Arctic carbon cycle is being affected by anthropogenic climate warming and extreme events such as heat waves and fires in boreal and tundra regions. These changes have a direct or indirectly impacting the Arctic carbon cycle through melting sea ice and glaciers, increased land runoff, warming and thawing of permafrost, and coastal collapse. These processes lead to increased physical and biological degradation, changes in hydrological flows, and changes in sediment dynamics in the landscape.

Carbon stocks in the Arctic:

• The Arctic is a heterogeneous system with diverse landscapes and ecosystems, where organic carbon (OC) is found in long-term and short-term reservoirs or is in motion.
• The main reservoirs of OC include terrestrial sediments, marine sediments, and the water column.
• Permafrost soils contain 540 ± 10 Pg C, representing 62 % of the total SOC stock in the northern circumpolar region. Of this, 174 ± 3 Pg C (20 %) is found in the seasonally frozen active layer and approximately 292 ± 7 Pg C (19 %) in unfrozen and seasonally frozen soils in non-permafrost areas. Deep terrOC (terrestrial organic carbon) stocks in permafrost can also be substantial, for example, Pleistocene ice-rich permafrost contains 297–436 Pg C.
• It is estimated that approximately 21 Pg C is located in the top 30 cm of permafrost, directly below the active layer.
• The total hydrologically accessible SOC (SOCHA) is 487 ± 8 Pg, which consists of stocks in the active layer (174 ± 3 Pg), the upper 30 cm of permafrost (21 ± 0.4 Pg), and non-permafrost soils (292 ± 7.4 Pg).

Carbon release:

• Thawing permafrost leads to the release of previously frozen organic carbon, which can be degraded and mobilized.
• Thermokarst phenomena, in which land collapse occurs due to melting ice, lead to lateral transport of SOC into aquatic ecosystems.
• Coastal erosion is estimated to release 4.9–14 Tg C per year. Remotely sensed observations of coastal erosion indicate that carbon loss ranges between 2.03 and 2.74 Tg C per year.
• River systems transport significant amounts of DOC (dissolved organic carbon), POC (particulate organic carbon) and DIC (dissolved inorganic carbon) to the Arctic Ocean. Annual river flows are estimated at 39.5 (17–90) Tg DOC, 8.21 (1.5–37) Tg POC and 65.4 (23–102) Tg DIC.

Marine sediments:

• Marine sediments are key receptors and storage sites for terrOC (terrestrial organic carbon).
• The total OC stock in the upper 1 cm of sediments for the seven Arctic shelf seas is estimated to be 529 ± 107 Tg C.
• The updated estimate suggests that the OC pool in marine sediments is 82 ± 35 Pg C at a depth of 0–100 cm.

Water column:

• DOC represents the largest proportion of OC in the water column.
• Annual average DOC stocks are estimated to be 506 ± 14 Tg for the six high Arctic shelf seas.
• Annual net primary production (NPP) in the Arctic Ocean is estimated to be 241 (138, 422) Tg based on remote sensing and 691 ± 18 Tg based on NEMO-PISCES modeling.

Carbon balance:

• The Arctic Ocean is a strong carbon sink, but the northern circumpolar region is a weak sink with large uncertainties.
• Emissions from land and inland water disturbance are estimated to reduce the terrestrial carbon sink.

Uncertainties and research priorities:

• The main uncertainties include the amount of carbon in deep soils (> 3 m), the amount and distribution of ice in soil and permafrost peatlands.
• Priorities include mapping soil ice, peatlands and wetlands across the Arctic and including soil organic carbon and ice content in the Global Terrestrial Permafrost Network (GTN-P).
• There is a need to improve observational coverage and characterization of landscape parameters to support large-scale modeling and detection of changes in heterogeneous Arctic regions.
• A better understanding of Arctic tipping points, climate feedbacks, and their links to carbon source-to-sink transitions is essential.

Overall, the complexity of the Arctic carbon cycle requires further research and monitoring to reduce uncertainties and improve projections of future developments. Spring

Glossary of key terms

• Permafrost: Permafrost is ground that remains below 0°C for at least two consecutive years.
• Organic carbon (OC): Carbon compounds that come from plant or animal sources.
• Soil organic carbon (SOC): Organic carbon stored in soil.
• Dissolved organic carbon (DOC): Organic carbon that is dissolved in water.
• Solid organic carbon (POC): Organic carbon that is in the form of particles in water.
• Dissolved inorganic carbon (DIC): Inorganic forms of carbon dissolved in water, such as carbonate, bicarbonate, and carbon dioxide.
• Primary production (PP): The rate at which plants or other organisms produce organic matter through photosynthesis.
• Thermocrat: A landscape characterized by an uneven surface due to the thawing of ice-rich permafrost.
• Top-down estimates: Carbon flux estimates based on atmospheric measurements and inversion models.
• Bottom-up estimates: Carbon flux estimates based on ecosystem-level measurements and scaling methods.
• Ebullitive emissions: The release of gases such as methane from water systems through bubbles.
• Inflow shelves: Shelves in the Arctic Ocean that receive waters from lower latitudes.
• Outflow shelves: Shelves in the Arctic Ocean, where Arctic waters flow south into the subpolar oceans.
• Organic carbon accumulation in sediments (OC MAR): The rate of organic carbon accumulation in seabed sediments.
• CO2, CH4: Carbon dioxide and methane