Warming above, cooling below: First model-based quantitative thermal- regime assessment and subsurface thermal evolution of Nivlisen Ice Shelf, East Antarctica, revealing non-equilibrium thermal adjustment and progressive thermal preconditioning

Geetha Priya M , Deva Jefflin A R, Adithya Sunil

We present the first quantitative thermal characterisation of Nivlisen Ice Shelf, central Dronning Maud Land, East Antarctica, using a one-dimensional heat-transfer model forced by ERA5 surface skin temperatures (1940–2025). For the primary scenario (H = 312 m), the steady-state solution yields a mid-column temperature of −4.89 °C, Péclet number Pe = 4 (intermediate conduction–advection regime), thermal equilibration timescale τ = 269 yr, basal conductive heat flux of 14.22 mW m⁻², and conductive basal melt rate of 1.49 mm yr⁻¹.
16 Seasonal temperature variations penetrate to an e-folding depth of 3.42 m, with the firn layer effectively isolating ice below ~20 m from seasonal forcing. Transient simulations reveal significant surface warming of +0.60 °C decade⁻¹ (R² = 0.622, p < 0.0001). Temperatures increase at +0.57 and +0.31 °C decade⁻¹ at 1 m and 10 m depth, respectively, but decrease at −0.23 °C decade⁻¹ at 100 m depth (R² = 0.995), producing a sign reversal at ~27 m depth. This non-equilibrium structure reflects an equilibration timescale substantially longer than the 86-year forcing record. Near-surface cold content at 1 m depth decreased by 36.1% (38.96 to 24.91 MJ m⁻³), the melt-energy barrier declined by 38.3% (40.05 to 24.72 MJ m⁻³), and the temperature deficit to the 0°C melt threshold decreased by 78.8% (2.89 to 0.61 °C). Together, these metrics quantify progressive thermal preconditioning through depletion of the near surface cold reservoir and melt-energy barrier. Model-class uncertainty assessed using the Fimbulisen S1 borehole analogue yielded a conservative upper-bound uncertainty of ±8.69 °C. The results demonstrate that NIS is undergoing progressive thermal preconditioning while remaining in a state of long-term thermal disequilibrium.