Climate pessimists have long postulated an arctic global warming catastrophe scenario that goes like this: trillions of tonnes of methane are locked up in the arctic permafrost in the form of hydrates, also known as clathrates. As the planet slowly warms, this methane gradually bubbles out, slowly at first but then reaches a tipping point at which the hydrates destabilize and a vast amount—perhaps 50 trillion tonnes—is released in one great methane ‘belch’.
Releasing so much methane all at once jump-starts what has been, so far, a fairly slow and steady climate warming process. What makes this scenario particularly worrisome is that methane is a much more potent greenhouse gas than CO2 – between 20 and 100 times more effective at warming the planet’s surface depending on whose estimates you believe. The net result of this nightmare scenario, also known as the clathrate gun hypothesis, is a rapid acceleration in global warming along with all the consequent bad effects for humankind.
Methane hydrates are found in subsea deposits in many parts of the world’s oceans including shallower areas of the Arctic Ocean as well as onshore in permafrost, but until recently this ‘arctic methane timebomb’ hypothesis that there could exist a tipping point at which gradual warming might suddenly trigger a mass release was just that, still a hypothesis.
But four years ago, a joint US-Russian expedition observed fountains of methane bubbles rising from the seafloor of the East Siberian Arctic Shelf. Last summer, a Swedish expedition found a whole lot more, with plumes up to a kilometre wide and methane concentrations 100 times the background concentration. More recently they have been observed offshore Svalbard in the Norwegian arctic too.
If that wasn’t enough, two years ago mysterious craters began appearing in the permafrost of the Yamal Peninsula, also in the Russian arctic. Just one or two at first, but last month twenty more were found in a survey of satellite images. The most credible explanation so far is that they are sinkholes that have filled with methane released from the surrounding permafrost, until the pressure build-up makes them explode outwards.
So is the arctic methane timebomb prophecy becoming a reality? Are we on the verge of climate calamity?
Unlike the near-universal agreement that humanmade CO2 is responsible for global warming, scientific opinion is sharply divided over whether catastrophic release of methane from the arctic is likely or even possible.
Carolyn Ruppel of the USGS Gas Hydrates Project concluded a recent Nature review article on the subject by saying ‘catastrophic, widespread dissociation of methane gas hydrates will not be triggered by continued climate warming at contemporary rates (0.2oC per decade; IPCC 2007) over timescales of a few hundred years. Most of Earth’s gas hydrates occur at low saturations and in sediments at such great depths below the seafloor or onshore permafrost that they will barely be affected by warming over even 103 years’, and made it even clearer in a follow-up TV interview by saying bluntly that the clathrate gun hypothesis was ‘nearly impossible’.
Others have supported this view by pointing out that if hydrates are so unstable one would expect to find high concentrations of methane during past global warm periods in records of past atmospheric composition, such as polar ice cores – yet no such correlation has been consistently observed.
On the other hand, computer simulations of warming seafloor gas hydrates have demonstrated that they can undergo rapid dissociation over timescales as short as decades, and the recent observations from the East Siberian Arctic unquestionably do show that significantly increased amounts of methane are being released as the arctic warms, although it is estimated to be still only 0.5 million tonnes per year.
Such gradual releases of methane from melting ice both on-land and subsea are to be expected as the climate warms, however that’s missing the point. The doomsday scenarios require not a gradual release but a rapid, catastrophic one – and as much as the climate pessimists may wish for it, there isn’t much hard evidence for such a scenario … yet.
There is no question that the arctic will play a key role in the future climate change of the planet, in fact it’s already happening thanks to the rapidly shrinking summer pack ice cover and the resulting change in albedo, leading to a warming of the Arctic Ocean and hence changing weather patterns. The resulting ‘wavier’ jet stream is thought to be the cause of the recent spate of more severe winter weather events in North America. But the jury is still out on whether runaway methane release is likely to add to the problem too.