Yokohata et al.
Progress in Earth and Planetary Science(2022) 9:39
https://doi.org/10.1186/s40645-022-00498-z

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

Assessing and projecting surface airtemperature conditions required to sustain
permafrost in Japan
Tokuta Yokohata1* , Go Iwahana2, Kazuyuki Saito3, Noriko N. Ishizaki1, Taiga Matsushita4 andTetsuo Sueyoshi3,5Abstract
Permafrost covers a wide area of the Northern Hemisphere, including high-altitude mountainous areas and even at
mid-latitudes. There is concern that the thawing of mountain permafrost can cause slope instability and substantially
impact alpine ecosystems, and because permafrost in mountainous areas is difficult to observe, detailed analyses
have not been performed on its current distribution and future changes. Although previous studies have observed
permafrost only at a limited number of points in Japan (e.g., Daisetsu Mountains, Mt. Fuji, and Mt. Tateyama in the
Northern Japan Alps), we show that permafrost potentially exists in nine domains in Japan (Daisetsu Mountains, Mt.
Fuji, Northern and Southern Japan Alps, Hidaka Mountains, Mt. Shiretokodake, Sharidake, Akandake, and Yotei). In the
Daisetsu Mountains and Mt. Fuji, the environmental conditions required for maintaining at least some permafrost are
projected to remain in the future if a decarbonized society is achieved (RCP2.6 or RCP4.5). However, if greenhousegas emissions continue to increase (RCP8.5), the environmental conditions required for sustaining permafrost are
projected to disappear in the second half of the twenty-first century. In other domains, the environmental conditions required for maintaining permafrost are either projected to disappear in the next ten years (Hidaka Mountains,Northern Japan Alps) or they have almost disappeared already (Southern Japan Alps, Mt. Shiretokodake, Sharidake,
Akandake, and Yotei). Our projections show that climate change has a tremendous impact on Japan’s mountainpermafrost environment and suggests the importance of monitoring the mountain environment and considering
measures for adapting to future climate change.
Keywords: Mountain permafrost, Climate change, Future projections
1 Introduction
Areas in which the temperature of the ground falls below
0 °C for two consecutive years are called permafrost(IPCC 2013). Thawing of permafrost in mountains (wecall mountain permafrost hereafter) areas has reportedly increased the frequency and scale of rockfalls andlandslides (Krautblatter et al. 2013; Lacelle et al., 2015;
Patton et al. 2019; Hjort et al. 2022). This threatens the*Correspondence: yokohata@nies.go.jp
1
Earth System Division, National Institute for Environmental Studies, 16‑2,
Onogawa, Tsukuba 305‑8506, Japan
Full list of author information is available at the end of the article

safety of hikers and mountaineers (Purdie et al. 2015)
and changes iconic mountaineering routes and suitableclimbing seasons (Mourey et al. 2019). In addition, thethawing of mountain permafrost has a substantial impact
on alpine ecosystems through changes in temperature,soil moisture and groundwater. The disappearance of
mountain permafrost, coupled with inadequate precipitation in summer and reduced snowmelt, can cause alack of water during the growing season, changes in species composition, and reduced greening and productivity(Trujillo et al. 2012; Sloat et al. 2015). Thawing of mountain permafrost can have a marked impact on mountainecosystems, including various kinds of alpine plants and© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

animals (Jones et al. 2018), which are valuable natural
resources that are actively sought out by climbers (Kuboet al. 2018; Mameno et al. 2022).
Studies on future projections of permafrost thawing
show that thawing proceeds from the lower latitudes andwarmer regions (Yokohata et al. 2020a; Hjort et al. 2022).
Therefore, it is considered important to estimate the current and future projected status of permafrost located at
the lower latitudinal limits of the permafrost distribution
to understand the changes in permafrost due to climatechange. According to a study on the global permafrost
distribution (Obu et al. 2019; Smith et al. 2022), thelowest-latitude permafrost in the Northern Hemisphereis located in the Himalayas (Iwata et al. 2003; Matsuoka
2002; 2003; Fukui et al. 2007). In Asia, Mt. Fuji (35° N),
Mt. Tateyama (37° N), and the Daisetsu Mountains (43°N) in Japan, and the Tianshan Mountains (42° N) andthe Daxing Anling Mountains (50° N) in China reportedly contain permafrost (Obu et al. 2019). Among theseareas, estimates of the permafrost distribution at the
1 km scale have been compared with observational data
for the Tibetan Plateau and mountains in China (Obuet al. 2019). Further, while permafrost in Japan has beenreported in the Daisetsu Mountains (Fukuda and Sone
1992; Ishikawa and Hirakawa 2000; Iwahana et al. 2011;
Sone and Watanabe 2018; Sone 2020), the lowland of
Hokkaido such as Shikaribetsu and Oketo (Sone 1996;
Sawada et al. 2003), Mt. Fuji (Higuchi and Fujii 1971;
Ikeda et al. 2012), and Mt. Tateyama (Fukui and Iwata
2000; Aoyama 2005) in the Northern Japan Alps, country-scale analyses have not been performed to date.
Future projections of the permafrost distribution havebeen performed using global climate models (GCMs)(Schneider von Deimling et al. 2015; Chadburn et al.
2017; Gasser et al. 2018; Yokohata et al. 2020b). However,since the general resolution of GCMs is approximately
100 km, it is difficult to estimate the current distribution and to perform future projections of mountainpermafrost, which involves more detailed estimates ofpermafrost at different altitudes (Hock et al. 2019). In aprevious study, we used bias-corrected and 1-km downscaled GCM projections (Ishizaki et al. 2020) to estimate
the size of areas where climate conditions are suitablefor permafrost in Japan’s Daisetsu Mountains (Yokohata
et al. 2021). In this study, such areas throughout Japan
were estimated by employing the methods used in ourprevious study (Yokohata et al. 2021).

2 Methods
2.1 Bias‑corrected and downscaled climate model output

We used the bias-corrected and downscaled climate
model outputs developed by Ishizaki et al. (2020) andIshizaki (2021), who generated two bias-corrected

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climate scenarios using different methods. We used climate scenarios based on the cumulative distributionfunction-based downscaling method (CDFDM, developed in previous studies; Iizumi et al. 2010, 2011, 2012).
Using the CDFDM, the cumulative distribution function for simulated daily mean data is corrected so thatit matches the 1-km resolution meteorological data forJapan (Ohno et al. 2016). Ishizaki et al. (2020) demonstrated that the CDFDM method is superior to othermethods, such as Gaussian-type scaling approaches(Haerter et al. 2011). Furthermore, Ishizaki et al. (2020)and Ishizaki (2021) corrected biases in historical simulations and future projections based on the RCP2.6 and
RCP8.5 scenarios for four CMIP5 GCMs (i.e., GFDLCM3; Donner et al. 2011, MIROC5; Watanabe et al. 2010,
HadGEM2-ES; Jones et al. 2011, MRI-CGCM3; Yukimoto et al. 2012, Nor-ESM; Bentsen et al. 2013), and the
RCP2.6, RCP4.5, and RCP8.5 scenarios for five CMIP6
GCMs (i.e., MIROC6; Tatebe et al. 2019, MRI-ESM2-0;
Yukimoto et al. 2019, ACCESS-CM2; Bi et al. 2020, IPSLCM6A-LR; Boucher et al. 2020, MPI-ESM1-2-HR; Müller
et al. 2018) for the Japan region at a resolution of 1 km.
Briefly, the reasons why Ishizaki et al. (2020) and Ishizaki(2021) selected nine GCMs from the CMIP5 and CMIP6
GCMs were to account for uncertainties in the ranges of
future surface air temperature and precipitation projections with the models and to ensure that they effectively
reproduce the twentieth-century climate.
Ishizaki et al. (2020) showed that bias-corrected historical climate scenarios accurately reproduced monthly
averaged values; extreme values, such as summer days,
and indicators defined by daily values, such as precipitation intensity. In this study, we utilized version 202,005
of CMIP5 (Ishizaki et al. 2020) and version 202,105 of
CMIP6 (Ishizaki 2021), in which the time window for
the cumulative distribution function (1 month) andthe reference period (1980–2018) were modified sothat the monthly values corresponded to those for theobservations.
2.2 Statistical method for inferring permafrost distribution

The freezing (thawing) index, defined as the cumulative
daily temperature below (above) the freezing point of
0 °C, has been used as a proxy for inferring the permafrost distribution. Saito et al. (2014) used this index todevelop a high-resolution (2 km) method for estimating the permafrost distribution in northeastern Asia,
including the permafrost in Japanese mountainous
areas investigated in the present study. We employed
the same method to infer the permafrost distribution by using the bias-corrected, 1 km resolution climate scenarios developed by Ishizaki et al. (2020). The

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

method of Saito et al. (2014) was also used to estimate
the permafrost environment for the past 122 kyr (Saito
et al. 2022).
Saito et al. (2014) classified the permafrost area innortheastern Asia into two categories based on freezing and thawing indices: (a) climate-driven permafrost (CDP), which are areas where climatic conditions
favor the development and/or maintenance of continuous permafrost, and (b) environmentally conditioned
permafrost (ECP), which are areas where permafrost
development and/or maintenance is dependent uponenvironmental factors such as ecosystem characteristics, topography or geology. In addition, Saito et al.
(2014) divided seasonally frozen ground into two subcategories: (c) ground that undergoes seasonal freezing(SF) and (d) ground that undergoes intermittent freezing (IF). These distinctions were made to distinguishbetween seasonal frost that is deep and/or persistentand frost that exists for a short time (i.e., < 2 weeks).
Using a freezing index (i.e., the number of days per
year when the surface air temperature is below 0 °C
multiplied by the surface air temperature), If, and thethawing index (i.e., the number of days per year when
the surface temperature is above 0 °C multiplied by thesurface air temperature), It, these permafrost classes
can be defined as follows.
(a) CDPIt < 0.9 If − 2300

(1)

0.9If − 2300 < It < 2.4If − 3300

(2)

2.4If − 3300 < It(3)(b) ECP(c) SFand 30 < If(d) IF

0 < If ≤ 30

(4)

For consistency with Saito et al. (2014), this study
used the monthly mean surface air temperature to calculate the freezing and thawing indices; previous studies have shown that the relative error is less than 5%
when using daily or monthly means (Frauenfeld et al.
2007). Saito et al. (2014) performed permafrost classification at a resolution of 2 km by considering the temperature decrease with altitude using spatially detailed
elevation data (ETOPO1; Amante and Eakins 2009)that were based on the results of the CMIP5 GCMs. In
the present study, a temperature decrease with altitude
was considered in the 1-km-mesh observational data

Page 3 of 10(Ohno et al. 2016) that were used for bias-corrected climate scenarios (Ishizaki et al. 2020; Ishizaki 2021).
In this study, the freezing and thawing indices for the
past 30 years were averaged to classify the permafrost
in each grid cell, similar to Saito et al. (2014). Averaging
over 30 years smooths the internal variability in surface
air temperature and corresponds to the delayed responseof permafrost to climate change.

3 Results and discussion
3.1 Surface air temperature conditions required to sustain
permafrost in Japan

Figure 1 (left) shows the altitude distribution of easternJapan. In general, the annual mean surface air temperature is low at high latitudes and at high altitudes. The
correspondence between altitude and the annual meansurface air temperature is shown in Additional file 1: Fig.
S1. In eastern Japan, regions with low surface air temperatures are widespread in Hokkaido (the northernmost island of Japan). As shown in Fig. 1a, in Hokkaido,
high-altitude mountains exist in the interior (Daisetsu
Mountains, 1 in Fig. 1b), southern central region (Hidaka
Mountains, 2 in Fig. 1b), and in the east (5, 6 and 8 inFig. 1b). On the main island of Honshu to the south of
Hokkaido, there are particularly high mountains in thecentral Chubu region (inset at bottom of Fig. 1a), due tothe influence of orogeny. In the Chubu region, the highmountainous areas are the Northern Japan Alps (3 inFig. 1c), the Southern Alps (7 in Fig. 1c), and Mt. Fuji (4in Fig. 1c).
Figure 1b and c shows an area with climatic conditions that are suitable for permafrost development in themountainous regions of Japan. These areas are probablelocations of permafrost based on nine bias-corrected anddownscaled climate scenarios (Ishizaki et al. 2020; Ishizaki 2021) in each grid cell. Permafrost classification inthis study is based on Saito et al. (2014), who classified
permafrost into CDP and ECP; however, only ECP is present in the mountainous regions of Japan. Given that theactual distribution of permafrost is determined by complex interactions between environmental phenomena,such as topography and geology (Ishikawa and Hirakawa
2000), grid cells that were classified as permafrost in thisstudy indicate only where the climatic conditions are
suitable for the maintenance of permafrost. We refer toareas containing these grid cells as an “ECP region”. In
addition to surface air temperature, snow cover is an
important factor affecting the distribution of permafrost(Ishikawa and Hirakawa 2000). In general, permafrost is
distributed in wind-blown gravel areas where the snow
cover is typically thin. Furthermore, it has been notedthat mountain permafrost distribution is greatly limited by thawing of frozen soil due to the percolation of

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

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Fig. 1 a Map showing the altitude [m] of northern and eastern Japan, and the distribution of regions with surface air temperatures that favor the
maintenance of permafrost (the Environmentally Conditioned Permafrost regions) in Japan; b Hokkaido and c the Chubu. The numbers indicate(1) Daisetsu Mountains, (2) Hidaka Mountains, (3) Northern Japan Alps, (4) Mt. Fuji, (5) Mt. Shiretokodake, (6) Mt. Shari, (7) Southern Japan Alps, (8)Mt. Akandake, and (9) Mt. Yotei. The distribution of the environmentally conditioned permafrost region is based on probabilities estimated by ninebias-corrected climate scenarios

rainfall during the summer (Fukui 2003, 2004; Ikeda andIwahana 2010, Ikeda et al. 2012). Due to the limited distribution of wind-blown gravel areas and the percolationof rainfall, the actual distribution of permafrost would
be smaller than the area estimated using only surface airtemperatures.
The ECP regions estimated in this study under current climate conditions, in descending order of area, are(1) the Daisetsu Mountains, (2) the Hidaka Mountains,(3) the Northern Japan Alps, (4) Mt. Fuji, (5) Mt. Shiretokodake, (6) Mt. Sharidake, (7) the Southern Japan Alps,(8) Mt. Akandake, and (9) Mt. Yotei. In general, ECP
regions are located in high-latitude and high-altitude
mountainous regions where the surface air temperature
is low throughout the year.
Figure 2 shows the details of the ECP regions in thenine areas. Regions identified as ECP regions with highconfidence in multi-model estimations (i.e., large value
in Fig. 2) are (1) the Daisetsu Mountains, (2) the Hidaka
Mountains, (3) the Northern Japan Alps, and (4) Mt. Fuji.
In Fig. 2, the points where permafrost was confirmed
by previous observational studies are shown in red (thedetails of the observational sites are shown in Additional
file 1: Table S1). The permafrost observations in previous
studies (Higuchi and Fujii 1971; Fukuda and Sone 1992;
Ishikawa and Hirakawa 2000; Fukui and Iwata 2000; Aoyama 2005; Iwahana et al. 2011; Ikeda et al. 2012; Soneand Watanabe 2018; Sone 2020) are consistent with thefindings of the present study, as shown in Fig. 2. The
ECP region exists at altitudes higher than 1600 m in the
Daisetsu and Hidaka Mountains in Hokkaido. Permafrost
has not been observed in the Hidaka Mountains to date,but our results indicate that permafrost could exist ataltitudes higher than 1600 m in the Hidaka Mountains.

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

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Fig. 2 Distribution of regions with surface air temperatures that favor the maintenance of permafrost (the Environmentally Conditioned Permafrost
regions) in Japan under the current climate conditions (averaged over 1999–2018); (1) Daisetsu Mountains, (2) Hidaka Mountains, (3) Northern
Japan Alps, (4) Mt. Fuji, (5) Mt. Shiretokodake, (6) Mt. Sharidake, (7) Southern Japan Alps, (8) Mt. Akandake, and (9) Mt. Yotei. The distribution of
the environmentally conditioned permafrost region is based on probabilities estimated by nine bias-corrected climate scenarios. Red points
indicate sites where permafrost has been observed. The observations of sporadic permafrost in Shikaribetsu and Oketo are shown as ▼, and otherobservations are shown as + . See Additional file 1: Table S1 for details. The contour interval is 400 m

Permafrost has been observed at Tateyama in the Northern Japan Alps (Fukui and Iwata 2000; Aoyama 2005),
but could exist on other mountains at altitudes above
2600 m. Mt. Fuji is located at a lower latitude than otherareas, but the area above 3000 m is identified as an ECP
region, which is consistent with previous studies (Ikeda
et al. 2012). We also confirmed that the annual mean surface air temperature at the field observation sites in thesemountains is consistent with the observational data, as
shown in Additional file 1: Fig. S2.
It should be noted that sporadic permafrost has been
observed in the lower altitudes of the mountain flanks(shown as ▼ in Fig. 2-1 at Shikaribetsu and Oketo withan altitude of 1254 m and 480 m, respectively), which
formed under exceptionally cold conditions at the specific geomorphological locations of Fuketsu, or wind

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

caves (Sone 1996; Sawada et al. 2003). Our analysis isbased on spatially averaged surface air temperatures on
the 1-km scale and thus does not represent permafrost insuch extraordinary conditions defined as extra-zonal permafrost (Ishikawa et al. 2003).
Regions with low confidence in projections from multiple bias-corrected and downscaled GCMs are (5) Mt.
Shiretokodake, (6) Mt. Sharidake, (7) the Southern
Japan Alps, (8) Mt. Akandake, and (9) Mt. Yotei (Fig. 2).
In these domains, ECP regions exist at altitudes above
1400 m (Mt. Shiretokodake), 1200 m (Mt. Sharidake),
3000 m (Southern Japan Alps), 1300 m (Mt. Akandake),and 1800 m (Mt. Yotei).
Since latitude and altitude are important geographicalfactors that determine the surface air temperature, theyalso affect the distribution of permafrost. Figure 3 showsa scatterplot of the latitude–altitude distribution in the
ECP regions under present climate conditions. We use
the grid mean altitudes (Ohno et al. 2016) for the scatterplot. In Fig. 3, the regression line is calculated by selecting points corresponding to the lowest altitude at each
latitude. Thus, the lower altitude limit for the ECP region
in mountainous regions of Japan in the present climate isgiven by:Altitude = 2997−(latitude − 35) × 177 [m](5)The lower altitude limit, which is approximately 2997 m
at a latitude of 35° N, decreases by 177 m as the latitude
increases by 1°. This is consistent with the latitudinal variation of the lower limit of mountain permafrost in Asia

Fig. 3 Latitude–altitude distribution of surface air temperaturethat maintains permafrost. The colors or shapes of symbols differ
for each of the nine regions shown in Fig. 1. The red regression line
is calculated using the points that take the lowest altitude at each
latitudePage 6 of 10

has a north–south tilt of 160 m per degree (Matsuoka
2002; 2003).
3.2 Projecting surface air temperature conditions required
to sustain permafrost in Japan

Figure 4 shows a time series of changes in the area of ECP
regions in Japan. As in Fig. 2, it shows the results for nine
domains beginning with the largest ECP region under thecurrent climate conditions. Based on the time series of
future projections, the ECP regions in Japan can be classified into the following three types: (a) long-term sustained: Daisetsu Mountains and Mt. Fuji, (b) Borderline:
Hidaka Mountains and Northern Japan Alps, and (c) In
crisis: other domains.
3.2.1 Long‑term sustained: Daisetsu Mountains and Mt. Fuji

For the Daisetsu Mountains and Mt. Fuji, the ECP
regions are projected to be sustainable over the longterm. This is because there is a large difference between
the mountaintop altitude and the lower limit altitude of
the ECP region under the current climate (Fig. 3). As thesurface air temperature rises in the future, the altitudeof the lower part of the ECP region will also rise. However, for the Daisetsu Mountains and Mt. Fuji, the time atwhich the lower altitude limit for the ECP region is projected to reach the mountain summit (i.e., when the ECP
regions will disappear) depends on the climate scenario.
Currently, the Daisetsu Mountains have the largest ECP
region in Japan; however, the spatial extent of this ECP
region is estimated to have decreased since around 2000,
and it is projected that the ECP regions will disappear
around 2070, regardless of the model used, under the
RCP8.5 scenario (Fig. 4). On the other hand, under the
RCP2.6 scenario, which stabilizes the future global meansurface air temperature rise to about 2 °C, the ensemble average of the ECP regions is projected to remain at
approximately 20 ­km2 by 2100. Under the RCP4.5 scenario, the ensemble average of the ECP regions is projected to remain at approximately 10 ­km2. The results of
RCP8.5 and 2.6 scenarios in the present study are consistent with Yokohata et al. (2021), which used only four
GCMs for the future projection, while those of RCP4.5
were not included in Yokohata et al. (2021).
Due to the low latitude of Mt. Fuji (approximately 35°
N), its ECP regions are relatively small (approximately
8 ­km2 in total based on the 2010 ensemble average).
However, due to its high altitude, the ECP region is projected to persist long into the future. Under the RCP8.5
scenario, the ECP regions on Mt. Fuji are projected todisappear at around 2100 based on the ensemble average.
The ECP regions are projected to be sustained at approximately 2 ­km2 under RCP2.6 and approximately 1 ­km2
under RCP4.5.

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

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Fig. 4 Time sequence of areas in mountainous regions of Japan with surface air temperatures that favor the maintenance of permafrost; (1)Daisetsu Mountains, (2) Hidaka Mountains, (3) Northern Japan Alps, (4) Mt. Fuji, (5) Mt. Shiretokodake, (6) Mt. Shari, (7) Southern Japan Alps, (8) Mt.
Akandake, (9) Mt. Yotei. Historical simulations (black) and future projections for the RCP2.6 (blue), RCP4.5 (green), and RCP8.5 (red) scenarios areshown. Shading indicates the minima and maxima of multi-model projections, and the thick line indicates the multi-model average

3.2.2 Borderline: Hidaka Mountains and Northern JapanAlps

The Hidaka Mountains and the Northern Alps are
domains where ECP regions are currently thought toexist, but which are likely to disappear by 2030 regardlessof the climate scenario (Fig. 4). This is because the lower
altitude limit of the ECP region in the current climate isrelatively close to the altitude of the mountain summit.
In the Hidaka Mountains and the Northern Alps, the
ECP regions are currently projected to be several tens ofsquare kilometers, and it is estimated that they will disappear in the next 10 years. It is projected that the ECP
regions in the Hidaka Mountains and Northern Japan
Alps will be lost due to the rapid increase in surface temperature in the high-latitude regions, which has been
occurring since the 2000s (Fig. 7 in Yokohata et al. 2021).
3.2.3 In crisis: other domains

In the five domains with low concordance with multimodel projections for ECP regions under the current climate (< 20%), including Mt. Shiretokodake, Mt. Shari, theSouthern Japan Alps, Mt. Akan, and Mt. Yotei, the ECP
regions are estimated to decrease rapidly beginning inthe 2000s (Fig. 4) and are projected to disappear before

2020. In these domains, the mountaintop altitudes areeven lower and the areas of the ECP region are smaller
than those for the Hidaka Mountains and the Northern
Alps (approximately several k­ m2), and thus, the rate ofdisappearance is projected to be even faster than those inthese latter two domains.

4 Conclusions
The analysis in this study clarified, for the first time, thedistribution of the temperature environment that sustains permafrost in mountainous areas throughout Japan.
Due to the difficulties associated with in situ observations of permafrost in mountainous areas, the existenceof mountain permafrost has only been reported in a limited number of places, such as in the Daisetsu Mountains,
Mt. Fuji, and Mt. Tateyama in the Northern Japan Alpsof Japan (Higuchi and Fujii 1971; Fukuda and Sone 1992;
Ishikawa and Hirakawa 2000; Fukui and Iwata 2000; Aoyama 2005; Iwahana et al. 2011; Ikeda et al. 2012; Sone and
Watanabe 2018; Sone 2020). However, the findings of thisstudy show that permafrost could exist in other regions,such as the Hidaka Mountains, Mt. Shiretokodake, Mt.
Sharidake, the Southern Japan Alps, Mt. Akandake and
Mt. Yotei (Fig. 2). Our results will therefore aid in theYokohata et al. Progress in Earth and Planetary Science(2022) 9:39

selection of candidate sites for mountain permafrost
observations in Japan.
Furthermore, for the first time, we performed projections of environmental conditions to identify mountainous areas that are well suited to sustaining permafrost
throughout Japan and showed that changes in the possible distribution of permafrost can vary greatly among
regions. At present, the domain with the largest ECP
region is the Daisetsu Mountains in Hokkaido, where thelatitude and altitude are both high and the annual averagetemperature is low. Predictions for the future ECP areaof Daisetsuzan will differ depending on the climate scenario that is applied. Under RCP8.5, where greenhousegas concentrations continue to increase, the ECP region
will disappear in the second half of the twenty-first century. However, under the RCP2.6 or the RCP4.5 scenarios, where the future global mean surface air temperaturechange is stabilized at 2 °C and < 3 °C, respectively, the
ECP regions are projected to remain at approximately
20 ­km2 and 10 ­km2, respectively. Similarly, at Mt. Fuji,
which has a low latitude but a high altitude, the ECP
regions are projected to disappear around 2100 under
the RCP8.5 scenario. However, if the RCP2.6 or RCP4.5
scenarios are realized, the ECP regions at Mt. Fuji areprojected to remain at approximately 2 ­km2 or 1 ­km2,
respectively. Successful attempts to stabilize the climate
in the twenty-first century may leave a small area capableof sustaining permafrost.
Although ECP regions are projected to persist in the
Daisetsu Mountains and Mt. Fuji after the latter halfof the twenty-first century, the risk of disappearance
is higher for the ECP regions in other domains. In the
Hidaka Mountains and the Northern Japan Alps, where
the latitude and mountaintop altitude are relatively high,the ECP regions are projected to disappear in the nextten years or so, irrespective of the climate scenario. In theother five domains, Mt. Shiretokodake, Mt. Sharidake,
Southern Japan Alps, Mt. Akandake, and Mt. Yotei,
most of the ECP regions are also projected to disappear
shortly. Indeed, the mode of seasonal freezing in these
domains may already have changed significantly.
The analysis presented in this study does not consider
detailed factors such as topography (slope direction andslope angle), soil properties (porosity and permeability),
vegetation, and meteorological conditions (precipitation,
local wind direction and snow cover), which are known
to play important roles in determining the distribution of
permafrost (e.g., Ishikawa and Hirakawa 2000). Using a
30-year average as the calculation period for the freezing
and thawing indices, hysteresis in surface air temperaturechanges is considered in our estimates for the ECP region(see Methods). However, the response of permafrost tochanges in climatic conditions may occur over a much

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longer timescale (Saito et al. 2014), and changes in permafrost distributions may be slower than those shown in
Fig. 4.
The climatic conditions under which permafrost canexist in Japan’s mountainous regions are expected todisappear, indicating the importance of monitoring theenvironment and developing adaptation measures for climate change. Specifically, monitoring the phenology anddistribution of alpine vegetation is considered importantfor clarifying the effects of permafrost thawing on alpine
ecosystems (Yokohata et al. 2021). Accurately monitoring environmental changes in mountainous areas is also
vital for addressing the problem of increases in the scale
and frequency of flow slopes and landslides (Krautblatter
et al. 2013; Lacelle et al. 2015; Patton et al. 2019). Thus,
applying a technique to determine the ground-surface
displacement in detail by using satellite data is considered necessary (Iwahana et al. 2016; Abe et al. 2020). In
addition to accurately monitoring changes in mountainenvironments, providing local governments with appropriate measures to prepare for major future environmental changes is an essential issue for future studies.
Abbreviations
CDP: Climate-driven permafrost; ECP: Environmentally conditioned permafrost; IF: Intermittent freezing; SF: Seasonal freezing.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00498-z.
Additional file 1, Information for locations of permafrost occurrence
based on observations (Table S1), distribution of altitude and annual
mean surface air temperature in Japan (Figure S1), and time series ofannual mean surface air temperatures at the locations where we detectedenvironmentally conditioned permafrost regions (Figure S2).
Acknowledgements
We thank Masao Uchida, Yusuke Satoh for their assistance with field observations in the Daisetsu Mountains. We also thank FORTE Science Communications for proofreading the English used in this manuscript.
Author contributions
T.Y. and K.S. proposed the topic and conceived and designed the study. G.I.
provided permafrost observation data. N.N.I. contributed to preparing thebias-corrected and downscaled climate model outputs. T.M. contributed todata analysis and drawing figures. All authors discussed and commented on
the outcomes. All authors have read and approved the final manuscript.
Funding
This work is supported by TOUGOU, an “Integrated Research Programfor Advancing Climate Models” administered by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant Number
JPMXD0717935715). Acquisition of permafrost observation data was supported by JSPS KAKENHI (18H03353). This work was supported in part by
the Arctic Challenge for Sustainability II (ArCS II) (Program Grant Number
JPMXD1420318865). This work was also supported in part by the Climate
Change Adaptation and Decarbonized and Sustainable Society Research
Program at the National Institute for Environmental Studies.

Yokohata et al. Progress in Earth and Planetary Science(2022) 9:39

Availability of data and materials
Data sharing is not applicable to this article as no datasets were generated oranalyzed during the current study. Please contact the author for data requests.
The bias-corrected meteorological dataset is available at https://​www.​nies.​go.​jp/​doi/​10.​17595/​20210​501.​001-e.​html.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Earth System Division, National Institute for Environmental Studies, 16‑2,
Onogawa, Tsukuba 305‑8506, Japan. 2 International Arctic Research Center,
University of Alaska Fairbanks, Fairbanks, USA. 3 Research Institute for Global
Change, Japan Agency for Marine-Earth Science and Technology, Yokohama,Japan. 4 Faculty of Life and Environmental Sciences, University of Tsukuba,
Tsukuba, Japan. 5 International Affairs and Research Development Office,National Institute of Polar Research, Tokyo, Japan.
Received: 15 April 2022 Accepted: 29 June 2022

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