Sixth in a series. Read 1 | 2 | 3 | 4 | 5 | 6 | 7
Permafrost is releasing significant amounts of greenhouse gases, and feedbacks are under way, but the dynamics are not yet well enough understood to be able to judge whether tipping points have been reached or not. As previously noted (in part 1 of this series), University of NSW researchers point out that: “We do not know exactly how close we are to a tipping point, or even whether we have already passed it… There are tipping points that while not yet triggered may already be fully committed to.”
As permafrost thaws, soil microbes awaken and feast on the warming biomass, creating heat as they do so: a positive feedback that drives more defrosting. Russian permafrost scientist Trofim Maximov describes the global feedback: thawing permafrost releases greenhouse gases which cause warmer temperatures, melting the permafrost further: “It’s a natural process… which means that, unlike purely anthropogenic processes, once it starts, you can’t really stop it.”
A 2018 study estimated that stabilisation of the climate at 2°C may eventually result in release of 225–345 gigatonnes (GtC) of thawed permafrost carbon. That is equivalent to two-to-three decades of human emissions at the current rate. Some scientists consider that 1.5°C appears to be something of a "tipping point” for extensive permafrost thaw.
The US government's National Oceanic and Atmospheric Administration Produces an annual Arctic Report Card. In 2019, it concluded that permafrost ecosystems could be releasing as much as 1.1 to 2.2 billion tons of carbon dioxide per year. The Washington Post quoted Prof. Ted Schuur:
“These observations signify that the feedback to accelerating climate change may already be underway… Together [the studies] really paint the picture [that] we’ve turned this corner for Arctic carbon… Together they complement each other nicely and really in my mind are a smoking gun for this change already taking place.”
Like other cryosphere systems, permafrost emissions are not well incorporated into climate models, especially emissions from deep permafrost, a problem exacerbated by evidence of abrupt thawing. Unusually warm summers, such as the record-breaking 2020 heatwave in Siberia and Svalbard, are happening more often, causing Arctic permafrost to thaw in some northern regions almost a century earlier than some climate models projected. Subsea permafrost is not included in models.
Abrupt thaw could shift the entire northern hemisphere peatland carbon sink into a net source of warming, dominated by methane, lasting several centuries. Arctic wildfires rapidly expand the layer of permafrost subject to thawing, and these remote, uncontrolled blazes are projected to increase 130 to 350 percent by mid-century, releasing more and more permafrost carbon.
Evidence is emerging that sub-sea methane clathrates are being released over a large area of the continental slope off the East Siberian coast, but there is not yet sufficient evidence on which to assess the system dynamics of the process.
Prof. Merritt Turetsky says that “permafrost is thawing much more quickly than models have predicted, with unknown consequences for greenhouse-gas release. Across the Arctic and Boreal regions, permafrost is collapsing suddenly as pockets of ice within it melt. Instead of a few centimetres of soil thawing each year, several metres of soil can become destabilized within days or weeks… Around 20% of frozen lands have features that increase the likelihood of abrupt thawing… the impacts of thawing permafrost on Earth’s climate could be twice that expected from current models.”
Permafrost carbon emissions and the dangerous climate feedback loops they will set off are not accounted for in most Earth system models or integrated assessment models, including those which informed the IPCC’s special report on global warming of 1.5°C, nor are they fully accounted for in global emissions budgets.
If carbon-cycle feedbacks are accounted for, "such as tipping points in forest ecosystems and abrupt permafrost thaw, the estimated remaining budget could disappear altogether”.
In conclusion: are permafrost and methane clathrates the "carbon bomb" that could drive the the "Hothouse Earth" scenario? Yes. Do we know that the feedbacks have already driven these systems to a tipping point? No. But when risks are existential, focus must be given to the high-end possibilities, and what needs to be done to prevent them being realised.
