Carbon levels are rising but things are starting to look a lot worse.
Climate scientists such as Columbia University’s James Hansen have long warned about “runaway” climate change – feedback loops where climate levers get pushed to the point where out planet enters a phase of unstoppable warming.
One of the most worrisome runaway warming scenarios involves the thawing Arctic permafrost. This causes microbes entombed in the frozen soil for millennia to begin releasing methane, a greenhouse gas with 20 times the warming power of carbon dioxide.
The thaw triggers a vicious cycle. The vented methane amps up the rate of warming. That, in turn, thaws more permafrost, triggering the release of more methane. Before we know it, the planet has left two degrees of warming in the dust.
Where the tipping point lies for runaway permafrost thaw is so uncertain that the Intergovernmental Panel on Climate Change doesn’t factor it into its reports. But new research shows we might reach it sooner than we think.
An American study in the Proceedings of the National Academy of Sciences in October showed that, once reawakened, the hungry microbes in permafrost can pump out greenhouse gases remarkably quickly. After only 200 hours of thawing almost half the carbon in a sample of 35,000-year-old Alaskan permafrost was released into the atmosphere.
And there’s a lot of patches to worry about. Permafrost accounts for 23 million square kilometres of the land surface inside and around the Arctic Circle. That’s around a quarter of the northern hemisphere’s landmass that is not under ice, including 85% of Alaska and around half of Canada and Russia.
Some permafrost regions are already emitting more carbon than they are absorbing.
Permafrost formed during the ice ages, when glaciers and ice sheets expanded and shrank, grinding the rock below into fine dust called glacial flour. Over tens of thousands of years plants and animals became part of the mix. Some permafrost patches are 1,500 metres thick. These vast tracts of frozen soil are thought to contain almost 1.7 trillion tonnes of carbon trapped within them – double the amount of carbon now in the atmosphere.
An April Nature review led by Northern Arizona University soil ecologist Ted Schuur calculated that almost a tenth of that carbon, 160 billion tonnes, might be released into the atmosphere from thawing permafrost between now and 2100. That first tranche of carbon could contribute up to a quarter of a degree of global warming on its own and “could have catastrophic global consequences”, says Max Holmes, a climate scientist at the Woods Hole Research Centre in Massachusetts – especially when humanity is already perilously close to pushing the planet beyond two degrees of warming.
It’s hard for the Intergovernmental Panel on Climate Change to factor permafrost into its climate models because the microbes that produce the greenhouse gas emissions are unpredictable.
The top, or active, permafrost layer thaws and re-freezes seasonally. The real trouble starts when heat seeps into the rock-hard layers below, which have been frozen for millennia. Like peas in your freezer the ensconced organic matter largely stays intact while it remains frozen. Ancient animals occasionally found in the permafrost are beautifully preserved, such as the 39,000-year-old Yuka woolly mammoth unearthed in Siberia in 2010 – complete with brain.
As these soils thaw and the cryogenically preserved microbes start to devour the plant and animal remnants around them, they release greenhouse gases including methane. But exactly what gases will be released and how much they will contribute to global warming is diabolically hard to predict.
For example, the type of gassy waste the microbes burp out depends on whether they are sitting in water. If they are dry the microbes have access to oxygen and emit carbon-dioxide. But if the microbes are smothered by water and oxygen-starved, methane-emitters or “methanogens” come to the fore.
Around 10% of the microbial population are methanogens, says Ben Woodcroft, a microbiologist at the University of Queensland who last year with colleagues identified a new species of methanogen in a patch of Swedish permafrost called the Stordalen Mire.
The amount of liquid water in the active layer also controls the microbes’ menu. In a 2014 Proceedings of the National Academy of Sciences paper, Florida State University geochemist Suzanne Hodgkins reported that when the active layer of Stordalen Mire is merely damp, that favours the growth of peat moss which is tough for microbes to break down. But when the active layer is very wet, it provides perfect conditions for grass-like sedges – the methanogens’ favourite food.
The permafrost also supports vast evergreen forests more than twice the size of the Amazon rainforest. They have made the Arctic a carbon sink, sucking in more carbon from the atmosphere than is released by the reawakened microbes. Global warming changes that equation. Schuur says some permafrost regions are already emitting more carbon than they’re absorbing – probably for the first time since the permafrost was formed.
“We are near that tipping point – and maybe over it already,” he says. “Arctic sea ice is shrinking. They know this because it’s been photographed since the 1970s. But we don’t know what the permafrost is doing. Do you think it’s been sitting there doing nothing the whole time?”
So how do we stop the vicious cycle? That’s the billion-dollar question, Woodcroft says. Measures we can take now include curbing fossil fuel use, keeping forests intact and limiting emissions of “black carbon” – sooty particles that darken snow and ice and absorb heat.
Ideally, climate scientists would like to model the rate at which the permafrost thaws, along with the carbon emissions it produces.
But it’s not that simple, Woodcroft says. Ecology can change completely within a couple of metres and new microbial effects, such as the heat-producers, are being uncovered all the time. The complex interactions in the Arctic environment muddy the waters.
“It’s actually really simple if you keep it frozen,” Woodcroft says. “But once you let it thaw, it becomes a lot more complicated.”
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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