Permafrost Experimental Protocol within the Tipping Point Modelling Intercomparison Project (TIPMIP)

Within the Tipping Point Modeling Intercomparison Project (TIPMIP; www.tipmip.org), we outline a coordinated experimental protocol designed for standalone land surface models (LSMs) as well as coupled Earth System Models (ESMs). The protocol targets key questions on permafrost feedbacks, thresholds, timescales, abrupt versus gradual, and irreversible changes, and interactions with other tipping elements, aiming to quantify plausible landscape transformations, associated greenhouse gas emissions, and their impacts on global energy, carbon, and hydrological cycles, as well as large scale circulation.

Building on TIPMIP-ESM, the TIPMIP-permafrost Tier 1 experiments, defined as the core, priority experiments required for MIP participation, aim to use LSMs to quantify the extent to which permafrost area, ground ice volume, and soil carbon stocks exhibit path dependent behavior and reversibility when subjected to idealized warming, stabilization, and subsequent cooling phases. This includes evaluating whether the permafrost returns to its initial state as temperatures decline, or the extent to which certain changes, such as area loss, carbon redistribution, hydrological reorganization, or ground subsidence, persist despite cooling and thus remain effectively irreversible over time scales spanning a century to a several hundred years. In addition, during the stabilization phase we will examine whether permafrost degradation continues even after warming ceases. This involves assessing time lagged thaw response, identifying critical thresholds that may trigger rapid acceleration of degradation, and determining whether internal processes, such as shifts in soil moisture, ground subsidence and thermokarst lake formation, or carbon redistribution, amplify change through self reinforcing feedbacks. The emergence of such behaviors would indicate nonlinear system dynamics and a heightened susceptibility to tipping point transitions.

Additional sensitivity experiments (Tier 2), aimed at understanding and quantifying specific processes that could induce permafrost tipping, will apply idealized forcing. The focus is on altering hydrologic conditions and modifying surface properties (e.g., vegetation, albedo, thermokarst lake distribution) to explore how these factors influence the onset and dynamics of tipping behavior.

Tier 3 experiments, also sensitivity oriented, investigate the coupling between permafrost dynamics and other climate tipping elements using ESMs, for example, assessing the consequences of an AMOC collapse for high latitude permafrost stability, as well as the broader Earth system impacts of abrupt permafrost tipping.