Chinese Science Bulletin
© 2008
SCIENCE IN CHINA PRESS
Springer
A possible important CO2 sink by the global water cycle
LIU ZaiHua1,2†, Wolfgang DREYBRODT3 & WANG HaiJing4
1
The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang
550002, China;
2
Karst Dynamics Laboratory, Ministry of Land and Resources, Guilin 541004, China;
3
Institute of Experimental Physics, University of Bremen, Bremen 28334, Germany;
4
School of Geographical Sciences, Southwest University, Chongqing 400715, China
The locations, magnitudes, variations and mechanisms responsible for the global CO2 sink are uncertain and under debate. Here, we show, based on theoretical calculations and evidences from field
monitoring results, that there is a possible important CO2 sink (as DIC-dissolved inorganic carbon) by
the global water cycle. The sink constitutes up to 0.8013 Pg C/a (or 10.1% of the total anthropogenic
CO2 emission, or 28.6% of the missing CO2 sink), and is formed by the CO2 absorption of water and
subsequent enhanced consumption by carbonate dissolution and aquatic plant photosynthesis. Of the
sink, 0.5188 Pg C/a goes to sea via precipitation over sea (0.2748 Pg C/a) and continental rivers (0.244
Pg C/a), 0.158 Pg C/a is released to the atmosphere again, and 0.1245 Pg C/a is stored in the continental
aquatic ecosystem. Therefore, the net sink could be 0.6433 Pg C/a. This sink may increase with the
global-warming-intensified global water cycle, the increase in CO2 and carbonate dust in atmosphere,
and reforestation/afforestation, the latter increasing soil CO2, and thus the concentration of the DIC in
water.
CO2 sink, global water cycle, CO2 absorption, carbonate dissolution, dissolved inorganic carbon, aquatic plant photosynthesis
One of the most important problems in the science of
global change is the balancing of the global budget for
―
atmospheric CO2[1 4]. Although anthropogenic activities
have clearly altered the global carbon cycle, significant
gaps exist in our understanding of this cycle. Of the CO2
emitted into the atmosphere as a result of burning fossil
fuels, roughly half remains in the atmosphere and the
other half is absorbed into the oceans and the terrestrial
―
biosphere[5 13]. The partitioning between these two sinks
is the subject of considerable debate. Without robust
accounting for the fate of CO2 leaving the atmosphere,
predictions of future CO2 concentrations that result from
different emission scenarios will remain uncertain[14].
This, in turn, weakens the link between energy policy
and climate change.
One of the major issues concerning carbon cycling is
the apparent missing carbon that is not accounted for in
the present carbon-cycle model. This is a major issue, as
it may define how the atmosphere will change as emis-
sions increase.
The balance[15] is as follows:
Atmospheric increase = emissions from fossil fuels +
net emissions from changes in land use-oceanic uptake - missing carbon sink:
3.2 = 6.3 + 1.6 − 1.9 − 2.8
(1)
(all values in Pg C/a, 1 Pg =1015 g).
What is the possible reservoir for the missing carbon
sink? Here, we show, based on theoretical calculations
and evidence from field monitoring results, that there is
an important potential CO2 sink by the global water cycle[16] (Figure 1), which has not been considered fully in
the present carbon-cycle model. This sink is formed by
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Chinese Science Bulletin | February 2008 | vol. 53 | no. 3 | 402-407
Received June 5, 2007; accepted August 20, 2007
doi: 10.1007/s11434-008-0096-9
†
Corresponding author (Email: [email protected])
Supported by the Hundred Talent Program of Chinese Academy of Sciences,
Knowledge Innovation Program the Chinese Academy of Sciences (Grant No.
KZCX2-YW-306), the National Natural Science Foundation of China (Grant No.
40572017), and the Ministry of Science and Technology of China (Grant No.
2005DIB3J067)
ARTICLES
The global water cycle and its CO2 sinks. The numbers without unit are in Pg C/a (Table 2); water fluxes are from ref. [16].
the CO2 absorption of water and subsequent enhanced
consumption by carbonate dissolution and aquatic plant
photosynthesis.
1 Solubility of CO2 in CO2-H2O and
CaCO3-CO2-H2O systems
CO2 is readily soluble in water. In solution, there is an
equilibrium mixture of carbonic acid and bicarbonate
and carbonate ions, which make up the “dissolved inorganic carbon” (DIC) fraction:
CO2+H2O⇔ H2CO3⇔ H++HCO3−⇔ 2H++CO32− (2)
The proportion of each species depends on pH. At
high pH the reactions shift to the right of eq. (2). At a
pH of approximately 7 to 9, most of the carbon in the
groundwater and ocean is in the form of bicarbonate
(HCO3−). Under certain conditions of very high pH, carbonate predominates.
At global mean annual air temperature of 15℃ and
global mean atmospheric CO2 partial pressure (Pco2) of
350 ppmv, the equilibrium value of the DIC for the
CO2-H2O system is calculated to be 0.018 mmol/L (Table 1)[17]. However, in CaCO3-CO2-H2O system, the
CO2 absorption of water is enhanced remarkably by calcite dissolution. For example, at 15℃ and Pco2 = 350
ppmv, the equilibrium value of the DIC for the
CaCO3-CO2-H2O system is 1.19 mmol/L, or about 65
times larger than that in the CO2-H2O system (Table 1).
If calcite dissolution happens in soil water or in
groundwater recharged by soil water with higher Pco2,
the amount of CO2 absorption by water is even larger.
For example, soil Pco2 can reach up to 105 ppmv[17,18].
In this case, equilibrium values of the DIC for CO2-H2O
and CaCO3-CO2-H2O systems are 4.70 and 15.75
mmol/L, respectively (Table 1).
LIU ZaiHua et al. Chinese Science Bulletin | February 2008 | vol. 53 | no. 3 | 402-407
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Figure 1
Table 1 Calculated equilibrium values of dissolved inorganic carbon
(DIC) in pure CO2-H2O and CaCO3-CO2-H2O systems under different
Pco2 (15℃) according to Dreybrodt[17]
Pco2 (ppmv)
350
Pure CO2-H2O system (mmol/L) 0.018
Pure CaCO3-CO2-H2O system
1.19
(mmol/L)
700
[HCO3−] and water stage after rainfall (Figure 3))[23].
5000 10000 100000
0.035
0.24
0.47
4.70
1.52
3.27
4.43
15.75
Calcite dissolution can be presented as
CaCO3+CO2+H2O=Ca2++2HCO3−
(3)
DIC can be also derived from the weathering and
dissolution of silicate minerals. The weathering process
also involves CO2. In a simplified form it can be presented as
2CO2+3H2O+CaSiO3=Ca2++2 HCO3−+ H4SiO4 (4)
CO2 is supplied either directly from the atmosphere or
from soils.
The above reactions are important in the sequestration
of atmospheric CO2 because all of the HCO3- for silicate
weathering and half in the case of calcite dissolution are
of atmospheric origin. The weathering products are
transported to the ocean and to the inland lakes (Figure
1), where they are used by plankton to build their skeleton and tissues. When the plankton dies and falls to the
seafloor and lake floor, they are buried in marine and
lake sediments. Because of the very low weathering kinetics of silicate minerals and their low solubility[19,20],
we concentrate on the CO2 consumption by the dissolution of carbonate rock in this study.
The relationship between DIC (mainly in bicarbonate
at 7<pH<9) and CO2 has also been evidenced by field
observations in the karst areas of North China and
Southwest China[21,22]. For example, in the Guilin Karst
Experimental Site of SW China, reforestation and temperature-induced annual soil CO2 increase and the increase in Pco2 in summer growing seasons enhanced
carbonate rock dissolution, and thus increased the bicarbonate concentrations of the epikarst spring S25
(Figure 2). Especially, in the Maolan Karst Experimental
Site, storm-scale variations in water stage, pH, [HCO3−],
calculated CO2 partial pressure (Pco2) and calcite saturation index (SIcalcite) for Maolan epikarst spring at
15-minute intervals, July 19―20, 2004, show that there
is an increase in CO2 sink by a factor of ~2 after rainfall
by rainwater absorption of soil CO2 (see from the increase in Pco2 and decrease in pH and SIcalcite, in Figure
3) and subsequent enhanced consumption by carbonate
rock dissolution (judged from the increase in both
404
Figure 2 Variations of monthly mean air temperature, soil Pco2 and
[HCO−3 ] of the epikarst spring S25, showing the annual and seasonal variations of bicarbonate with soil Pco2, at the Guilin Karst Experimental Site,
Guangxi, SW China (Modified after ref. [21]).
Moreover, we also found that the annual mean calcite
saturation indexes of these karst waters were close to
zero, similar to the most cases in the world karst areas
reviewed by Dreybrodt[17], Ford and Williams[18], and
White[24]. This implies that most of the karst waters in
the world are at or near to the equilibrium of calcite dissolution due to the quick dissolution kinetics of calcite
(in the order of 104 s)[17]. Therefore, in the following
calculation of CO2 sinks by the global water cycle, the
equilibrium DIC values of calcite dissolution are used
for non-rain waters.
2 Calculation of CO2 sinks by the global
water cycle
To calculate the CO2 sink (as DIC) by the global water
cycle, we use the various fluxes among atmosphere,
ocean and continent provided by Shiklomanov [16] in the
global water cycle model (Figure 1). In this model, precipitation fluxes between the atmosphere and internal
LIU ZaiHua et al. Chinese Science Bulletin | February 2008 | vol. 53 | no. 3 | 402-407
Storm-scale variations in water stage, pH, [HCO−3 ], calculated
CO2 partial pressure (Pco2), and calcite saturation index (SIcalcite) for
Maolan epikarst spring (Guizhou, SW China) at 15-minute intervals, on
July 19―20, 2004, showing an increase (by a factor of ~2) in CO2 sink
after rainfall by rainwater absorption of soil CO2 and subsequent consumption by carbonate rock dissolution (Modified after ref. [23]).
continent, between the atmosphere and external continent, and between the atmosphere and the ocean are
9000, 110000 and 458000 km3, respectively. Given an
atmospheric Pco2 equivalent to 350 ppmv and the global
mean air temperature of 15℃, we calculated an equilibrium value of DIC of 0.018mmol/L (Table 1) for pure
rain water-CO2 system. However, according to Snoeyink
and Jenkins[25], the mean DIC for the global precipitation is about 0.1mmol/L, much higher than the equilibrium value of the pure rain water-CO2 system due to
presence of carbonate dusts dissolution in the atmosphere. The mean DIC value, however, is lower than the
equilibrium value (1.19 mmol/L (Table 1)) of the pure
CaCO3-CO2-H2O system due to the short rainwatercarbonate dusts interaction time (<<104s, the time to
LIU ZaiHua et al. Chinese Science Bulletin | February 2008 | vol. 53 | no. 3 | 402-407
405
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GEOCHEMISTRY
Figure 3
reach calcite dissolution equilibrium[17]) and the low
content of carbonate dusts in the atmosphere. By using
the mean DIC of 0.1mmol/L for the global precipitation,
then the CO2 sinks by absorption of these precipitation
fluxes will be 0.0054, 0.066 and 0.2748 Pg C/a, respectively (Figure 1, Table 2). According to Baumgartner
and Reichel[26] and Shiklomanov[16], the annual global
internal runoff and external runoff are 2000 and 44800
km3, respectively. Given the global mean soil Pco2 of
5000 ppmv[17,27] and the global mean air temperature of
15℃, we got the equilibrium values of DIC of 0.24 and
3.27 mmol/L for the CO2-H2O system and
CaCO3-CO2-H2O system, respectively (Table 1). According to Ford and Williams[18], carbonate rock occupies 12% of the continental land area. Then the CO2
sinks by absorption of internal runoff and external runoff (12% in dissolution equilibrium with soil CO2 and
CaCO3 in carbonate rock area, and other 88% in dissolution equilibrium with soil CO2 in non-carbonate rock
area) will be 0.0098 and 0.219 Pg C/a, respectively (Table 2). These values for CO2 sinks are close to those obtained by Yoshimura and Inokura[28], but it is minima.
On account of the huge area of soil rich in pedogenic
carbonate in arid climate (especially of loess origin),
―
even in non-karst areas of the world[29 31], the CO2 sinks
by these runoffs will be much larger. According to Ford
and Williams[18] and Jia[30], the area of karst regions and
soil with pedogenic carbonate in dryland areas except
karst regions could be 50% of the continent. Then, the
CO2 sinks by internal runoff and external runoff are estimated to be 0.0225 and 0.504 PgC/a, respectively
(Figure 1, and Table 2). Therefore, the total CO2 sink as
DIC by the global water cycle could be 0.8013 Pg C/a
(0.0225+0.504+0.2748=0.8013, where the sinks by the
precipitation in internal and external runoff areas have
been incorporated into those by internal and external
runoff), which is about 10.1% of the total anthropogenic
CO2 emission, or 28.6% of the missing CO2 sink[15],
showing the significance of the global water cycle in the
atmospheric CO2 sink. These DIC will be eventually
mainly consumed by the aquatic plant photosynthesis in
―
the ocean and on the continent[32 34]. Marine ecosystems
play an important role in the carbon cycle through the
so-called “biological pump”[34] acting as follows: Organisms occupy the well-mixed surface layers of the
ocean and photosynthesize and grow at a rate, which
varies according to the nutritional state of the ocean.
Dead biota and feces fall through the water column, with
some of them reaching the seabed, thus removing carbon
from the surface layers and hence reducing the partial
pressure of CO2 there. This enables uptake of new CO2
from the atmosphere, and new DIC from the rivers. Thus,
the ocean’s sink strength is increased by biological activity.
Table 2 Calculation of carbon sinks by precipitation and runoff in the
global water cycle
[DIC]
Carbon sinkf)
Flux
Item
3
(km /a)
(mmol/L)
(1015g C/a)
Precipitation in internal runoff
a)
c)
0.1
0.0054
9000
area
Precipitation in external runoff
0.1c)
0.066
110000a)
area
a)
c)
0.1
0.2748
Precipitation over sea
458000
Internal runoff
External runoff
Total
2000b)
44800a)
0.8148d)
0.0098
1.875e)
0.0225
0.8148d)
0.219
1.875e)
0.504
0.8013
−
−
a) From ref. [16]; b) from [26]; c)from ref.[25]; d) calculated value on
account of the mixing of karst water and non-karst water on the land
(3.27×12%+0.24×2×88%=0.8148, where 0.24 and 3.27 are calculated
equilibrium concentrations (mmol/L) of dissolved inorganic carbon (DIC)
in pure CO2-H2O and CaCO3-CO2-H2O systems under global mean soil
CO2 partial pressure of Pco2=5000 ppmv and global mean air temperature
of 15oC respectively, 12% and 88% are percentage of karst area and
non-karst area in the continent respectively[18], and factor 2 comes from
the fact that all of the HCO3- for non-karst area and half in the case of
calcite dissolution are of atmospheric origin); e) calculated value on account of the mixing of water from (karst areas + soil areas with pedogenic
carbonate in dryland areas except karst areas) and water from other areas
on the land (3.27×50%+0.24×2×50%=1.875, where 0.24 and 3.27 are
calculated equilibrium concentrations (mmol/L) of dissolved inorganic
carbon (DIC) in pure CO2-H2O and CaCO3-CO2-H2O systems under
global mean soil CO2 partial pressure of Pco2=5000 ppmv and global
mean air temperature of 15oC respectively, both 50% is percentage of
(karst areas + soil areas with pedogenic carbonate in dryland areas except
karst areas) and other areas in the continent respectively[18,30], and factor 2
comes from the fact that all of the HCO3- for the other areas is of atmospheric origin); f) carbon sink=0.5 × [DIC] × flux × 12 × 109 (g), where the
factor 0.5 results from the fact that in the case of carbonate dissolution
(CaCO3 +CO2 + H2O＝Ca2+ + 2HCO3-) only half of the [HCO3-] is of
atmospheric origin, and 12 is the atomic weight of carbon. Total carbon
sink by the global water cycle is 0.8013 (0.2748+0.0225+0.504=0.8013
Pg C/a, or 1015g C/a), where the sinks by the precipitation in internal and
external runoff areas have been incorporated into those by internal and
external runoffs.
The surface areas of river and inland lakes are not
very great compared with those of land and ocean, and
therefore the direct exchanges of carbon between rivers/lakes and atmosphere are possibly less important[35,36]. However, the rivers of the world are conduits
406
between the land and ocean, or to the inland lakes, carrying large amounts of carbon and nutrients to the sea or
―
to the inland lakes[36 38].
Given 0.244 PgC/a of the global inputs to oceans of
riverine atmospheric DIC[38], and the maximum release
of carbon between rivers/lakes and atmosphere (30%, or
(0.0225+0.504)×30%=0.158 PgC/a)[35,36], then at least
0.1245 PgC/a (0.0225+0.504−0.244−0.158 = 0.1245)
will remain in the continental aquatic ecosystem, possibly in the inland lakes and rivers, or being stored in the
large but slowly cycled groundwater reservoir, which
needs to be confirmed by future observation data.
To summarize, the atmospheric CO2 sink as DIC by
the global water cycle is 0.8013 Pg C/a, of which 0.5188
PgC/a goes to sea via precipitation over sea (0.2748
PgC/a) and continental rivers (0.244 PgC/a)[38], 0.158
PgC/a is released to the atmosphere again, and 0.1245
PgC/a is stored in the continental aquatic ecosystem.
Therefore, the net sink could be 0.6433 PgC/a.
3 Future of the CO2 sinks by the global
water cycle
In the end, it is inferred from this study that the CO2 sink
by the Global Water Cycle may increase with strengthening global-warming-intensified global water cycle[39]
(Table 2), the increase in CO2 and carbonate dust in atmosphere (Table 1), and reforestation/afforestation, with
the latter increasing the soil CO2, and thus the concentration of DIC in water[21] (Table 1). Moreover, aquatic
plants in both ocean and land can be fertilized by the
increased DIC. Therefore, the Global Water Cycle and
aquatic plants act as a regulator of atmospheric CO2 with
the interaction of carbonate weathering. In other words,
these processes would jointly provide a negative climate
feedback mechanism that counteracts the increasing atmospheric CO2.
However, for our full understanding and more accurate assessment of these processes, the temporal and
spatial variations in DIC of precipitation and runoffs in
the world, which are related to the temporal and spatial
distribution in carbonate dust and CO2 in the atmosphere,
and pedogenic carbonate and CO2 in the soil respectively,
remain to be determined in the first instance in future. In
addition, the role of organic carbon in the DIC cycle
needs also to be resolved. Therefore, this study does not
mean to give an accurate estimation of CO2 sink by the
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Special thanks are given to Dr. Russell Drysdale (The University of Newcastle, Australia) for his comments and suggestions in improving the
manuscript.
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global water cycle, which is not possible at the present
stage, but to suggest a new direction to searching for the
missing carbon sink.