Analysis of Permafrost Thermal Dynamics and Response to Climate Change in the CMIP5 Earth System Models

TLDR

Differences in coupling between near‑surface air and shallow soil temperatures, or between shallow and deeper soils, driven by snow physics and soil hydrology, explain many between‑model discrepancies. The study analyzes CMIP5 soil temperature predictions to evaluate current‑climate soil thermal dynamics and permafrost thaw projections for the twenty‑first century. The authors benchmark CMIP5 models against observations of active layer thickness, air and soil temperatures, and theoretical relationships between active layer thickness and air temperature statistics to assess permafrost‑relevant physics. CMIP5 models exhibit large variability in current permafrost extent, active‑layer statistics, and soil‑air temperature coupling, and predict permafrost loss ranging from 2%–66% (RCP2.6) to 30%–99% (RCP8.5), with an average loss of 1.6 ± 0.7 million km² per 1 °C high‑latitude warming.

Abstract

Abstract The authors analyze global climate model predictions of soil temperature [from the Coupled Model Intercomparison Project phase 5 (CMIP5) database] to assess the models’ representation of current-climate soil thermal dynamics and their predictions of permafrost thaw during the twenty-first century. The authors compare the models’ predictions with observations of active layer thickness, air temperature, and soil temperature and with theoretically expected relationships between active layer thickness and air temperature annual mean- and seasonal-cycle amplitude. Models show a wide range of current permafrost areas, active layer statistics (cumulative distributions, correlations with mean annual air temperature, and amplitude of seasonal air temperature cycle), and ability to accurately model the coupling between soil and air temperatures at high latitudes. Many of the between-model differences can be traced to differences in the coupling between either near-surface air and shallow soil temperatures or shallow and deeper (1 m) soil temperatures, which in turn reflect differences in snow physics and soil hydrology. The models are compared with observational datasets to benchmark several aspects of the permafrost-relevant physics of the models. The CMIP5 models following multiple representative concentration pathways (RCP) show a wide range of predictions for permafrost loss: 2%–66% for RCP2.6, 15%–87% for RCP4.5, and 30%–99% for RCP8.5. Normalizing the amount of permafrost loss by the amount of high-latitude warming in the RCP4.5 scenario, the models predict an absolute loss of 1.6 ± 0.7 million km 2 permafrost per 1°C high-latitude warming, or a fractional loss of 6%–29% °C −1 .