Simulating the current and future northern limit of permafrost on the Qinghai–Tibet Plateau

Abstract. Permafrost has been warming and thawing globally, with subsequent
effects on the climate, hydrology, and the ecosystem. However, the permafrost
thermal state variation in the northern lower limit of the permafrost zone
(Xidatan) on the Qinghai–Tibet Plateau (QTP) is unclear. This study
attempts to explore the changes and variability in this permafrost using
historical (1970–2019) and future projection datasets from remote-sensing-based land surface temperature product (LST) and climate
projections from Earth system model (ESM) outputs of the Coupled Model
Intercomparison Project Phase 5 and 6 (CMIP5, CMIP6). Our model considers
phase-change processes of soil pore water, thermal-property differences
between frozen and unfrozen soil, geothermal flux flow, and the ground ice
effect. Our model can consistently reproduce the vertical ground temperature
profiles and active layer thickness (ALT), recognizing permafrost
boundaries, and capture the evolution of the permafrost thermal regime. The
spatial distribution of permafrost and its thermal conditions over the study
area were controlled by elevation with a strong influence of slope
orientation. From 1970 to 2019, the mean annual ground temperature (MAGT) in
the region warmed by 0.49 ∘C in the continuous permafrost zone and
0.40 ∘C in the discontinuous permafrost zone. The lowest elevation
of the permafrost boundary (on the north-facing slopes) rose approximately
47 m, and the northern boundary of discontinuous permafrost retreated
southwards by approximately 1–2 km, while the lowest elevation
of the permafrost boundary remained unchanged for the continuous permafrost
zone. The warming rate in MAGT is projected to be more pronounced under
shared socioeconomic pathways (SSPs) than under representative
concentration pathways (RCPs), but there are no distinct discrepancies in the areal
extent of the continuous and discontinuous permafrost and seasonally frozen
ground among SSP and RCP scenarios. This study highlights the slow delaying
process of the response of permafrost in the QTP to a warming climate,
especially in terms of the areal extent of permafrost distribution.
