[A-CC32] Glaciology

[ACC32-12] Current Distribution and Future Projection of Mountain Permafrost in Japan: Estimation Based on Land Surface Physical Model and Observations

*Tokuta Yokohata¹, Go Iwahana², Kazuyuki Saito³, Tetsuo Sueyoshi⁴, Takashi Arakawa⁵, Tomoko Nitta⁶, Yukihiko Onuma⁷, Irina Melnikova¹, Noriko N Ishizaki¹, Haruto Okubo⁸, Yuka Sawa⁸, Kumiko TAKATA⁹ (1.National Institute for Environmental Studies, 2.University of Alaska Fairbanks, 3.Japan Agency for Marine and Earth Science and Technology, 4.National Institute for Polar Research, 5.CliMTech, 6.University of Tokyo, 7.Japan Aerospace Exploration Agency, 8.University of Tsukuba, 9.Azabu University)

Keywords:Permafrost, Future projections

Permafrost is defined as ground that remains below 0°C for more than two consecutive years and is widely distributed in the tundra and taiga regions of the high latitudes in the Northern Hemisphere. Japan is currently located at the southernmost boundary of the global permafrost distribution. Therefore, permafrost has only been confirmed at a very limited number of sites in high-latitude and high-altitude mountainous regions with extremely cold climate conditions in Japan. Since permafrost formulation is a subsurface phenomenon, it is inherently difficult to observe. This challenge is further compounded in mountainous regions, making it difficult to obtain a comprehensive understanding of permafrost distribution. Previous studies have assessed the current state and projected future changes of permafrost in regions such as Europe and the Tibetan Plateau. However, in East Asia, including Japan, very few studies have focused on future permafrost projections. In our previous research, we evaluated the current state and future projections of permafrost in Japan by defining "permafrost regions" as areas with temperature conditions suitable for maintaining permafrost (Yokohata et al. 2022, PEPS). This study was achieved using high-resolution (1 km) temperature projections and empirical relationships. However, these empirical relationships were derived from permafrost distributions in environments different from Japan, potentially leading to an overestimation of permafrost extent. Therefore, in this study, we estimated the distribution of permafrost in Japan by explicitly calculating subsurface temperatures at high resolution.

In this study, by giving 1 km resolution temperature projection for Japan into the land surface physical model ILS (Integrated Land Simulator), we estimated subsurface physical processes in Japan. ILS utilizes atmospheric physical variables at the surface, such as air temperature, precipitation, radiation, wind speed, and humidity, to simulate near-surface physical processes. This allows for the calculation of ground temperature, soil moisture, river discharge, and runoff. The 1 km resolution temperature projection used in this study was developed using the latest climate projections and bias correction techniques to support various stakeholders, including local governments and private companies, in implementing climate change adaptation measures. In this study, we estimated the permafrost distribution in Japan by giving surface physical projection from the pre-industrial era to the present into ILS.

The ILS simulation results revealed that permafrost exists in a much narrower range compared to the distribution estimated using temperature projection data and empirical relationships in our previous studies (Yokohata et al. 2022). This discrepancy is likely due, at least in part, to the fact that the empirical relationships used in previous studies were developed based on data from environments such as Alaska and Siberia, which differ from those in Japan. On the other hand, previous observational studies have shown that permafrost in areas such as the Daisetsu Mountains tends to be found in wind-exposed areas (wind-blown sites) where strong winter winds prevent significant snow accumulation. To account for this, we conducted numerical experiments by modifying model parameters to adjust the amount of snow depth within the model. The results indicated that reducing snow depth (which represents wind-blown conditions) led to a broader distribution of permafrost. By using model parameters that successfully reproduced the observed permafrost distribution, we conducted both past climate reconstruction and future projection experiments to estimate the current distribution of permafrost in Japan and project its future changes.