A dive deep into 27,000 years worth of muck piled up on the bottom of the Arctic Ocean has spurred researchers to renew warnings about a potential surge of greenhouse gas emissions from thawing permafrost.
By tracking chemical and organic fingerprints in long-buried layers of sediments remaining from previously frozen ground, the scientists showed that ancient phases of rapid warming in the Arctic, such as occurred near the end of the last ice age, released carbon on a massive scale. Vast frozen landscapes collapsed, turned to mud and flowed into the sea, releasing carbon dioxide and methane into the atmosphere along the way.
The study, published today in Science Advances, shows that only a few degrees of warming in the Arctic is enough "to abruptly activate large-scale permafrost thawing," suggesting a "sensitive trigger" for greenhouse gas emissions from thawing permafrost. The results also support climate models that have shown "large injections of CO2 into the atmosphere" when glaciers, and the frozen lands beneath them, melted.
The vast permafrost regions of the Arctic hold more carbon than the Earth's atmosphere, and a rapid and a large release of CO2 that exceeds current projections would dangerously accelerate global warming. Understanding how that happened in the past helps researchers make more accurate projections for the future. In the Northern Hemisphere, permafrost spreads over about 9 million square miles—in total, an area nearly as big as the United States, China and Canada combined.
One of the three warming phases the researchers studied was about 14,700 years ago, when temperatures in the Arctic suddenly warmed by 1.8 degrees Fahrenheit, said lead author Jannik Martens, an Arctic researcher with the University of Stockholm. It was such a distinct climate disruption that scientists named it the Bölling-Alleröd warming for two sites where distinctive soil layers show the ancient warming.
"If we consider the magnitude and the speed of anthropogenic climate warming, by 1 degree Celsius (1.8 Fahrenheit) globally and 2 degrees Celsius (3.6 Fahrenheit) in the Arctic, during the past 150 years, and compare this with the first abrupt temperature increase of about 1 degree Celsius at the Bölling-Alleröd, it appears likely that large-scale permafrost thawing and carbon release is going to happen again," he said. "Our study indeed suggests that abrupt permafrost thawing represents a tipping point in the climate system."
Even with the new research, it's still hard to say exactly where that tipping point is, even though warming is already penetrating the upper layers of the frozen soils fast and cliffs of coastal permafrost are collapsing at an accelerating rate. There are still a lot of uncertainties, including how much carbon could be recaptured by expanding peatlands, which can also sequester huge quantities of greenhouse gases, and how much will be absorbed by the greening of Arctic tundra as shrubs and trees move north in the rapidly warming region.
"The same uncertainties apply to our study," he said of the research into the ancient tipping point. "We can't resolve the effect of the northward shift of biomes at the end of the last ice age, or the large growth of peatlands ... both effects took up large amounts of carbon, while permafrost thawing released carbon at the same time."
Extrapolating the new research to the vast regions where permafrost is prevalent presents other uncertainties. The new study looked only at ocean-bottom sediment cores from a limited area of Siberia, so they don't necessarily show regional variations in permafrost response to warming.
That challenge, to some degree, has been resolved by other recent studies showing how permafrost is thawing at the global scale, and also how some parts of the Arctic are increasingly vulnerable to sudden and catastrophic permafrost collapse. The new research is yet more evidence that the amplified warming in the Arctic can release carbon at a massive scale, said UAF permafrost researcher Vladimir Romanovsky, who was not involved in the research.
"It's a very important paper. They did a really good job of looking at the fate of terrestrial carbon, and showing that huge amounts of terrestrial carbon were released," he said. Chemical analysis of the materials in the sediments using the latest instruments and methods and matched against other climate records like gas bubbles trapped in old ice helped paint a clearer picture of how much CO2 can be released. "That's what they calculated really well," he added.
Key to the Carbon Cycle?
The new study "supports the idea that permafrost is an important and significant source of carbon," Romanovsky said, alluding to ongoing research about the relative regional roles of permafrost and the oceans in the carbon cycle. "That permafrost is the main source, or one of the main sources. That's what they are trying to prove."
Along with increasing the amount of greenhouse gases in the atmosphere, thawing permafrost also has immediate effects on people living in the Arctic. Indigenous communities have been hardest hit; when frozen ground slumps and caves in, infrastructure they depend on fails. Roads collapse and power lines fall.
"It's a little scary because it's happening under our feet," said Romanovsky, who, while in Fairbanks this summer, observed signs that even permafrost that has survived thousands of years is starting to thaw.
If Alaska continues warming at the same pace as the last five years, there will be widespread thawing of ancient permafrost, he warned. "We're very, very close, and when it does, a lot of dramatic things will happen to landscapes and infrastructure, including roads and oil and gas developments. We will see huge changes."
The new research gives more detailed information of what happens to the carbon locked into permafrost when it thaws, said Merritt Turetsky, director of the Institute of Arctic and Alpine Research at the University of Colorado, Boulder.
"What is not clear to me is whether we fully understand how past cycles of permafrost thaw can be used to infer changes observed today," said Turetsky, who was not part of the study.
Today's permafrost may hold more organic carbon that could be set free than in the past, but on the other hand, she said, the historic record shows that there were more hotspots in the past that released methane—a far more potent greenhouse gas than carbon dioxide, so more study is needed to understand how the different processes interact and affect greenhouse gas releases.
But human-caused global warming has already triggered thawing that may be hard to stop, even if the climate cools.
"There is momentum in the climate system and also in permafrost thaw trajectories," she said. "Once permafrost begins to thaw in some settings, a number of self-organizing feedbacks kick in and that means that thaw may continue to occur even if the climate cooled again. We need to aim as a society to keep permafrost carbon in the ground and out of the atmosphere by keeping it frozen," she said.