Estimating Vulnerable Carbon in Thawing Northern High-Latitude Permafrost Using CMIP6 Climate Projections

Permafrost degradation across the Northern Hemisphere is projected to continue and intensify under ongoing climate warming, with important implications for the global carbon cycle. Thawing permafrost exposes previously frozen soil organic carbon (SOC) to microbial decomposition, resulting in the release of carbon dioxide and methane, thus potentially amplifying climate change through positive feedbacks. Robust projections of permafrost thaw are therefore essential for improving estimates of future carbon emissions and the global carbon budget.

Using output from the latest generation of global climate models participating in CMIP6, we assess changes in the annual active layer thickness (ALT), defined as the maximum seasonal thaw depth of permafrost, under four Shared Socioeconomic Pathway (SSP) scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). We show that ALT estimates derived directly from CMIP6 soil temperature fields exhibit substantial deviations from observed ALT values, which can lead to inconsistent estimates of permafrost carbon release.

To address this limitation, we propose a simplified, observation-constrained approach that focuses on projected changes in ALT rather than absolute model-derived values. These projected ALT changes, combined with present-day ALT observations, are used to estimate vulnerable carbon under future climate projections. We validate our ALT-based approach through comparison with simulations from the Lund–Potsdam–Jena managed Land (LPJmL) dynamic global vegetation model, which explicitly represents permafrost and soil thermal processes. This comparison shows consistent spatial patterns of active layer thickness, supporting the robustness of our simplified estimation framework.

CMIP6 models project ALT changes of 0.1–0.3 m per degree rise in local temperature, resulting in an average deepening of approximately 1.2–2.1 m in the northern high latitudes under different scenarios. With increasing temperatures, permafrost thawing initiates in southern Siberia, northern Canada, and Alaska, and progressively extends poleward, reaching much of the Arctic under high-emissions scenarios (SSP5-8.5) by the end of the century. Using projections of ALT changes and vertically resolved SOC data, we estimate ensemble-mean decomposable carbon stocks in thawed permafrost of approximately 115 GtC under SSP1-2.6, 180 GtC under SSP2-4.5, 260 GtC under SSP3-7.0, and 300 GtC under SSP5-8.5 by 2100.