The Batagaika crater in eastern Russia was formed when land began to sink in the 1960s owing to thawing permafrost. Permafrost collapse is accelerating carbon release The sudden collapse of thawing soils in the Arctic might double the warming from greenhouse gases released from tundra, warn Merritt R. Turetsky and colleagues. his much is clear: the Arctic is warming the permafrost region hold twice as much to track these slow changes. fast, and frozen soils are starting to carbon as the atmosphere does — almost But models are ignoring an even more thaw, often for the first time in thou- 1,600 billion tonnes1. troubling problem. Frozen soil doesn’t just Tsands of years. But how this happens is as What fraction of that will decompose? Will lock up carbon — it physically holds the land- murky as the mud that oozes from permafrost it be released suddenly, or seep out slowly? We scape together. Across the Arctic and Boreal when ice melts. need to find out. regions, permafrost is collapsing suddenly as As the temperature of the ground rises Current models of greenhouse-gas pockets of ice within it melt. Instead of a few above freezing, microorganisms break down release and climate assume that perma- centimetres of soil thawing each year, several YURI KOZYREV/NOOR/EYEVINE organic matter in the soil. Greenhouse gases frost thaws gradually from the surface metres of soil can become destabilized within — including carbon dioxide, methane and downwards. Deeper layers of organic days or weeks. The land can sink and be inun- nitrous oxide — are released into the atmos- matter are exposed over decades or even dated by swelling lakes and wetlands. phere, accelerating global warming. Soils in centuries, and some models are beginning Abrupt thawing of permafrost is dramatic 32 | NATURE | VOL 569 | 2 MAY 2019 ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. COMMENT to watch. Returning to field sites in Alaska, for released with abrupt thawing? As a first step, often using data from oil exploration or road example, we often find that lands that were this year we synthesized results from pub- surveys. Researchers need to establish how forested a year ago are now covered with lished studies of abrupt thawing across the much permafrost carbon is displaced and lakes2. Rivers that once ran clear are thick permafrost zone. We asked how this type of what happens after it has thawed. For exam- with sediment. Hillsides can liquefy, some- thawing influences plants, soils and moisture ple, it is not known how much will stay in the times taking sensitive scientific equipment in the ground. The studies revealed patterns ground or be buried, and how much will enter with them. of collapse and recovery. This international the atmosphere as a greenhouse gas11,12. And This type of thawing is a serious problem project was supported by the Permafrost Car- what happens to this material if it flows into for communities living around the Arctic (see bon Network (www.permafrostcarbon.org), rivers, lakes and estuaries? ‘Arctic permafrost’). Roads buckle, houses part of the multimillion-dollar global Study Third, we need to identify the extent to become unstable. Access to traditional foods of Environmental Arctic Change (SEARCH). which plant growth will offset the carbon is changing, because it is becoming danger- Lakes and wetlands are a big part of the that is released by permafrost3. Over time, ous to travel across the land to hunt. Families problem because they release large amounts lakes are invaded by wetland plants, and cannot reach lines of game traps that have of methane, a greenhouse gas that is much eventually drain and convert back to tundra. supported them for generations. more potent than CO2 (ref. 5). Erosion from Eroded areas are colonized by plants, which In short, permafrost is thawing much more hills and mountains is also problematic: helps to stabilize soils and speed their quickly than models have predicted, with when hillsides thaw and break up, much CO2 recovery. Researchers need to monitor unknown consequences for greenhouse-gas is released as material is destabilized, decom- how thawed ecosystems evolve, the rate at release. Researchers urgently need to learn posed or washed into streams or rivers6. which vegetation stabilizes, and how these more about it. Here we outline how. We estimate that abrupt permafrost plants accumulate biomass. Vegetation also thawing in lowland lakes and wetlands, responds to rising CO2 and nutrients, longer TWICE THE PROBLEM together with that in upland hills, could growing seasons and changing levels of soil Permafrost is perennially frozen ground. It release between 60 billion and 100 billion moisture. Modellers will need to predict is composed of soil, rock or sediment, often tonnes of carbon by 2300. This is in addition changing feedbacks between ecological com- with large chunks of ice mixed in. About to the 200 billion tonnes of carbon expected munities and geomorphology as permafrost one-quarter of the land in the Northern to be released in landscapes transform. Hemisphere is frozen in this way. Carbon has other regions that “Understanding Fourth, the distribution of ice in the ground built up in these frozen soils over millennia will thaw gradually. abrupt is the main factor influencing the fate of per- because organic material from dead plants, Although abrupt thawing must mafrost carbon. Yet observations of ground animals and microbes has not broken down. permafrost thaw- be a research ice are sparse. More-widespread geophysical Modellers attempt to project how much ing will occur in less priority.” measurements could map pockets of ice of this carbon will be released when the than 20% of frozen below the surface, revealing where it con- permafrost thaws. It is complicated: for land, it increases permafrost carbon release centrates and how quickly it melts. Machine- example, they need to understand how much projections by about 50%. Gradual thaw- learning techniques might even be developed of the carbon in the air will be taken up by ing affects the surface of frozen ground and to predict where most ice is buried, by plants and returned to the soil, replenishing slowly penetrates downwards. Sudden col- analysing soils and topography at the surface. some of what was lost. Predictions suggest lapse releases more carbon per square metre that slow and steady thawing will release because it disrupts stockpiles deep in frozen NEXT STEPS around 200 billion tonnes of carbon over layers. To plug these knowledge gaps, we have five the next 300 years under a business-as- Furthermore, because abrupt thawing recommendations. usual warming scenario3. That’s equivalent releases more methane than gradual thawing to about 15% of all the soil carbon currently does, the climate impacts of the two processes Extend measurement technology. There stockpiled in the frozen north. will be similar7. So, together, the impacts of should be better tracking of permafrost But that could be a vast underestimate. thawing permafrost on Earth’s climate could and carbon across the Arctic, especially in Around 20% of frozen lands have features be twice that expected from current models. regions undergoing abrupt thawing. It is that increase the likelihood of abrupt thaw- Stabilizing the climate at 1.5 °C of important to establish baselines of perma- ing, such as large quantities of ice in the warming8 requires massive cuts in carbon frost and ecosystem change against which ground or unstable slopes2. Here perma- emissions from human activities; extra future measures can be compared. This will frost thaws quickly and erratically, triggering carbon emissions from a thawing Arctic require aircraft-based lidar (light detection landslides and rapid erosion. Forests can make that even more urgent. and ranging, a surveying technique that uses be flooded, killing large areas of trees. pulsed laser light), drone-based surveys and Lakes that have existed for generations can RESEARCH GAPS better algorithms for image analysis. disappear, or their waters can be diverted. Our estimates are rough and need refining. Worse, the most unstable regions also tend However, they show that understanding Fund monitoring sites. River chemistry can to be the most carbon-rich2. For example, abrupt thawing must be a research priority. be a sensitive indicator of abrupt thawing, but 1 million square kilometres of Siberia, Can- First, climate and soil scientists need to find many monitoring stations are being aban- ada and Alaska contain pockets of Yedoma out where the greatest emissions of methane doned13. Instead, there should be increased — thick deposits of permafrost from the and CO2 will come from. Although we have national and international investment in last ice age4. These deposits are often 90% a good idea of current numbers of thaw lakes long-term sites that link land-based obser- ice, making them extremely vulnerable to and wetlands9, and how many existed in the vations with aquatic and marine measure- warming. Moreover, because of the glacial past10, we need to be able to project where new ments. Better recordings of organic matter dust and grasslands that were folded in ones will appear. We also need to know how and nutrients in rivers would shed light on when the deposits formed, Yedoma contains quickly they will drain as the climate warms. how permafrost plant and microbial commu- 130 billion tonnes of organic carbon — the Second, the erosion of thawed soils on hill- nities respond to abrupt and gradual thawing. equivalent of more than a decade of global sides is poorly understood. Because collapsing human greenhouse-gas emissions. slopes are hard to detect using satellites, only Gather more data. Regions that are vul- How much permafrost carbon might be a few large-scale studies have been done, nerable to abrupt thawing need more ©2019 Spri nger Nature Li mited.