The permafrost carbon cycle, also called the Arctic carbon cycle, is part of the larger global carbon cycle. Permafrost is material below the ground that stays below 0°C (32°F) for at least two years. Because permafrost soil remains frozen for long periods, it holds large amounts of carbon and other nutrients inside its frozen structure during that time. Permafrost is a major carbon storage area, one that was often not considered much in early research about global land-based carbon storage. However, since the start of the 2000s, more attention has been given to this topic, with a large increase in both general interest and scientific studies about it.
The permafrost carbon cycle involves the movement of carbon from permafrost soil to plants and microorganisms on land, to the atmosphere, back to plants, and finally back to permafrost soil through burial and sedimentation caused by freezing processes. Some of this 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 areas and the atmosphere, as well as the movement of carbon between land and water in the forms of methane, dissolved organic carbon, dissolved inorganic carbon, particulate inorganic carbon, and particulate organic carbon.
Storage
Soils are the largest storage places for carbon in land ecosystems. This is also true for Arctic soils that contain permafrost. In 2003, Tarnocai and others used a database called the Northern and Mid Latitudes Soil Database to study carbon amounts 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 the soil’s organic carbon. These numbers show that permafrost soils are important storage places for carbon. These soils not only contain large amounts of carbon but also trap it through natural mixing processes and freezing-related actions.
Carbon is not created by permafrost. Instead, organic carbon from plants must enter the soil and then become part of permafrost to be stored. Because permafrost changes slowly with climate, carbon remains locked away 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.
It is estimated that the total soil organic carbon in northern permafrost regions is about 1,460–1,600 Pg (1 Pg = 1 Gt = 10 g). Including the Tibetan Plateau, the total carbon in Northern Hemisphere permafrost is likely around 1,832 Gt. This amount is more than double the carbon currently in the atmosphere.
Soil layers in permafrost are usually divided into three sections: 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 contains an estimated 500 Pg of organic carbon, and the 0–300 cm layer holds about 1,024 Pg of organic carbon. These numbers more than double previous estimates of carbon in permafrost soils. Additional carbon is found in yedoma (400 Pg), carbon-rich loess deposits in Siberia and parts of North America, and deltaic deposits (240 Pg) across the Arctic. These deposits are deeper than those studied in earlier research. Many concerns arise because of the large amount of carbon stored in permafrost soils. Until recently, climate models and global carbon budgets did not include the carbon found in permafrost.
Carbon release from the permafrost
Carbon moves between soil, plants, and the atmosphere as part of a natural cycle. As global temperatures rise in the Arctic, permafrost melts more, and the layer of soil that thaws each year becomes deeper. This exposes old carbon that has been stored for decades or even thousands of years, allowing it to enter the atmosphere through natural processes. Scientists predict that the amount of permafrost in the top 3 meters of ground could 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 global warming in recent decades has caused permafrost temperatures to rise significantly. 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. The layer of soil that thaws each year has grown thicker in the Arctic and at high elevations in Europe and Asia 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 and evidence from Earth’s history, it is very likely that permafrost will continue to shrink as the planet warms.
Carbon released from melting permafrost contributes to the warming that causes the melting, creating a cycle that worsens climate change. Warming also increases rainfall in the Arctic, which can deepen permafrost melting. The amount of carbon released depends on how deep the permafrost melts, 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 stop 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 it can also boost methane production by specific microbes. These processes are not yet fully understood.
Even though large amounts of carbon are stored in permafrost, it is unlikely that all of it will enter the atmosphere. While temperatures will rise, not all permafrost will melt completely. Much of the ground with permafrost will stay frozen, even if melting increases. Minerals like iron and aluminum in soil can trap some carbon before it reaches the air, especially in layers of sand above permafrost. Once permafrost thaws, it may not refreeze for centuries even if temperatures drop again, making it an example of a climate tipping point.
A 1993 study found that the tundra was absorbing carbon until the late 1970s but had become a net source of carbon by the time the study ended. The 2019 Arctic Report Card estimated that permafrost releases between 0.3 and 0.6 petagrams of carbon each year. A 2019 study confirmed the 0.6 petagram figure, based on the difference between winter carbon emissions (1.66 petagrams) and summer carbon absorption by plants (1 petagram). Under a scenario of high greenhouse gas emissions (RCP 8.5), winter carbon emissions from northern permafrost could increase by 41% by 2100. Under a scenario with slower emissions (RCP 4.5), emissions could rise by 17%. A 2022 study challenged these estimates, finding that permafrost regions may have been absorbing more carbon than previously thought, while forested areas may have released more carbon due to increased plant respiration.
Carbon from permafrost can be released as carbon dioxide or methane. Aerobic respiration produces carbon dioxide, while anaerobic respiration produces methane. Methane is a more powerful greenhouse gas than carbon dioxide, though it stays in the atmosphere for fewer years. Most permafrost soil is oxygen-rich, supporting aerobic respiration, so carbon dioxide emissions dominate. Some debate exists about whether emissions from permafrost come from ancient carbon or modern plant material, as warmer soils may increase microbial activity. Studies from the early 2020s suggest that microbes primarily use modern plant carbon during spring and summer 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 growth can reduce the amount of carbon released. However, in areas near water, more leaf litter enters waterways, increasing dissolved organic carbon. Warming also speeds up the release of carbon from soil and increases the risk of wildfires, which burn stored carbon and expose soil to more heat, further thawing permafrost. Changes in soil moisture affect how much carbon is broken down aerobically or anaerobically.
A hypothesis by Sergey Zimov suggests that the decline in large herbivores, like reindeer, has altered the balance of energy in the tundra, increasing permafrost melting. He is testing this idea in an experiment at Pleistocene Park, a nature reserve in northeastern Russia.