It was probably more hope than expectation, but in the early years of the 21st century, it looked as if atmospheric concentrations of the hyper-greenhouse gas, methane had pretty much stabilised. This was good news as the gas has the capability of sending planetary heating into overdrive. In the short term – say a decade or two – methane is capable of warming the planet up to 86 times more rapidly than carbon dioxide. The gas doesn’t hang around in the atmosphere for much more than ten years or so, but then it breaks down into carbon dioxide and water – both greenhouses – which means that its warming influence continues. Even after 100 years, in fact, the global warming potential of the gas is still more than 30 times that of carbon dioxide.
Now, both the hope and expectation seem short-sighted as new research reveals that methane levels in the atmosphere are on the rise again. A new open access paper published by the American Geophysical Union (1) provides evidence for atmospheric methane levels starting to climb once more in 2007 and accelerate significantly for the period 2014 – 17. Such a hike is unexpected and was not factored into the calculations that came up with the emissions reductions framework for the Paris Climate Agreement. Consequently, the probability that global average temperatures will rise far above the 2°C dangerous climate change guard rail is now even greater.
A big concern is that it is not clear where the methane is coming from. There seems to have been an especially significant increase in the gas across the tropics and sub-tropics and at northern mid-latitudes, and more intensive farming and the warming of methane-hosting swamps and bogs have been fingered as possible culprits. Far more worrying is the possibility that chemical changes in the atmosphere, as it warms, might make it more difficult to break down methane. If true, this would be very bad news indeed, because it would mean that this extremely potent greenhouse gas would hang around for longer, thereby significantly increasing its global warming potential.
And there could be plenty more methane to come. Trapped beneath the vast tracts of permafrost at high latitudes are colossal quantities of the gas. The geographic region of most concern is probably the submarine permafrost that floors the East Siberian Continental Shelf, where an estimated 1400 billion tonnes of carbon, in the form of methane, is lurking beneath a frozen carapace that is thawing rapidly. According to one research team as much as 50 billion tonnes of this is available for sudden release at any time, which would – at a stroke – hike the methane content of the atmosphere 12 times. A discrete methane ‘burp’ on this scale could, it has been estimated, advance global warming by 30 years and cost the global economy USD60 trillion – a figure close to four times the US national debt. The occurrence of such an outburst is far from certain and there are other issues to consider, including how much methane is absorbed by the ocean as it bubbles upwards. Nonetheless, this cataclysmic scenario provides yet another reason – if more were needed – why we must slash our own emissions to zero as soon as we can.
(1) Very strong atmospheric methane growth in the four years 2014‐2017: Implications for the Paris Agreement
Bill McGuire is Professor Emeritus of Geophysical & Climate Hazards at UCL and author of Waking the Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanic Eruptions. He was a contributor to the IPCC 2012 report on Climate Change & Extreme Events and Disasters.