Permafrost degradation as a heterogeneous mosaic of tipping and non-tipping behaviour

Helen Millman , Andrew M Cunliffe, Annett Bartsch, Gustaf Hugelius, Jesse Abrams, Timothy M. Lenton

Permafrost thaw can occur as widespread gradual thaw, or as abrupt thaw.
Abrupt thaw can occur on decadal or shorter timescales and produce large,
often irreversible changes to landscapes, ecosystems, and infrastructure. These
changes are frequently described as tipping points, yet the underlying mechanisms differ substantially in their feedback structure, reversibility, and capacity
for self-sustained propagation. Existing discussions therefore often conflate fundamentally different modes of permafrost degradation. Here we propose a
four-class framework that distinguishes gradual, reversible non-linear, threshold,
and tipping-system behaviour based on realised system dynamics. The framework classifies process behaviour within its environmental context, recognising
that the same physical mechanism may occupy different classes of behaviour
under different combinations of ice content, thermal margin, structural integrity,
and propagation potential. We apply the framework to the principal mechanisms of permafrost thaw, assess which processes satisfy strict tipping-element
system criteria, and identify the state variables governing class membership. We
argue that permafrost instability is best understood as a heterogeneous mosaic of locally conditioned behaviours, of which some include tipping. We show that
tipping potential is strongly pre-conditioned by glacial and depositional inheritance, meaning that vulnerability is spatially heterogeneous in ways that cannot
be inferred from contemporary climate forcing alone. The framework provides a
basis for improved integration of permafrost instability into Earth system models,
hazard assessment, and early warning systems.