Permafrost

Permafrost is ground that remains at or below 0 °C for at least two consecutive years. The definition rests on temperature alone: ice content, moisture, overlying snow and geographic location do not enter the test. Above the permanently frozen material lies a seasonally thawed active layer. The frozen ground may consist of soil, sand, sediment or rock, and its thickness ranges from less than 1 metre in some places to greater than 1,500 metres in others. Permafrost occurs both on land and beneath offshore Arctic continental shelves.

The term joins perma-, meaning permanent, with frost. The name overstates its subject: the oldest permafrost has been continuously frozen for only about 700,000 years. Scientists also avoid saying that thawing permafrost melts, because frozen soil that has thawed is still solid. Although temperature is the sole criterion, some permafrost carries more than 30 percent ice in lenses and layers, and ground ice can reach up to 90 percent of the volume in syngenetic permafrost deposits.

Permafrost is commonly divided into continuous and discontinuous zones, a classification that turns on how much of the ground remains frozen throughout the year. Lakes and rivers that do not freeze to the bottom interrupt the frozen ground and leave gaps within the continuous zone.

Thickness varies enormously, from under 1 m to more than 1,500 m. The deepest known sections lie in northern Siberia at 1,500 metres and in northern Alaska at 740 metres, and the frozen layer thins progressively toward the south. Permafrost is widespread in 85 percent of Alaska and 55 percent of Russia and Canada, and it probably underlies all of Antarctica. In the Northern Hemisphere it covers about 22 million km2, roughly 15% of the land surface. It underlies more than 50 percent of the unglacierized land north of 60 degrees north, while mountain permafrost accounts for roughly a quarter to three tenths of the global area.

Ground ice is classified relative to dry soil mass. When ice exceeds 250 percent of the dry soil mass it is termed massive ice, with beds at least 2 m thick and 10 m wide. Ice content can reach up to 90 percent of volume in syngenetic deposits, so the ice itself, not only the frozen soil, governs how the ground behaves when it thaws.

The thermal state of permafrost is now measured directly, and the record shows sustained change. In 2016 permafrost temperatures reached record highs across the Arctic, with the largest increases since 2000 recorded in the northern circumpolar region. Ground temperatures at 20 m depth on the North Slope tracked air temperature trends closely over the same period.

The active layer, the shallow horizon that thaws each summer, offers the most immediate signal. At 20 North Slope sites in 2016 average active-layer thickness was 0.52 m, 4 cm above the 21-year average and a 21-year maximum.

Warming of the ground translates into carbon loss, and the assessed scale is large. About 25% of the carbon held in the top 3 m volume is lost per 1 °C of warming. Of the total carbon stored in permafrost, 65 to 70 percent, or 1,035 plus or minus 150 Pg, lies in the surface 0 to 3 m layer alone. AR6 assesses the permafrost carbon dioxide feedback at 18 PgC per 1 C of warming by 2100 with a 3.1 to 41 PgC range, and a separate methane term of 2.8 PgC-equivalent per 1 C.

Confidence in the direction of the feedback is not matched by confidence in its details. AR6 concludes there is high confidence that thawing terrestrial permafrost will release carbon, but only low confidence on timing, magnitude and the split between carbon dioxide and methane. Abrupt thaw processes can contribute up to half of the total net greenhouse gas release from permafrost loss, with the rest from gradual thaw. Emissions from abrupt thaw across 2.5 million square kilometres could give a climate feedback as large as gradual thaw across the whole 18 million square kilometre permafrost region under the RCP8.5 pathway. After allowing for stabilisation, lake drainage and plant regrowth, models that consider only gradual thaw substantially underestimate permafrost carbon emissions.

Observations suggest the release may already be under way. Independent flux measurements suggest the permafrost region may already be releasing a net 0.3 to 0.6 Pg of carbon a year to the atmosphere. A pan-Arctic synthesis found carbon release in winter 2 to 3 times higher than ground-based estimates, enough for the whole region to be a source of about 0.6 Pg of carbon a year. Because deep anoxic sediments are missing from carbon feedback estimates, the impact of permafrost thaw may be more substantial than currently anticipated.

Across 70 permafrost and non-permafrost ecosystems, the annual carbon sink grew in non-permafrost sites but not in permafrost sites despite similar gains in summer uptake. That gap is attributed to non-growing-season carbon losses, and water and nutrient availability are expected to predict how these ecosystems respond to further warming.

A distinct methane pathway emerges from lakes. The new lake methane source added by the model, roughly 27 Tg a year by mid-century under the strong-warming pathway, is nearly triple the 10 Tg rise in global methane sources seen over the preceding decade.

Precipitation-driven slumps become rarer northwards while temperature-driven ones rise sharply. At Batagaika, described by USGS as a widening chasm and the world's largest permafrost crater, the feature has been tracked in satellite imagery from declassified photographs of the 1960s. NASA Earth Observatory describes it as a megaslump whose cliff faces top 85 metres in places, the biggest permafrost crater in the world, retreating backwards into the hillside. As melting debris comes loose at the crater floor, more face is exposed to the air, which speeds thaw, so the feature is expected to eat through the whole hillslope. The slump has disgorged Pleistocene animals since it began growing, likely in the early 1980s, including a steppe bison and a horse species, plus cave lion and wolf remains.

A pan-Arctic study from 2022 puts the cost of permafrost degradation to infrastructure at 182 billion dollars by mid-century under a moderate pathway and 276 billion under a high-emissions one. A 1.5 C rise in mean annual air temperature is estimated to be enough to trigger deformation of almost all foundations in Yakutsk. Nearly 70 percent of Arctic infrastructure is at high risk by mid-century, including 30 to 50 percent of critical infrastructure, with costs in the tens of billions. On the Qinghai-Tibet Plateau, 30 percent of 9,389 km of roads already show permafrost thaw damage, and about 60 percent of infrastructure would be at high risk by the end of the century. Holding warming at 1.5 C instead of 2 C sharply changes the outlook there: fewer than 20 percent of railways at high risk by 2100 at 1.5 C, against 60 percent at 2 C.

The thaw-vulnerable carbon held in the yedoma region totals about 211 Gt, a figure derived by applying field data and geospatial extrapolations to the northern permafrost deposits. That total divides into 83 Gt stored in yedoma deposits and 128 Gt in thermokarst deposits.

The scale of this reservoir becomes apparent when the fate of the carbon is considered. Winter carbon dioxide loss from permafrost regions could rise 41 percent over the next century if emissions continue at their current pace, or 17 percent if they are modestly reduced. Sue Natali of the Woods Hole Research Center, the lead author of the winter flux study, says winter loss may already be offsetting growing-season carbon uptake across the Arctic.

Thaw does not release carbon only gradually. Thaw slumps released about 0.002 Pg of carbon a year, roughly 0.2 percent of estimated annual gradual thaw emissions, so slumping is a small but measurable part of the picture.

Within that region the yedoma deposits account for 83 Gt and the thermokarst deposits for 128 Gt, the latter exceeding the former. Together they make up the approximately 211 Gt total assessed by Strauss and colleagues.

A deep-learning survey of ArcticDEM identified 2,747 active retrogressive thaw slumps across the Northern Hemisphere, which together lost 317 million cubic metres of ground over the preceding decade.

Those thaw slumps released about 0.002 Pg of carbon a year, roughly 0.2 percent of estimated annual gradual thaw emissions, so slumping is a small but measurable part of the picture.

The drivers of thaw slumping vary with latitude: precipitation-driven slumps become rarer northwards while temperature-driven ones rise sharply.

Projected Feedback and Model Gaps.

Thawing terrestrial permafrost is expected with high confidence to release carbon, yet the assessment assigns only low confidence to the timing, the magnitude and the division of that release between carbon dioxide and methane.

Landforms and Ground Ice Features.

The Batagaika depression is described by USGS as a widening chasm and the world's largest permafrost crater. NASA Earth Observatory characterizes the same feature as a megaslump whose cliff faces top 85 metres in places, the biggest permafrost crater in the world, retreating backwards into the hillside.

Since it began growing the slump has disgorged Pleistocene animals, among them a steppe bison and a horse species, together with cave lion and wolf remains. The crater has been tracked in satellite imagery from declassified photographs of the 1960s.

Yakutsk and Norilsk are the only two large cities built in continuous permafrost.

By 2022 up to 80 percent of buildings in some northern Russian cities had already suffered damage, and 45 percent of oil and gas facilities are believed to be at risk. On the Qinghai-Tibet Plateau 30 percent of 9,389 km of roads already show permafrost thaw damage, and about 60 percent of infrastructure would be at high risk by the end of the century.

Environmental Hazards and Deep Biology.

The hazards that permafrost poses are not confined to the slow deformation of thawing ground. In 2020 a diesel tank at a power plant in Norilsk collapsed, releasing 6,000 tonnes of fuel onto land and 15,000 into water, a spill that reached Lake Pyasino.

Permafrost is not sterile: a gram of frozen soil may hold between 1 and 1,000 million microorganisms, many of them unculturable bacteria and fungi. The depth of that preservation is shown by the revival in 2012 of Silene stenophylla by Russian researchers, working from 30,000-year-old tissue found in an Ice Age squirrel burrow in Siberian permafrost. That tissue is the oldest plant tissue ever revived.