TCThe CryosphereTCThe Cryosphere1994-0424Copernicus GmbHGöttingen, Germany10.5194/tc-9-479-2015Editorial: Organic carbon pools in permafrost regions on the Qinghai–Xizang
(Tibetan) PlateauMuC.ZhangT.tjzhang@lzu.edu.cnWuQ.PengX.CaoB.ZhangX.CaoB.ChengG.https://orcid.org/0000-0002-2758-6211College of Earth and Environmental Sciences, Lanzhou University,
Lanzhou Gansu 730000, ChinaState Key Laboratory of Frozen Soil Engineering, Cold and Arid
Regions Environmental and Engineering Research Institute, CAS, Lanzhou Gansu
730000, ChinaT. Zhang (tjzhang@lzu.edu.cn)6March20159247948627August201429September201427January201511February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.the-cryosphere.net/9/479/2015/tc-9-479-2015.htmlThe full text article is available as a PDF file from https://www.the-cryosphere.net/9/479/2015/tc-9-479-2015.pdf
The current Northern Circumpolar Soil Carbon Database did not include organic
carbon storage in permafrost regions on the Qinghai–Xizang (Tibetan) Plateau
(QXP). In this study, we reported a new estimation of soil organic carbon
(SOC) pools in the permafrost regions on the QXP up to 25 m depth using a
total of 190 soil profiles. The SOC pools were estimated to be
17.3 ± 5.3 Pg for the 0–1 m depth, 10.6 ± 2.7 Pg for the
1–2 m depth, 5.1 ± 1.4 Pg for the 2–3 m depth and
127.2 ± 37.3 Pg for the layer of 3–25 m depth. The percentage of SOC
storage in deep layers (3–25 m) on the QXP (80 %) was higher than that
(39 %) in the yedoma and thermokarst deposits in arctic regions. In
total, permafrost regions on the QXP contain approximately 160 ± 87 Pg
SOC, of which approximately 132 ± 77 Pg (83 %) stores in
perennially frozen soils and deposits. Total organic carbon pools in
permafrost regions on the QXP was approximately 8.7 % of that in northern
circumpolar permafrost region. The present study demonstrates that the total
organic carbon storage is about 1832 Pg in permafrost regions on northern
hemisphere.
Introduction
Soil organic carbon (SOC) storage in permafrost regions has received
worldwide attention due to its direct contribution to the atmospheric
greenhouse gas contents (Ping et al., 2008a; Tarnocai et al., 2009; Zimov et
al., 2009). Climate warming will thaw permafrost, which can cause previously
frozen SOC become available for mineralization (Zimov et al., 2006).
Permafrost has potentially the most significant carbon-climate feedbacks not
only due to the intensity of climate forcing, but also the size of carbon
pools in permafrost regions (Schuur et al., 2008; Mackelprang et al., 2012;
Schneider von Deimling et al., 2012).
Recently, carbon stored in permafrost regions has created many concerns
because of the implication on global carbon cycling (Ping et al., 2008; Burke
et al., 2012; Zimov et al., 2006; Michaelson et al., 2013; Hugelius et al.,
2013). It has been estimated that permafrost regions of circum-Arctic areas
contain approximately 1672 Pg of organic carbon, which include 495.8 Pg for
the 0–1 m depth, 1024 Pg for the 0–3 m depth and 648 Pg for 3–25 m
depth. Based on newly available regional soil maps, the estimated storage of
SOC in 0–3 m depth is estimated to 1035 ± 150 Pg (Hugelius et al.,
2014), about 1 % higher than the previous estimate by Tarnocai et
al. (2009). The thawing of permafrost would expose the frozen organic carbon
to microbial decomposition, and thus may initiate a positive permafrost
carbon feedback on climate (Schuur et al., 2008). The strength and timing of
permafrost carbon feedback greatly depend on the distribution of SOC in
permafrost regions. Therefore, understanding soil carbon storage in
permafrost regions is critical for better predicting future climate change.
However, the present knowledge of SOC pool in permafrost regions only limited
to the circum-Arctic areas. Little is known about the SOC pools in the
low-altitude permafrost regions.
The Qinghai–Xizang (Tibetan) Plateau (QXP) in China has the largest extent of
permafrost in the low-middle latitudes of the world, with permafrost regions
of about 1.35 × 106 km2 and underlying ∼ 67 %
of the QXP area (Ran et al., 2012). It has been suggested that SOC in
permafrost regions on the QXP was very sensitive to global warming, due to
the permafrost characteristics of high temperature
(<∼ 2.0∘), thin thickness (< 100 m) and unstable
thermal states (Cheng and Wu, 2007; Li et al., 2008; Wu and Zhang, 2010).
Mean annual permafrost temperatures at 6.0 m depth increased by a range of
0.12∘ to 0.67∘ from 1996 to 2006 (Wu and Zhang, 2008), and
increased ∼ 0.13∘ from 2002 to 2012 (Wu et al., 2015). Active
layer thickness increased, on average, approximately
∼ 4.26 cm yr-1 along the Qinghai–Tibetan Highway from 2002 to
2012 (Wu et al., 2015). In addition, the carbon stored in permafrost area was
labile and a great part of the carbon was mineralizable (Mu et al., 2014; Wu,
et al., 2014).
Some studies have been conducted on SOC pools in 0–1 m depth on the QXP
(Wang et al., 2002, 2008; Yang et al., 2008, 2010; Liu et al., 2012; Wu et
al., 2012). It was estimated that total SOC for the top 0.7 m was about
30–40 Pg in the grassland of the plateau. The disagreement among the
studies on the SOC pools was attributed to the limited sampling points and
the quality of the SOC data gathered to date. Despite the importance of SOC
in permafrost areas, there are still few reports to the SOC storage in
permafrost regions of the QXP. So far, the current Northern Circumpolar Soil
Carbon Database does not include the SOC in permafrost regions on the QXP
(Tarnocai et al., 2009).
Perennially frozen soils are important earth system carbon pools because of
their vulnerability to climate change (Koven et al., 2011). Some of the
movement of SOC from surface to few meter depth is accomplished through
cryoturbation (Bockheim et al., 1998), which is caused by cracking due to
soil freeze-thaw cycles and by soil hydrothermal gradients (Ping et al.,
2008b). It was reported that the total yedoma region contains
211 + 160/-153 Pg C in deep soil deposits (Strauss et al., 2013).
Current studies have shown the importance of deep organic carbon in
permafrost regions and its feedback with climate change (Hobbie et al., 2000;
Davidson and Janssens, 2006; Schuur et al., 2009). Deep organic carbon can be
more sensitive to temperature increasing compared with that in the active
layer (Waldrop et al., 2010). Therefore, it is essential to study the
distribution of organic carbon content in deep layers of permafrost regions.
For the top layer, important factors controlling SOC pools are vegetation
type and climate (Jobbagy and Jackson, 2000). The vegetation type and climate
conditions related closely to each other on the QXP (Wang et al., 2002). Thus
it is possible to calculate the SOC pools at 0–2 m depth according to the
area of vegetation type (Chinese Academy of Sciences, 2001) in the permafrost
regions (LIGG/CAS, 1988). For deep layers, the geomorphology and lithological
conditions play an important role in the distribution of SOC pools (Hugelius
et al., 2013). Thus it is reasonable to estimate the SOC pools at 2–25 m
depth according to the area of Quaternary geological stratigraphy in
permafrost regions on the QXP.
The objective of this study is to assess the SOC pools in permafrost regions
on the QXP, based on the published data and new field sampling through deep
drilling from this study. The new estimation focuses on the permafrost
regions and includes deeper layers, down to 25 m. SOC storages of the
plateau were estimated using the published data of 190 soil profiles and 11
deep sampling sites from this study in combination with the vegetation map,
permafrost map and geological stratigraphy map of the QXP (Figs. 1–3). The
result would update current estimation of surface organic carbon pools and
deep organic carbon storage in permafrost regions of the QXP, which can
provide new insights in permafrost carbon on the global scale.
Organic carbon pools in the 0–1 m depth with different vegetation
type on the QXP.
VegetationReferencesAnalytical StudySite dataArea SOC stock SOC storagetypesmethodsarea(n)(× 106 km2)(kg m-2)(Pg)AlpineYang et al., (2010)Wet oxidationQXP220.2249.3 ± 3.910.7 ± 3.8meadowOhtsuka et al. (2008)Heat combustionQXP113.7Dorfer et al. (2013)Heat combustionQXP210.4Mu et al. (2013)Heat combustionHHRB110.006539.0 ± 17.50.3 ± 0.1Liu et al. (2012)Wet oxidationSLRB–420.0138.7 ± 1.20.1 ± 0.02Alpine Yang et al. (2010)Wet oxidationQXP330.7723.7 ± 2.05.3 ± 2.8steppeWu et al. (2012)Wet oxidationwestern QXP527.7 ± 3.2Liu et al. (2012)Wet oxidationSLRB–429.2 ± 1.1Alpine Wu et al. (2012)Wet oxidationwestern QXP250.1753.3 ± 1.50.7 ± 0.3desertLiu et al. (2012)Wet oxidationSLRB∼ 424.4 ± 0.7
Location of sampling sites on the QXP, shown on the background of
QXP permafrost distribution (blue points were sampling sites in Yang et al.,
2010; orange points were in Wu et al., 2012; red box was Shule River basin
(SLRB) in Liu et al., 2012; black box was Heihe River basin (HHRB) in Mu et
al., 2013).
Location of sampling sites on the QXP, shown on the background of
QXP vegetation atlas at a scale of 1: 400 000 (Chinese Academy of Sciences,
2001). (Sampling sites were the same as those shown on the background of
permafrost distribution.)
Materials and methodsSoil carbon database in previous reports
The soil carbon databases in 0–1 m depth were retrieved from the previous
reports (Yang et al., 2010; Liu et al., 2012; Wu et al., 2012; Dorfer et
al.,2013; Mu et al., 2013) (Table 1). We integrated the databases from Yang
et al. (2010), Dorfer et al. (2013) and Ohtsuka et al. (2008) because these
studies were all performed in the middle and eastern parts of the QXP. The
data of Wu et al. (2012), Liu et al. (2012) and Mu et al. (2013) in the soil
carbon database in 0–1 m depth were calculated separately, since their
study regions of western QXP, Shule River basin (SLRB) and Heihe river
basin (HHRB) belonged to the isolated permafrost zone and the climate
conditions differed greatly with the continuous permafrost zones of the QXP.
The total organic carbon pools in 0–1 m depth in permafrost regions on the
QXP were calculated using 190 profile sites from published sources.
Field sampling
To calculate the deep carbon pools (2–25 m) in permafrost regions, 11
boreholes on the QXP were drilled from 2009 to 2013 (Fig. 1). Geographic
location for the 11 boreholes, together with the active layer depth, sampling
depth, vegetation type, geological stratigraphies, SOC contents, bulk
density, water contents and soil texture are provided in the supplement
materials.
Location of sampling sites on the QXP, shown on the background of
the QXP Quaternary geological map. (Sampling sites were the same as those shown on
the background of permafrost distribution.)
The deep sampling sites were mainly located in three vegetation types of
alpine meadow, alpine stepper and alpine desert (Fig. 2). Three sampling
sites (KXL: KaiXin Ling, HLH-1: HongLiang He-1, HLH-2:
HongLiang He-2) were located in the vegetation type of alpine
steppe. Another site was near ZhuoEr Hu (ZEH) in Kekexili,
with soil formed from lacustrine deposits. It was typical alpine desert and
perennially frozen, containing less amounts of organic carbon. Five sampling
sites (KL150: KunLun150, KL300: KunLun300, KL450:
KunLun450, WDL: WuDao Liang, XSH: XiuShui He) were
located in the vegetation type of alpine meadow. In addition, two sites in
permafrost regions of the Heihe river basin (HHRB: Heihe-1, Heihe-2) with
vegetation type of alpine meadow were rich in organic carbon with high soil
water contents (Mu et al., 2013).
The deep sampling sites were mainly distributed in three geological
stratigraphies: ZEH, WDL, XSH, Heihe-1 and Heihe-2 were in the Quaternary
stratigraphy, KL150, KL300, KL450, HLH-1 and HLH-2 were in the Triassic
stratigraphy, and KXL was in the Permian stratigraphy (Fig. 3).
Analytical methods
For SOC analyses, the homogenized samples were quantified by dry combustion
on a vario EL elemental analyzer (Elemental, Hanau, Germany). During
measurement, 0.5 g dry soil samples were pretreated by HCl (10 mL
1 mol L-1) for 24 h to remove carbonate (Sheldrick, 1984). Bulk
density was determined by measuring the volume (length, width, height) of a
section of frozen core, and then drying the segment at 105∘ (for
48 h) and determining its mass.
Calculation of soil carbon pools
For the stock of soil organic carbon (SSOC, kg m-2), it was calculated
using the Eg. (1) (Dorfer et al., 2013):
SSOC=C×BD×T×(1-CF),
where C was the organic carbon content (wt %), BD was the bulk density
(g cm-3), T was the soil layer thickness and CF was the coarse
fragments (wt %). Using this information, the SSOC was calculated for the
0–1, 1–2, 2–3 and 3–25 m depths, respectively. Then, SOC storage (Pg)
was estimated by multiplying the SSOC at different depth by the distribution
area.
Permafrost organic carbon storage to the depth of 25 m on the QXP.
For the organic carbon storage in 0–1 m depth, the reported SOC densities
data of 190 sampling sites were collected through their distribution in
permafrost regions (Fig. 1). The area of alpine meadow, alpine steppe and
alpine desert in permafrost regions was calculated through overlaying the
vegetation map over the QXP permafrost regions (Fig. 2). For the organic
carbon storage in 1–2 m depth, the organic carbon densities of 11 boreholes
were extrapolated to the located vegetation-type area.
For the organic carbon storage in 2–3 and 3–25 m depths, the area of
permafrost regions in the Quaternary, Triassic and Permian stratigraphies on
the QXP was calculated through overlaying the distribution of geological
stratigraphies over the permafrost map (Fig. 3). The organic carbon pools of
2–3 and 3–25 m depth were estimated through deep organic carbon densities
multiplied by the area of geological stratigraphies. The three geological
stratigraphies had thick sediments of about 25 m (Fang et al., 2002, 2003;
Qiang et al., 2001). As for other geological stratigraphies, the poor soil
development was reported and soil thickness was usually less than 3 m (Wu et
al., 2012; Yang et al., 2008; Hu et al., 2014). Thus other stratigraphies
were not considered in the estimation of deep organic carbon pools in the
permafrost regions.
ResultsOrganic carbon pools in the 0–1 m depth
Based on the vegetation data on the QXP (Figs. 1, 2), the area of permafrost
regions in the alpine meadow, alpine steppe and alpine desert are
0.302 × 106 km2, 0.772 × 106 km2 and
0.175 × 106 km2, respectively, with a total area of
approximately 1.249 × 106 km2.
Organic carbon storage of the permafrost regions in the 0–1 m depth on the
QXP was approximately 17.3 ± 5.3 Pg, of which approximately
11.3 ± 4.0 Pg (65 %) in the alpine meadow, 5.3 ± 2.8 Pg
(31 %) in the alpine steppe, and 0.7 ± 0.3 Pg (4 %) in the
alpine desert, respectively (Table 1). There were great variations in SOC
contents among the sites under alpine meadow area. SOC store in the HHRB
(39.0 ± 17.5 kg m-2) was much higher than that of most sites in
the predominately continuous permafrost zone on the QXP. In contrast, the SOC
stores showed little variation over the sites in the alpine steppe and alpine
desert areas, with the ranges of 6.9 ± 3.6 and
3.9 ± 1.5 kg m-2, respectively.
Distribution of deep organic carbon
According to the distribution of sampling sites at the geological
stratigraphies, for the Quaternary stratigraphy, average SOC contents at 2–3
and 3–25 m depths were 0.8 ± 0.6 and 0.8 ± 0.7 %. For the
Triassic stratigraphy, average SOC contents at 2–3 and 3–25 m depths were
1.1 ± 0.3 and 1.2 ± 0.6 %. For the Permian stratigraphy,
average SOC contents at 2–3 and 3–25 m depths were 1.5 ± 0.4 and
1.1 ± 0.3 %. As for the permafrost regions in HHRB, the SOC
contents (Heihe-1, Heihe-2) were higher than those of predominately
continuous permafrost zone on the QXP, with a range of 5.1 ± 3.7 and
2.7 ± 2.4 % to depth of 19 m. SOC contents decreased with depth in
most deep boreholes, while SOC contents in deeper layers were higher than
those in the top layer at the XSH, KL150 and KL300 (Fig. 4).
With the deep soil data, a relationship between SOC contents (SOC %) and
soil depth (h) in deep soils of permafrost regions can be characterized by
a power Eq. (2) (Fig. 4):
SOC%=14.11h-1.20(R2=0.68,p<0.01,n=362).
Deep organic carbon pools
Based on the Quaternary stratigraphies data in permafrost regions of the QXP
(Fig. 3), the area of permafrost regions in the Quaternary, Triassic and
Permian stratigraphies are 0.194 × 106,
0.238 × 106 and 0.135 × 106 km2
respectively, with a total area of approximately
0.567 × 106 km2, about 45 % of permafrost regions on
the QXP.
Distributions of soil organic carbon contents in deep soils in
permafrost regions on the QXP.
Organic carbon storages in permafrost regions on the QXP were approximately
10.6 ± 2.7 Pg in the 1–2 m depth, 5.1 ± 1.4 Pg in the 2–3 m
depth and 127.2 ± 37.3 Pg in deep depth of 3–25 m (Table 2). In
total, it contains approximately 160 ± 87 Pg of organic carbon at
depth of 25 m in permafrost regions on the QXP.
Active layer thickness on the QXP varies from 0.8 to 4.6 m, and in most
regions, active layer thickness was about 2 m (Cheng and Wu, 2007; Wu and
Zhang, 2008; Zhao et al., 2010; Wu et al., 2012). Thus we consider the upper
2 m as the active layer. According to this depth, the organic carbon storage
in permafrost layers of 132 ± 77 Pg was approximately five times of
that (28 ± 6 Pg) in the active layer.
SOC storages in Quaternary, Triassic and Permian stratigraphies were
31 ± 17, 69 ± 53 and 32 ± 20 Pg at depth of 2–25 m,
respectively. More than a half of organic carbon is stored in permafrost
layers which belonged to the Triassic stratigraphy.
Discussions
Our estimates indicate that organic carbon storage in permafrost regions in
the 0–1 m depth on the QXP was approximately 17.3 ± 5.3 Pg. However,
previous soil carbon pools on the alpine grasslands of the whole QXP were
estimated to be 33.5 Pg of 0–0.75 m (Wang et al., 2002), and 10.5 Pg of
0–0.30 m (Yang et al., 2010). The difference, in large part, between our
new estimate and previous reports can be explained as follows: (i) area of
vegetation types in permafrost regions was recalculated. The area of
permafrost regions of about 1.249 × 106 km2 was smaller
than that of Wang et al. (2002) (1.63 × 106 km2) and Yang
et al. (2010) (1.26 × 106 km2). (ii) Carbon density data
of sampling sites located in permafrost regions was collected. The
integration of carbon data from the results of recent publications (Ohtsuka
et al., 2008; Dorfer et al., 2013; Wu et al., 2012) and our field data
resulted in a higher carbon density than those of previous reports (Wang et
al., 2002; Yang et al., 2010). (iii) The regions of SLRB and HHRB were not
considered in previous SOC pool estimate. The organic carbon storages of
0.43 ± 0.11 Pg in SLRB and 0.25 ± 0.11 Pg in HHRB were added in
the present study.
It is worth to mention that there were wide variations in organic carbon
contents in permafrost regions on the QXP in previous reports (Wang et al.,
2002; Yang et al., 2010; Liu et al., 2012; Wu et al., 2012; Dorfer et al.,
2013; Ohtsuka et al., 2008; Mu et al., 2013). A possible explanation is the
spatial heterogeneity of SOC contents in permafrost regions of the QXP. In
addition, the different analytical methods may also contribute to the
differences of carbon contents (Table 1). It has been demonstrated that if
taking the dry combustion method as standard, the recovery of organic carbon
was 99 % for wet combustion and 77 % for the Walkley–Black procedure (Kalembasa and
Jenkinson, 1973; Nelson and Sommers, 1996).
The SOC stocks at 0–1 m depth (17.3 kg m-2) in the alpine meadow on
the QXP is higher than that in subarctic alpine permafrost
(0.9 kg m-2) (Fuchs et al., 2014), and similar to that of the
lowland and hilly upland soils in the North American Arctic region
(55.1, 40.6 kg m-2) (Ping et al., 2008a). It implies
that SOC of the alpine meadow in permafrost regions has a large proportion in
permafrost carbon pools. The SOC contents at 0–1 m depth
(3.9 ± 1.5 kg m-2) in the alpine desert on the QXP was similar to that (3.4, 3.8 kg m-2) in rubble-land and mountain
soils in the North American Arctic region (Ping et al., 2008a). These results
suggest that the SOC stocks are closely related to the vegetation type in the
permafrost regions.
SOC decreases with the depth on the QXP (Fig. 4), which is in good agreement
with those reported in circum-Arctic regions (Strauss et al., 2013; Zimov et
al., 2006). This could be explained by the dynamics of Quaternary deposit and
SOC formation in permafrost regions (Strauss et al., 2013). However, the
organic carbon contents of deep layers in some sites (XSH, KL150 and KL300)
were higher than those in the top layers (Fig. 4), which may be caused by the
cryoturbation and sediment burying process (Ping et al., 2010), and
Quaternary deposits following the uplift of Tibetan Plateau (Li et al., 1994,
2014). Overall, SOC decreases exponentially with depth (Eq. 1) in permafrost
regions on the QXP, which is in agreement with results from other regions
(Don et al., 2007). Certainly, more efforts are still needed in studying the
distribution of deep organic carbon density in permafrost regions.
In the present study, it is the first time to study the deep organic carbon
in permafrost regions, and quantify the carbon storage below 1.0 m depth on
the QXP. The mean SOC content of 11 boreholes in permafrost regions on the
QXP (2.5 wt %) was similar to that in the yedoma deposits (3.0
wt %) (Strauss et al., 2013), and that of lowland steppe-tundra soils in
Siberia and Alaska (2.6 wt %) (Zimov et al., 2006). Since it has been
pointed out that yedoma deposits contain a large amount of organic carbon, it
would be reasonable to infer that deep soil carbon in permafrost regions on
the QXP may also have a great contribution to carbon pools. Our estimations
indicate that the soils on the QXP contains 33.0 ± 13.2 Pg of organic
carbon in the top 3.0 m of soils, with an additional 127.2 ± 37.3 Pg
C distributed in deep layers (3–25 m) of the Quaternary, Triassic and
Permian stratigraphies in permafrost regions. In northern circumpolar
permafrost region, 1024 Pg of organic carbon was in the 0–3 m depth and
648 Pg (39 %) of carbon was stored in deep layers of yedoma and deltaic
deposits (Tarnocai et al., 2009). The percentage of SOC storage in deep
layers (3–25 m) on the QXP (80 %) is much higher than that (39 %)
in the yedoma and thermokarst deposits in Siberia and Alaska. This could be
explained as that the paleoenvironment of the QXP was wet and warm, or
lacustrine sediment in most regions (Zhang et al., 2003; Lu et al., 2014),
which always links to the well formation of soil organic matter (Kato et al.,
2004; Piao et al., 2006; Chen et al., 1990).
In total, there is approximately 160 ± 87 Pg of organic carbon stored
at 0–25 m depth in permafrost regions on the QXP, which would update the
total carbon pools to 1832 Pg in permafrost regions of northern hemisphere.
The total carbon pools on the QXP permafrost regions account for
approximately 8.7 % of the total carbon pools in permafrost regions in
northern hemisphere. Since the permafrost region on the QXP was about 6 %
of the northern permafrost area (Ran et al., 2012), it could be seen that SOC in
permafrost regions on the QXP should be paid more attention in the future
studies.
Conclusions
According to the organic carbon data in previous analysis
and field exploration of deep boreholes in permafrost regions, the organic
carbon storages in permafrost regions on the QXP were estimated to
approximately 17.3 ± 5.3 Pg in the 0–1 m, 10.6 ± 2.7 Pg in
the 1–2 m, 5.1 ± 1.4 Pg in the 2–3 m and 127.2 ± 37.3 Pg in
deep depth of 3–25 m.
The percentage of SOC storage in deep layers (3–25 m) of permafrost
regions on the QXP was 80 %, which was higher than that in the yedoma and
thermokarst deposits in Siberia and Alaska.
In total, organic carbon pools in permafrost regions on the QXP are
approximately 160 ± 87 Pg, of which 132 ± 76 Pg occurs in
permafrost layers. The total carbon pools in permafrost regions in northern
hemisphere are now updated to 1832 Pg.
The Supplement related to this article is available online at doi:10.5194/tc-9-479-2015-supplement.
Acknowledgements
This work was supported by the National Key Scientific Research Project
(Grant 2013CBA01802), National Natural Science Foundation of China (Grants
91325202, 41330634), and the Open Foundations of State Key Laboratory of
Cryospheric Sciences (Grant SKLCS-OP-2014-08) and State Key Laboratory of
Frozen Soil Engineering (Grant SKLFSE201408). The authors gratefully
acknowledge the reviewers, Gustaf Hugelius and Chien-Lu Ping, as well as the
editor, Steffen M. Noe, for their constructive comments and suggestions.
Edited by: S. M. Noe
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