Recent Intensification (2004–2020) of Permafrost Mass‐Wasting in the Central Mackenzie Valley Foothills Is a Legacy of Past Forest Fire Disturbances.

The effects of recent climate change are accelerating permafrost thaw, including ice‐rich landscapes of the western Canadian Arctic. However, regional drivers of permafrost slope failure in hillslopes with warm, thin permafrost remain poorly understood. Repeat satellite imagery (1984–2020) indicates rapid increases in retrogressive thaw slumps (RTSs) and deep‐seated permafrost landslides (DSPLs) since 2004, indicating a change in slope stability thresholds in an area that otherwise appeared thaw stable. The widespread occurrence of DSPL represents a contrasting geomorphic response to the RTS‐dominated ice‐rich permafrost landscapes. In this study area, RTS and DSPL occur predominantly in areas that were burned by forest fires in the 1990s, indicating a legacy thermal disturbance that preconditioned permafrost hillslopes for failure. The relations between historic fires and the later development of widespread permafrost slope failures represent an outstanding example of the complex interactions between inherited landscape sensitivity in ice‐rich terrain and ongoing climate change. Plain Language Summary: We characterize the type and nature of permafrost slope failures in the central Mackenzie Valley, NWT and find hillslope failures have increased rapidly over the past ∼15 years. This growth occurred in conjunction with increasing air temperatures and summer precipitation but the distribution of permafrost slope failures in this area is largely constrained by forest fire extents that affected the area in the 1990s. These fires have led to increased landscape sensitivity of this ice‐rich terrain through their impacts on ground temperatures. Key Points: Recent increases in permafrost mass‐wasting frequency (278%) and magnitude (602%) reveal a permafrost landscape in geomorphic transitionMore than 80% of failures occur in areas burned in the 1990sCompounding effects of legacy thermal disturbance and climate drivers of thaw likely to increase slope instability of warm permafrost [ABSTRACT FROM AUTHOR]