The permafrost carbon cycle, also called the Arctic carbon cycle, is part of the larger global carbon cycle. Permafrost is underground material that stays below 0°C (32°F) for at least two years in a row. Because permafrost remains frozen for long periods, it holds large amounts of carbon and other nutrients inside its frozen ground during that time. Permafrost is a major carbon reservoir, but it was often overlooked in early studies that measured global carbon storage on land. Since the year 2000, however, more attention has been given to this topic, with a significant increase in both public interest and scientific research.

The permafrost carbon cycle involves the movement of carbon from permafrost soil to plants, microbes, and the atmosphere. Some carbon returns to plants and eventually back to permafrost soil through burial and sedimentation caused by freezing processes. Some carbon moves to the ocean and other parts of the world through the global carbon cycle. This cycle includes the exchange of carbon dioxide and methane between land and the atmosphere. It also includes the movement of carbon between land and water in forms such as methane, dissolved organic carbon, dissolved inorganic carbon, particulate inorganic carbon, and particulate organic carbon.

Storage

Soils are the largest storage areas for carbon in land ecosystems. This is also true for Arctic soils that have permafrost beneath them. In 2003, Tarnocai and others used the Northern and Mid Latitudes Soil Database to calculate how much carbon is stored in cryosols—soils with permafrost within two meters of the surface. Permafrost-affected soils cover about 9% of Earth’s land area but hold between 25% and 50% of all soil organic carbon. These numbers show that permafrost soils are a major carbon storage area. These soils not only contain large amounts of carbon but also trap carbon through processes like cryoturbation and cryogenic activity.

Carbon is not created by permafrost. Instead, organic carbon from plants on land must enter the soil and then be trapped in permafrost to be stored. Because permafrost changes slowly with climate, carbon stored in it remains out of the atmosphere for long periods. Radiocarbon dating shows that carbon in permafrost is often thousands of years old. Two main processes are responsible for storing carbon in permafrost.

• The first process is syngenetic permafrost growth. This happens when the active layer of soil (the top layer that thaws seasonally) stays active, allowing energy and materials to move between the permafrost, active layer, plants, and atmosphere. This process causes the soil surface to rise over time. This rise is caused by wind-blown or water-deposited sediments and/or peat buildup. Peat forms at a rate of up to 0.5 mm per year, while sediment buildup can raise the soil surface by 0.7 mm per year. Thick layers of silt from wind-blown dust during the Last Glacial Maximum form carbon-rich soils called yedoma. As this happens, organic and mineral soil is trapped in the rising permafrost.

• The second process is cryoturbation, which mixes soil through freezing and thawing. This movement carries carbon from the surface deeper into the soil. Frost heaving is the most common type of cryoturbation. Eventually, carbon from the surface moves deep enough to be trapped in permafrost. When cryoturbation and sediment buildup work together, carbon storage increases.

It is estimated that the total organic carbon in northern permafrost regions is about 1,460 to 1,600 petagrams (Pg). Including the Tibetan Plateau, the total carbon in Northern Hemisphere permafrost is likely around 1,832 gigagrams (Gt). This amount of carbon stored in permafrost is more than double the carbon currently in the atmosphere.

Permafrost soil columns are usually divided into three layers: 0–30 cm, 0–100 cm, and 1–300 cm. The top layer (0–30 cm) holds about 200 Pg of organic carbon. The 0–100 cm layer holds an estimated 500 Pg, and the 0–300 cm layer holds about 1,024 Pg. These numbers are more than double previous estimates of carbon in permafrost. Additional carbon is stored in yedoma (400 Pg), carbon-rich dust deposits in Siberia and parts of North America, and deposits in Arctic river deltas (240 Pg). These deposits are deeper than the 3 meters studied in earlier research. Scientists are concerned about the large amount of carbon in permafrost because it was not included in climate models or global carbon calculations until recently.

Carbon release from the permafrost

Carbon moves continuously between soil, plants, and the air. As Earth's temperature rises, especially in the Arctic, permafrost (frozen ground) thaws more deeply, exposing old carbon that has been stored for decades to thousands of years. This carbon can then enter the atmosphere through natural processes. Scientists predict that the amount of permafrost in the top 3 meters of ground may decrease by about 25% for every 1 degree Celsius (1.8 degrees Fahrenheit) of global warming. According to the IPCC Sixth Assessment Report, there is strong evidence that rising temperatures in recent decades have caused permafrost to warm widely. For example, in parts of Northern Alaska, temperatures increased by up to 3 degrees Celsius (5.4 degrees Fahrenheit) between the early 1980s and mid-2000s, while in parts of Russia’s European North, temperatures rose by up to 2 degrees Celsius (3.6 degrees Fahrenheit) between 1970 and 2020. In the Arctic and high-elevation areas of Europe and Asia, the layer of ground that thaws each year has grown thicker since the 1990s. In Yukon, the area where permafrost is continuous may have shifted 100 kilometers (62 miles) toward the poles since 1899, though reliable records only exist for the past 30 years. Based on scientific models, understanding of natural processes, and evidence from Earth’s past, it is very likely that permafrost will shrink further as global temperatures rise.

When permafrost thaws, it releases carbon dioxide and methane, which contribute to warming, creating a cycle that speeds up further thaw. Warmer temperatures also increase rainfall in the Arctic, which can deepen permafrost thaw. The amount of carbon released depends on how deep the thaw is, the carbon content of the soil, changes in the environment, and the activity of microbes and plants. Microbial respiration is the main way old permafrost carbon enters the atmosphere. The speed of this process depends on factors like soil temperature, moisture, nutrients, and oxygen. In some permafrost soils, iron oxides can slow microbial activity and prevent carbon from moving into the air. However, this protection is temporary, as bacteria can break down the iron oxides over time. In certain soils, iron(III) oxide can increase the conversion of methane to carbon dioxide but may also boost methane production by specific microbes. These processes are still not fully understood.

Even though large amounts of carbon are stored in permafrost, it is unlikely that all of it will be released into the atmosphere. While temperatures will rise, not all permafrost will melt completely. Much of the ground with permafrost will stay frozen, even if thawing increases. Additionally, elements like iron and aluminum in soil can trap some carbon before it reaches the air, especially in layers of mineral sand above permafrost. Once permafrost thaws, it may not refreeze for centuries, even if temperatures drop again. This makes permafrost thaw one of the clearest examples of a tipping point in the climate system.

A study from 1993 found that the tundra was a carbon sink (absorbing more carbon than it released) until the late 1970s but became a net carbon source (releasing more carbon than it absorbed) by the time the study ended. In 2019, the Arctic Report Card estimated that permafrost releases 0.3 to 0.6 petagrams of carbon per year. Another study from the same year confirmed this estimate, noting that winter emissions of carbon dioxide from permafrost areas (October–April) are about 1,66 petagrams per year, while vegetation absorbs about 1 petagram of carbon during the growing season. Under a scenario of continued greenhouse gas emissions (RCP 8.5), winter carbon dioxide emissions from northern permafrost areas are expected to rise by 41% by 2100. Under a less extreme scenario (RCP 4.5), where emissions peak and then decline, emissions would increase by 17%. A 2022 study challenged these estimates, using atmospheric data from 1980 to 2017, and found that permafrost regions may be absorbing more carbon than previously thought, as models underestimated carbon uptake in permafrost areas and overestimated it in forests.

The impact of permafrost thaw on climate change depends not only on the amount of carbon released but also on whether it is released as carbon dioxide or methane. Aerobic respiration (with oxygen) produces carbon dioxide, while anaerobic respiration (without oxygen) produces methane. Methane is a powerful greenhouse gas, with a warming effect 80 times greater than carbon dioxide over 20 years and 28 to 40 times greater over 100 years, even though it stays in the atmosphere for less than 12 years. Most permafrost soils are oxygen-rich, so carbon dioxide emissions dominate. Some debate exists about whether emissions from permafrost come mainly from ancient carbon or from modern carbon (like leaf litter) due to warmer soils increasing microbial activity. Studies from the early 2020s suggest that microbes primarily use modern carbon during the growing season but switch to ancient carbon during winter.

Thawed permafrost areas often see more plant growth, as plants can grow deeper roots and absorb more carbon. This is a key counterbalance to carbon emissions. However, in areas near water, more leaf litter enters streams, increasing dissolved organic carbon. Warming also speeds up the release of carbon from permafrost soils through erosion along riverbanks and makes thawed areas more prone to wildfires. These fires burn stored organic carbon, remove protective soil layers, and expose the ground to more heat, increasing soil temperature and thaw depth. Changes in soil moisture also affect the balance between oxygen-rich and oxygen-poor decomposition.

Sergey Zimov proposed a hypothesis that the decline in large herbivore populations has altered the energy balance of the tundra, increasing the likelihood of permafrost thaw. He is testing this idea in an experiment at Pleistocene Park, a nature reserve in northeastern Russia.