Earth's methane emissions are rising and we don't know why
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Levels of a powerful greenhouse gas jumped again last year, continuing a surge in the past few years that researchers still cannot fully explain.
Atmospheric concentrations of methane climbed by 10.77 parts per billion in 2018, the second highest annual increase in the past two decades, according to provisional data released recently by US agency NOAA.
Methane is a shorter-lived but much more powerful greenhouse than carbon dioxide. The amount finding its way from human and natural sources, which can include everything from oil and gas wells to wetlands, has been rising since 2007. The rate has accelerated in the past four years.
Researchers warned earlier this year that if methane levels keep increasing at current rates then the Paris climate deal’s goals – of limiting global warming to 2°C and pursuing efforts to keep below 1.5°C – would be very difficult to meet.
Euan Nisbet of Royal Holloway University of London says researchers are very worried about the latest rise. Perhaps even more concerning is the fact no one is entirely sure what is driving the trend.
“The disturbing aspect is, we do not know which processes are responsible for methane increasing as rapidly as it is,” says Ed Dlugokencky of the US’s National Oceanic and Atmospheric Administration.
Keith Shine at Reading University echoes that view. “The fact that growth rates in the atmospheric concentrations of methane are approaching the levels we saw in the 1980s, after a period of relatively slow growth, is deeply concerning. The fact that we don’t understand the reasons for this surge deepen that concern.”
One possibility is that a warmer world is causing more methane to be released from wetlands in the tropics, fuelling even more warming. That would suggest a feedback loop is underway. “I’m not sure but it looks as if the warming is feeding the warming,” says Nisbet. More evidence is needed to prove the idea though.
Rebecca Fisher of Royal Holloway University of London says: “We still do not know whether the growth is primarily an increase in ‘natural’ emissions, such as from warmer or wetter wetlands, or increased anthropogenic emissions such as rice agriculture or fossil fuels.” It could also be a change in the atmospheric sinks of methane or, she says, most likely a combination of reasons.
The methane surge gains added significance from the fact researchers have been discovering in recent years that the gas has a more powerful warming effect than previously thought. In the first report by the UN climate science panel, in 1990, 21 tonnes of methane was considered to have the same global warming potential as one tonne of carbon dioxide. That was upgraded to 28 tonnes of methane in the most recent major report, and could rise as high as 35 tonnes in the next big assessment in 2022.
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