DATE: May 13, 2019
SNIP: Every week, dozens of metal flasks arrive at NOAA’s Earth System Research Laboratory in Boulder, Colorado, each one loaded with air from a distant corner of the world. Research chemist Ed Dlugokencky and his colleagues in the Global Monitoring Division catalog the canisters, and then use a series of high-precision tools — a gas chromatograph, a flame ionization detector, sophisticated software — to measure how much carbon dioxide, nitrous oxide, and methane each flask contains.
The air in the flasks shows that the concentration of methane in the atmosphere had been steadily rising since 1983, before levelling off around 2000. “And then, boom, look at how it changes here,” Dlugokencky says, pointing at a graph on his computer screen. “This is really an abrupt change in the global methane budget, starting around 2007.”
The amount of methane in the atmosphere has been increasing ever since. And nobody really knows why. What’s more, no one saw it coming. Methane levels have been climbing more steeply than climate experts anticipated, to a degree “so unexpected that it was not considered in pathway models preparatory to the Paris Agreement,” as Dlugokencky and several co-authors noted in a recently published paper.
As the years plod on and the methane piles up, solving this mystery has taken on increasing urgency. Over a 20-year-time frame, methane traps 86 times as much heat in the atmosphere as carbon dioxide. It is responsible for about a quarter of total atmospheric warming to date. And while the steady increases in atmospheric carbon dioxide and nitrous oxide are deeply worrying, they are at least conforming to scientists’ expectations. Methane is not. Methane — arguably humanity’s earliest signature on the climate — is the wild card.
Thanks to the careful measurements of NOAA scientists and others, we know that there are about 1,850 molecules of methane in the atmosphere for every billion molecules of air — typically shorthanded as parts per billion, or ppb — in today’s atmosphere. That’s compared to about 700 parts per billion in the pre-industrial era.
Getting answers is not simply an academic exercise; it’s crucial to knowing just what humanity might be facing as the planet continues to warm. “We need to have process representation to understand these mechanisms,” says Eric Kort, an atmospheric scientist at the University of Michigan, “so we can say, for example, with certain changes to temperature and the hydrological cycle, we’d expect methane emissions to increase by X amount.” Without that understanding, Kort suggests, we’re unable to answer some important questions about what looms ahead. “Is atmospheric methane increasing as a consequence of climate change, not of our direct emissions? Are some thresholds being passed?”
The first theory to gain traction pinned the blame on fossil fuels, based on some suspicious timing: The use of horizontal drilling and hydraulic fracturing — a method of harvesting buried hydrocarbons that involves blasting deep layers of rock with a cocktail of water, sand, and chemicals — surged in the U.S. oil and gas industry right around the time atmospheric methane levels shot up. Other scientists, however, are convinced that growing herds of livestock, which produce methane-rich belches and manure — are to blame. Some researchers pore over satellite data for evidence that methane production from natural sources, such as wetlands and wildfires, is changing.
And still others argue that the culprit isn’t a surging source at all, but the steady, or perhaps very sudden disappearance of a traditional methane “sink.” After an average residence time of about a decade, methane is oxidized into carbon dioxide and water vapor through chemical reactions with hydroxyl radical (OH). This atmospheric removal process may be weakening, though, possibly because OH levels are declining due to reactions with other anthropogenic pollutants.