The Arctic is warming. several times faster than the rest of the globeThis Arctic warming is rapidly changing hydrometeorological conditions, with consequences for cold terrestrial and aquatic ecosystems. Permafrost, defined as soil or rock frozen for at least two consecutive years, covers approximately 14 million km²These permafrost-affected soils contain vast reserves of organic carbon, estimated at 1,100 to 1,500 Pg, which is about twice the amount in the pre-industrial atmosphere. In addition, subaquatic permafrost on the Arctic shelf floor stores additional 2,800 Pg of carbon.

Cold and frozen conditions in permafrost slow the decomposition of organic matter. As temperatures rise, this organic matter can be released into the atmosphere as CO₂ or CH₄, greenhouse gases amplifying climate changeAlthough net carbon fluxes from permafrost are currently not very significant, the current rate of warming and thawing is unprecedented in the past few thousand years, suggesting possible change in the future. The reformation of thawed permafrost carbon is an extremely slow process, making these changes virtually impossible. irreversible on human (multi-decadal) time scales. Therefore, the permafrost system is proposed as flip element in the Earth system. The tipping element is a component of the Earth system that is susceptible to a tipping point, which is a critical threshold beyond which a system reorganizes, often suddenly and/or irreversibly.

Thawing of permafrost can occur gradually, as deepening of the active layer, or locally, by sudden surface deformation (thermokarst) caused by the melting of excess ice in the soil. Thermokarst often leads to the formation of thermokarst lakes or retrograde melt slides (RTS). These sudden thawing processes, although not currently included in all models, can lead to significant greenhouse gas emissions and are considered to be local tipping elements within the permafrost system.

Arctic lakes, many of them thermokarst, play an important role in the carbon cycle, releasing CO₂ and CH₄ from decaying organic matter. Observations of changes in the lake suggest gradual rather than sudden transitionsOverall, the area of open water in the Arctic is both increasing and decreasing, with a tendency towards smaller total area, which signals regional drying. Time series analysis of lake water levels suggests that Lake hydrology is changing in permafrost regionsA sign of decreasing system resilience may be increased temporal autocorrelation lake water levels, which is a harbinger of a possible shift to another state. These observations are currently primarily available for larger, glacially formed lakes, while smaller thermokarst lakes require higher resolution satellite data.

Earth Observation (EO) is useful for monitoring ongoing changes in permafrost and freshwater systems, especially extreme events and disruptionswhich can be indicators of a possible turning pointExtreme events such as heat waves, floods and fires can trigger sudden thawing of permafrost. Space-based monitoring, despite current limitations in resolution and cloud cover, is improving with new satellites such as Copernicus Sentinel-1C. When analyzing changes in water surface from satellites, it is important separately assess lakes from floodplains.

Model simulations suggest that changes in permafrost hydrology may have significant effects on hydroclimate in the tropics and subtropics, affecting other potential tipping elements such as the Amazon rainforest or the vegetation of the Sahel. Although no clear thresholds or abrupt changes in climate response have been identified, neither gradual changes in climate may lead to nonlinear changes in CO₂ and CH₄ emissions due to nonlinearities in ecosystem responseAlthough the release of carbon from permafrost simulated by existing models is rather gradual, the existence multiple stable states in permafrost systems suggests a possibility local ecosystem and soil carbon stock tipping, albeit on centennial rather than decadal time scales. The question remains whether the Arctic will become drier or wetter in the future, and this change in moisture is important for hydrometeorological changes far beyond the Arctic.

In conclusion, although pan-Arctic carbon release appears to be gradual so far, there is the possibility of local switch points and mode changes, especially in lake hydrological systems. Continued monitoring using Earth Observation and improved models are key to understanding these dynamics and determining whether the permafrost system as a whole can achieve a nonlinear response. Spring

Article published in a magazine Surveys in Geophysics

Glossary of key terms

• Active layer: The layer of soil above permafrost that freezes and thaws seasonally each year.
• Arctic amplification: A phenomenon in which the Arctic is warming much faster than the global average.
• Biogeophysical feedbacks: Interactions between biological and physical components of the Earth system that can affect climate.
• Temporal autocorrelation (TAC): A measure of correlation between values in a time series with a time lag; used to identify early warning signals of impending regime changes.
• Irreversibility: A state in which the recovery of a system from a disturbed state due to natural processes is significantly slower than the time frame of interest.
• Permafrost: Ground (soil or rock) that remains frozen (at a temperature of 0°C or below) for at least two consecutive years.
• Tipping Point: A critical threshold beyond which a small change in an external force can lead to a large-scale, often sudden, and irreversible change in a system.
• Tipping Element: A major component of the Earth system that is susceptible to a tipping point.
• Retrograde thermal slides (RTS): A type of thermokarst formation characterized by slope failure caused by melting ground ice, resulting in a deforested scarped area.
• SAR (Synthetic Aperture Radar): A type of radar used in Earth observation that provides high-resolution images of the Earth's surface, unaffected by cloud cover.
• SWI (Superfine Water Index): An index derived from multispectral data to estimate the percentage of surface water coverage.
• Like this: A layer of unfrozen soil in a permafrost region that persists year-round; often found beneath lakes or rivers.
• Thermokarst: The process of melting permafrost leading to surface subsidence and the formation of irregular terrain, such as thermokarst lakes and depressions.
• Earth Observation (EO): The collection and analysis of information about the Earth using remote sensing, especially from satellites.