Projected changes of seasonally frozen ground over the northeastern Qinghai-Xizang Plateau

Based on 22 bias-corrected and downscaled high-resolution simulations from NEX-GDDP-CMIP6, this study projects changes in the annual maximum freezing depth, freezing start date, thawing end date, and areal extent of seasonally frozen ground over the northeastern Qinghai-Xizang Plateau under SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios for the mid-21st century (2025-2060) and late-21st century (2061-2100). The results show that, across all three scenarios, the annual maximum freezing depth is projected to decrease significantly by 9.8-14.9 cm during the mid-21st century relative to the historical reference period. Concurrently, the freezing start date is projected to delay at a rate of 1-3 days per decade, while the thawing end date to advance at a rate of 2-4 days per decade, with the advance occurring nearly twice as rapidly as the delay. The shortening of the frozen period becomes more pronounced under higher emission scenarios. Under SSP1-2.6 scenario, changes in seasonally frozen ground remain relatively stable in the late 21st century. Under SSP2-4.5 scenario, the rates of change in maximum freezing depth and freeze-thaw timing resemble those projected for the mid-21st century under SSP1-2.6. Under SSP5-8.5 scenario, the maximum freezing depth continues to decrease substantially, accompanied by a significant shortening of the frozen period. Across different ecological functional zones, the annual maximum freezing depth of seasonally frozen ground decreases most rapidly in the eastern agricultural area during the mid- and late-21st century under SSP1-2.6 and SSP2-4.5 scenarios, whereas the Three River Source Region experiences the fastest rate of decline under SSP5-8.5. Moreover, the frozen peiod shortens most significantly in the Three River Source region across all scenarios. The area of seasonally frozen ground is projected to expand by 14.4×104-19.8×104 km2 in the mid-21st century across all scenarios relative to the historical reference period. This expansion continues into the late-21st century, with further increases of 2.2×104 km2, 8.6×104 km2, and 12.4×104 km2 under SSP1-2.6, SSP2-4.5, SSP5-8.5, respectively. Overall, seasonally frozen ground in the northeastern Qinghai-Xizang Plateau will be profoundly affected by future climate change, with the most pronounced reductions in maximum freezing depth and frozen period duration under SSP5-8.5 alongside accelerated permafrost-to-seasonally-frozen-ground conversion, and energy conservation and emission reduction measures can effectively mitigate this degradation trend.