Arctic Permafrost Carbon Feedback

Arctic and sub-Arctic permafrost holds ~1,700 Gt of frozen organic carbon — twice the carbon currently in the atmosphere. As the Arctic warms 2–3× faster than the global mean, this carbon is thawing and releasing CO₂ and CH₄. This feedback is absent from IPCC AR6 central projections and could add 0.2–0.4 °C of additional warming by 2100 on top of modelled trajectories.

1,700 Gt C
Total permafrost carbon stock
Schuur et al. 2022; ~2× atmospheric CO₂
2.5×
Arctic warming amplification
vs global mean; IPCC AR6 WG1
+0.2–0.4 °C
Additional warming by 2100
Not in AR6 central projections (Gasser 2018)
~2×
Abrupt thaw multiplier
Thermokarst doubles gradual estimates (Turetsky 2020)
20%
Released as CH₄
Anaerobic decomposition in wetlands / lakes; GWP100 = 28×
5%
Labile top-layer carbon
~85 Gt C mobilisable on century timescales
Uncertainty notice: Annual flux estimates carry ±50–100% uncertainty. The Koven et al. sensitivity (37 Gt C / °C) has a published range of 30–100 Gt C / °C. Abrupt thaw (thermokarst, retrogressive slumps) is poorly represented in global models. All projections here are scenario guidance for policy-relevance, not validated Earth System Model output. The IPCC AR6 central range for permafrost additional warming is +0.1–0.5 °C under SSP2-4.5.
Overview
Annual Flux
Cumulative Release
Additional Warming
Mechanisms
Timeline
Scientific Context
The missing feedback: Every IPCC AR6 scenario that hits 1.5 °C or 2 °C assumes a certain residual budget. But permafrost carbon release adds GHGs on top of human emissions — and the models used to set those budgets mostly don't include it. Effectively, the IPCC budgets are generous by 40–100 Gt CO₂e under moderate warming pathways.

Annual CO₂e Flux by Scenario (2024–2100)

Low Warming Moderate High Abrupt Thaw

Cumulative CO₂e Released (Gt) by 2100

Scenario Summary

Scenario 2050 cum. (Gt CO₂e) 2100 cum. (Gt CO₂e) Peak flux (Mt/yr) Add. warming 2100 (°C) Tipping threshold yr
Low Warming 42.529 114.193 1705.1 (2040) +0.188 —
Moderate Warming 61.272 202.941 3280.2 (2060) +0.335 2099
High Warming 78.343 252.052 4535.4 (2058) +0.416 2080
Abrupt Thaw 148.644 305.409 7297.7 (2047) +0.504 2058
CO₂ vs CH₄: About 80% of released carbon exits as CO₂ from aerobic (surface) decomposition. The remaining ~20% is released as CH₄ from anaerobic conditions in thermokarst lakes and water-saturated wetlands. At GWP₁₀₀ = 28, CH₄ accounts for a disproportionate share of near-term CO₂e flux.

Annual CO₂ Flux by Scenario (Mt/yr)

Annual CH₄ as CO₂e by Scenario (Mt CO₂e/yr)

Total Annual CO₂e Flux (all scenarios, Mt/yr)

The 200 Gt threshold: CE uses 200 Gt cumulative CO₂e as a heuristic tipping indicator — roughly the point at which permafrost feedback begins to meaningfully alter the remaining 1.5 °C budget. This is not a formal tipping point; it is a policy-relevance marker.

Cumulative Permafrost CO₂e Release (Gt CO₂e)

% of Total 1,700 Gt C Stock Released

Even the high-warming scenario releases less than 10% of the total stock by 2100 — but this is a slow-moving feedback that accelerates over centuries. The concern is not a one-time pulse but a persistent source term that outlasts any net-zero transition.

Context: Permafrost Release vs Human CO₂ Budget

QuantityGt CO₂eNote
Remaining 1.5 °C budget (66% prob)~400IPCC AR6, as of 2024
Remaining 2.0 °C budget~1,150IPCC AR6, as of 2024
Permafrost by 2100 — Low~114.193This model
Permafrost by 2100 — Moderate~202.941This model
Permafrost by 2100 — High~252.052This model
Permafrost by 2100 — Abrupt~305.409This model
How to read this: The "additional warming" curves show how much extra global temperature rise the permafrost feedback itself causes, over and above the scenario's assumed warming trajectory. A +0.3 °C permafrost contribution means a scenario targeting 2.0 °C effectively delivers 2.3 °C — pushing past critical thresholds.

Additional Warming from Permafrost Feedback (°C)

Global Temperature Trajectory per Scenario (°C above pre-industrial)

Gradual Active-Layer Deepening

The active layer is the seasonal thaw zone above perennially frozen ground. It deepens ~0.3–0.5 cm per year currently, but accelerates with warming. As organic material above the permafrost table is exposed to microbial decomposition, CO₂ is released aerobically.

This is the "baseline" thaw mode — slow, geographically widespread, well-characterised. It underlies the Koven et al. sensitivity of ~37 Gt C per °C of sustained warming.

Thermokarst (Abrupt Thaw)

Ice wedges and massive ground ice, when melted, cause ground subsidence and collapse — forming thermokarst lakes, retrogressive thaw slumps, and boggy depressions. These expose deep, previously stable carbon to rapid anaerobic decomposition.

Turetsky et al. 2020 showed thermokarst affects ~20% of permafrost area but contributes ~40–50% of total carbon release. It is rapid (years to decades, not centuries) and largely unrepresented in global models.

Yedoma — The Deep Frozen Reserve

Yedoma is exceptionally ice-rich (40–80% ice by volume) Pleistocene-age loess deposit found across Siberia and Alaska. It contains ~130 Gt C in particularly labile form — 10× more reactive than typical permafrost carbon.

Yedoma thaw events (e.g. the 2020 Siberian heat wave, +38 °C) can release CH₄ bubbles from lake bottoms ("ebullition"). These point sources are intermittent but potentially very large.

Self-Reinforcing Feedback Loop

Permafrost methane warms the Arctic faster than the global mean (amplification factor ~2.5×). Faster Arctic warming → deeper active layer → more carbon release → more warming. This creates a self-reinforcing loop that, once triggered, cannot be halted by cutting human emissions alone.

The loop accelerates when: (1) warming exceeds ~2 °C globally; (2) sea ice loss exposes dark ocean, adding further Arctic warming; (3) boreal forest fire frequency increases, burning the insulating peat layer above permafrost.

Why It's Excluded from IPCC Models

Earth System Models (ESMs) used for IPCC AR6 represent permafrost carbon with varying fidelity. Most models simulate gradual active-layer deepening but omit thermokarst, yedoma, and deep carbon pools below 1 m depth.

IPCC AR6 WG1 Box 5.1 explicitly notes that the "permafrost carbon feedback is not included in the IPCC assessed likely ranges." The best estimate (+0.2 °C under SSP2-4.5) is given as "medium confidence" in a separate box — not baked into headline temperature projections.

This means every 1.5 °C and 2.0 °C budget number in AR6 is implicitly optimistic relative to a world that properly accounts for permafrost feedbacks.

Key Milestones in Permafrost Carbon Science and Policy

YearEventDetail
1970s Permafrost Monitoring Begins International Permafrost Association establishes borehole networks across Siberia, Alaska, and Canada. Data reveal consistent active-layer deepening of 0.3–0.5 cm/yr.
2008 Permafrost Carbon Feedback Recognised in AR4 IPCC AR4 acknowledges permafrost as a potential carbon feedback but does not quantify it in model scenarios — a gap that persists through AR6.
2015 Koven et al. — Sensitivity Quantified First systematic estimate of permafrost carbon–climate feedback: 30–100 Gt C released per °C of sustained global warming. Implies 0.1–0.5 °C additional warming by 2100 under RCP 4.5.
2018 Schuur et al. — Stock Revised Upward to 1,700 Gt C Revised soil carbon inventories (to 3 m depth) raise total permafrost carbon estimate from ~1,000 Gt C to ~1,700 Gt C — double previous IPCC assessment values.
2020 Turetsky et al. — Abrupt Thaw Doubles Estimates Nature paper shows thermokarst (ground collapse from ice wedge melt) and retrogressive thaw slumps release carbon ~2× faster than gradual models predict. Affects ~20% of permafrost area but contributes disproportionately to flux.
2021 IPCC AR6 WG1 — Feedback Excluded from Central Projections Box 5.1 explicitly states permafrost feedbacks are not included in the IPCC 'medium confidence' projections due to model limitations. Best estimate: +0.2–0.4 °C additional warming by 2100 under SSP2-4.5.
2024 IPA State of the Permafrost — Record Active Layer Depths International Permafrost Association 2024 report documents record active-layer thickness at 70% of monitoring sites across the Arctic. Siberian permafrost showing first signs of deep (>3 m) carbon mobilisation.
2027+ Tipping Point Risk Window Multiple models suggest that warming above 2 °C global mean triggers self-reinforcing permafrost feedback: released methane warms Arctic faster than global mean, accelerating further thaw. No international policy mechanism currently addresses this feedback loop.
Model scope: This model uses a simplified stock-flux approach based on published sensitivity parameters (Koven et al. 2015; Gasser et al. 2018). It does not resolve spatial heterogeneity, soil type, or drainage. The abrupt thaw scenario applies a uniform multiplier (Turetsky et al. 2020 central estimate) rather than spatially explicit thermokarst modelling. All values should be treated as order-of-magnitude guidance consistent with published ranges.

Sources & References

SourceDescriptionKey Contribution
Schuur et al. 2022 Permafrost Carbon Network synthesis — "Permafrost and Climate Change: Carbon Cycle Feedbacks" 1,700 Gt C stock estimate (to 3 m depth); revised upward from prior ~1,000 Gt C
IPCC AR6 WG1 (2021) Ch.5 Box 5.1 — Permafrost carbon feedbacks +0.2 °C best estimate under SSP2-4.5; explicitly excluded from headline ranges
Turetsky et al. 2020 Nature — "Carbon release through abrupt permafrost thaw" Thermokarst doubles previous gradual-thaw estimates; ~20% area → ~50% flux contribution
Koven et al. 2015 Nature Clim. Change — "A simplified, data-constrained approach to estimate the permafrost carbon feedback" Sensitivity 30–100 Gt C / °C warming; central ~37 Gt C / °C used here
Gasser et al. 2018 Nature Geoscience — "Path-dependent reductions in CO₂ emission budgets caused by permafrost carbon release" Permafrost adds +0.2–0.4 °C by 2100 under RCP 4.5–8.5
Nitzbon et al. 2020 Nature Comm. — "Pathways of ice-wedge degradation in polygonal tundra under different hydrological conditions" Thermokarst dynamics under RCP 8.5; quantifies ice wedge melt accelerations
IPA State of Permafrost 2024 International Permafrost Association annual monitoring report Record active-layer depths at 70% of monitoring sites; 2023 Siberian anomalies
Millar et al. 2017 Nature Geoscience — TCRE (Transient Climate Response to Cumulative Emissions) ~1.65 °C per 1,000 Gt CO₂; used for additional-warming conversion in this model