Simulation of Permafrost and Seasonally Frozen Ground Conditions and their Response to Recent Climate Warming in the Tibetan Plateau

Abstract:

Significant changes have occurred in permafrost and seasonally frozen ground conditions in the Tibetan Plateau over the last few decades. These changes could have large impacts on the land surface energy and water balance, with potential influences on regional climate, hydrological and ecosystem processes. In this study, we investigated recent changes in permafrost and seasonally frozen ground conditions in the plateau from recent climate warming using simulations based on the Geophysical Institute of Permafrost Laboratory (GIPL2) model. We calibrated and validated GIPL2 at selected CAMP/Tibet (2002-2004) sites, and performed sensitivity analysis at a multiscale soil moisture and temperature monitoring network in the central Tibet (Nagqu site, 2010-2012). Our initial sensitivity analysis indicated that better simulations of soil thermal regime were obtained through using in-situ surface ground temperature (T_0cm) measurements than using surface air temperature. Our simulations from 1987 to 2014 showed a significant increase in the active layer thickness (~0.043 m yr^-1, p<0.001) in discontinuous permafrost areas (Wudaoliang and Tuotuohe sites) and decrease in the maximum soil frozen depth (0.028~0.064 m yr^-1, p<0.001) in seasonally frozen soils (Amdo, Nagqu and Damxung sites). Such distinct soil thawing is mainly caused by strong climate warming, as indicated from in-situ measurements of both in-situ air temperature and T_0cm. The climate records suggest significant advance of spring thawing (-0.27~-0.65 day yr^-1, p<0.1) and delay in autumn surface freezing (0.12~0.33 day yr^-1, p<0.1 except for Nagqu site). In addition, the climate records show much stronger warming rate in the surface ground than overlying surface air, especially in the spring and fall, likely due to changes in the surface energy balance with potential large influence on the soil thermal regime and freeze/thaw processes. Our future research will include mapping the permafrost vulnerability to climate change over the whole plateau using regional simulations based on GIPL2.