, the Arabian Peninsula, central Australia, Namibia, Central Asia, and the American southwest were far more strongly affected than others, in some cases by a factor of 10.
What these areas have in common, Collins says, is that they are deserts with bright, reflective surfaces.
“Sunlight that wasn’t absorbed on its way down through the atmosphere is reflected and can be absorbed on its way up,” he says. “That means there’s two chances for the light to be absorbed.”
Also, the air in these places is dry, a factor that increases the effectiveness of methane in absorbing shortwave radiation.
Putting it all together, Collins says, this means that climate change projections such as those from the Intergovernmental Panel on Climate Change (IPCC) have been underestimating the effects of methane.
That’s bad news for those worried about the future of the Earth’s climate.
But there’s a silver lining, Collins says, because it also means we’ve been underestimating the benefits of reducing methane emissions.
That’s important, Shine adds, because climate accords use a “multi-gas” framework, in which reductions of different greenhouse gases can be made on a carbon-dioxide-equivalent scale.
He compares the process to currency exchange rates. At present, he says, each kilogram reduction in methane emissions is valued as equivalent to 28 kilograms of carbon dioxide. Thanks to his and Collins’s papers, that number may go up to 32.
“And in fact,” he adds, “other improvements in understanding could push it even higher, to 35 or so.”
All in all, he says, that might lead to countries putting a greater emphasis on cutting methane emissions, something that is especially useful for nations with large agricultural economies, in which methane can form a large fraction of their total climate emissions.
Worldwide, in fact, agriculture is by far the leading source of methane emissions.
The second highest source is leakage from oil and gas production.
What the new studies mean, therefore, is that the Trump Administration’s much publicised September 18 decision to repeal an Obama-era rule designed to reduce methane emissions from oil and gas by an estimated 180,000 tons per year may have a bigger negative impact on climate change than was previously thought.