Simulation of present-day and future permafrost and seasonally frozen ground conditions in CCSM4

The combined impact of advances in CLM and a better Arctic climate simulation, especially for air temperature, improve the permafrost simulation in CCSM4 compared to CCSM3. Present-day continuous plus discontinuous permafrost extent is comparable to that observed [12.5 × 10⁶ versus (11.8-14.6) × 10⁶ km²], but active-layer thickness (ALT) is generally too thick and deep ground (>15 m) temperatures are too warm in CCSM4.

Present-day seasonally frozen ground area is well simulated (47.5 × 10⁶ versus 48.1 × 10⁶ km²). ALT and deep ground temperatures are much better simulated in offline CLM4 (i.e., forced with observed climate), which indicates that the remaining climate biases, particularly excessive high-latitude snowfall biases, degrade the CCSM4 permafrost simulation. Near-surface permafrost (NSP) and seasonally frozen ground (SFG) area are projected to decline substantially during the twenty-first century [representative concentration projections (RCPs); RCP8.5: NSP by 9.0 × 10⁶ km², 72%, SFG by 7.1 × 10⁶, 15%; RCP2.6: NSP by 4.1 × 10⁶, 33%, SFG by 2.1 × 10⁶, 4%].