Permafrost regions account for about 22% of the exposed land area in the
Northern Hemisphere (Obu et al., 2019). As one of physical characteristics in the
cold environment, permafrost is sensitive to climate change. During the past decades,
permafrost in high latitude and high-altitude regions shows obvious degradation,
which is indicated by increasing ground temperature, deepening active layer,
shrinking of permafrost area, and development of thermokarst features (Biskaborn
et al., 2019).
Permafrost is distributed beneath the earth’s surface. Permafrost can regulate the
regional water cycle and ecology from several mechanisms. First, as a weak
impermeable layer, permafrost can prevent water vertical infiltration and increase
the surface soil water content. Second, the freeze-thaw cycles of the active layer can
store excess water from summer rainfall as ice during winter, and the melting of this
ice can supply soil water in the following summer. Third, ground ice melting can
provide soil water for plant growth (Sugimoto et al., 2003). Permafrost regions also
store a large amount of soil organic carbon, which is almost twice as the carbon
currently contained in the atmosphere (Mishra et al., 2021). These carbon pools have
been gradually accumulated and preserved during the past thousands of years due to
the low-temperature limiting the microbial decomposition of organic matter. The
permafrost degradation may remobilize these carbon pools by releasing greenhouse
gases into the air. This process contributes one of the great uncertainties in the
terrestrial carbon cycle feedback (Schuur et al., 2015). In addition, permafrost regions
also store a large number of pollutants and heavy metals (e.g., mercury) which have
been sequestrated for a long time. Permafrost degradation poses environmental risksand thawing permafrost may release these biological or
chemical substances that can affect human health (Schuster
et al., 2018; Miner et al., 2021).
To address the issues on how permafrost environment is
changing, to what extent the changing permafrost may affect
the hydrology, ecology, carbon cycle, and pollutants, eleven
multi-discipline studies are collected in this special topic on
permafrost environment changes in a warming climate.
Permafrost regions have been warming at two to three
times the global average (Hu et al., 2021). Using the monthly
air temperature reanalysis dataset from the Climate Research
Unit (CRU, University of East Anglia), it was found that the
air freezing index in the Mongolian Plateau decreased by
4.1 C d yr-1, and the air thawing index increased by
2.3 C d yr−1 during 1901–2019. The northern permafrost
regions showed large variabilities in freezing and thawing
index than the southern non-permafrost regions (Ma et al.
). Based on the meteorological station records from 1957 to
2019, the annual mean air temperature has increased by 0.
031–0.039°C yr−1 in the hinterland of the Qinghai-Tibet
Plateau. The ground temperature within the active layer at
1 m depth increased at an average rate of 0.05°C yr−1 (Zhou
et al.). Along with climate warming, frequency of extreme
events also changed. On the Qinghai-Tibet Plateau, the
warmth indices such as warm days, warm nights, summer
days, and tropical nights increased at rates of 1.1, 1.6, 1.4 and
0.3 days per decade from 1960 to 2016. Meanwhile, cold
indices including the number of cool days, cool nights, ice
days, and frost days decreased significantly (Gong et al.).
These results confirmed the rapid warming of the
permafrost environment during the past decades and also
provide useful data to understand the changing patterns and
future projections of permafrost.
Three studies (i.e., Yang et al.; Rossi et al.; Polyakov et al.
) examined the detecting permafrost and soil mapping
method in permafrost regions. The equivalent anti-flux
opposing coils were used to eliminate the blind area for
the transient electromagnetic method, and the results
showed that this method solved the problem of the
shallow detection blind area, eliminated the interference
caused by the primary field, and improved the horizontal
and vertical resolutions (Yang et al.). In the Russian Arctic,
geophysical and geocryological methods including
landscape microzonation, borehole drilling, ground
temperature measurements, and geoelectric surveys were
employed to investigate the active layer thickness. The
results showed that the multidisciplinary approach can be
also useful for other areas (Rossi et al.). In permafrost
regions, soil type is one of the most fundamental
properties because it is an important parameter for Earth
System Models as well as the carbon stocks estimation.
However, due to the harsh natural conditions, field
investigation of soil types is usually costly and difficult.
Using the unmanned aerial vehicle (UAV) imaging data in
the Lena River Delta, classical soil sections,
geomorphological observation, and determination of the
main chemical parameters of soils are presented.
Although accurate mapping of soil types should be based
on chemical analysis, this result suggests that the highresolution
soil-geomorphological maps based on the
Geographic Information System and UAV data are useful
for the mapping of soil types under the high variability of the
watershed dan cryogenic landscapes (Polyakov et al.).
Permafrost significantly affects ecology and hydrology
(Woo et al., 2008). A review paper in this topic summarizes
that soil water potential is widely used to describe the energy
state of liquid water. The movement of liquid water in the soil
is mainly determined by soil matric potential. The process of
ice lenses development in permafrost has been explained by
mathematical models, however, existing models might be too
simplified (Fu et al.). Therefore, new model development for
ice formation for micro landscapes is still largely needed. To
investigate the effects of hydrology on peat permafrost and
carbon process, a process-based model, i.e., HPM-Arctic, was
used the simulate the past and future changes in a peatland
ecosystem in the Canadian Arctic. The results showed that the
regional hydrology and basin characteristics strongly
determined peat accumulation history and its future
changes in organic carbon stocks under different climate
scenarios (Treat et al.). For the carbon cycle in the Arctic
permafrost, a pilot study showed that extensively grazing by
large animals can cool the ground temperature by modifying
ground cover properties. In addition, the soil organic carbon
content is also higher in the extensively grazing sites than that
of non-grazing sites, which is likely attributed to the higher
carbon input (Windirsch et al.).
Heavy metals are anthropogenic contaminants that can be
transported for long distances. Due to the atmospheric
circulation and deposition, large heavy metals have been
transported to the Arctic, Antarctic, as well as the Qinghai-
Tibet Plateau. A review paper in this topic issue pointed out that
heavy metals on the Qinghai-Tibet plateau are mainly from
surrounding heavily-polluted regions. The shrinkage of the
cryosphere may increase the release of these heavy metals in
the future. This work highlights the importance of heavy metals
in permafrost environments.
This special topic has collected the studies of permafrost
regions located in the Arctic, Mongolia, and the Qinghai-
Tibet Plateau. The results deepen our understanding of
changing trends of climate and permafrost, interactions
among permafrost, hydrology, ecology, carbon cycle, and
risks of heavy metals. We hope this special topic could
provide valuable references to the researchers with
relevant interest and play an active role in promoting the
research of permafrost changes and their environmental
impacts.