Armed with an aeroplane loaded with sampling equipment, UK scientists have taken the most detailed measurements yet of the methane in the skies over tropical Africa. The data should help researchers to understand a mysterious spike in atmospheric concentrations of the powerful greenhouse gas, which began in 2007.
The flights started in Uganda in January and ended this week in Zambia. Researchers sampled methane emissions emanating from papyrus swamps, burning farm fields and flatulent livestock. Early results confirm that Africa is playing a major, yet poorly documented, role in the global methane cycle, with enormous consequences for the global climate.
In between dodging thunderstorms, the team found massive methane plumes rising above wetlands in both Uganda and Zambia. Researchers targeted this particular habitat because previous studies1 suggested that wetland microbes might be responsible for the methane spike. Project scientists were also able to measure the methane contained in smoke plumes emanating from agricultural fires around Lake Victoria and in northern Uganda.
“We’ve already learned a lot,” says Euan Nisbet, an earth scientist at Royal Holloway, University of London in Egham, UK, who leads a consortium of 17 research institutions involved in the campaigns. In 2016, with £5 million (US$6.5 million) in core funding from the UK Natural Environment Research Council in Swindon, the consortium started fieldwork in Africa and elsewhere, in addition to computer modelling work, to try to understand what is driving the global methane increase. The project is scheduled to wrap up in 2020.
Instruments on board the plane that the team flew over Uganda and Zambia collected data on the stew of gases and pollutants in the atmosphere. But the key to unravelling the mystery of the methane spike might be in the air samples that Nisbet’s team gathered. The researchers will return to Britain this week with hundreds of flasks and plastic bags filled with the samples taken from the aircraft and by teams on the ground.
Nisbet and his colleagues will be looking at the isotopic signature of the methane emissions contained in their samples. Bacteria that consume carbon and produce methane in wetlands, for instance, tend to take up more of the lighter carbon-12 isotope. Fossil-fuel operations tend to release methane that contains more of the heavier carbon-13 isotope. Methane produced in a fire falls somewhere in between.
Scientists plug these chemical fingerprints into computer models to analyse the global methane trends documented at dozens of air-sampling sites around the world. But data from the tropics, and from Africa in particular, is sparse, says Nisbet.
Globally, atmospheric methane concentrations have more than doubled since the pre-industrial era to around 1,860 parts per billion (p.p.b.). Methane levels remained fairly constant from 1999–2006, averaging 1,774 p.p.b. — but then they started rising again. Many researchers initially thought fossil-fuel emissions could be responsible, but the evidence from subsequent isotope studies2 suggests that atmospheric methane is getting lighter, not heavier.
“A lot of the extra emissions seem to come from the tropical region, and that points to a larger contribution from wetlands,” says Stefan Schwietzke, an atmospheric scientist who is now with the Environmental Defense Fund, an advocacy group based in New York City. He was the lead author of the study that suggested microbes in wetlands might be responsible for the spike in atmospheric methane.
Other studies have suggested that the methane spike could be due to increased emissions from agriculture in Southeast Asia3 or to a subtle shift in the rate at which methane breaks down in the atmosphere4,5. And work6 led by John Worden, an atmospheric physicist at the Jet Propulsion Laboratory in Pasadena, California, indicates that shifts in the amount of methane from sources including fires and fossil fuels could explain the increase, as well as changes in the isotopic signature of the greenhouse gas, in the atmosphere.
But the uncertainties in all these analyses are large enough that any of the proposed theories could be at least partially correct, Worden says.
With the isotopic signatures that Nisbet’s team will pull from their air samples from this and related field expeditions, modellers should be able to produce more-accurate estimates of global methane emissions, including in the tropics. This should help scientists in their efforts to understand what is happening globally and, ultimately, how climate change will affect the methane cycle in the future.
But Nisbet cautions that campaigns such as the Africa project provide snapshots in time, and partial ones at that. “We’re just scratching at the surface with this trip,” he says. “We’re going to have to go back again.”
Nature 566, 165-166 (2019)