Accepted: February 13, 2023 | Published Online: February 15, 2023

Citation: Liu Y, He Y, Wang Y, Gao Z (2023) Rapid Stabilization on Global Atmospheric CO₂ Concentrations by Cheaply Enhanced Tibetan Geological Carbon Sink. Int J Earth Sci Geophys 9:064.

Copyright: © 2023 Liu Y, et al. This This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

Tibetan plateau is the only nascent carbon reservoir worldwide due to uniquely flat subduction of the Indian continent beneath Tibetan plateau, controlling global CO₂ levels since the Oligocene. However, it remains obscure whether massive anthropogenic emissions are rapidly sequestered by the growing Tibetan plateau. Multidiscipline pieces of evidence are present here for extremely large carbon uptake during summer and relatively low carbon uptake during winter. Comprehensive investigations have further revealed that during summer, plants are highly physiological active in the tectonically active freshwater-enriched silicate regions. Huge amounts of anthropogenic emissions are thus rapidly turned into massively secondary carbonates and minor organic carbon matters by enhanced subsurface silicate chemical weathering through coupled the active tectonics and the physiologically active plant roots during summer. During winter, chemical sedimentation in salty lakes and the planting wetlands is greatly raised to transform additional anthropogenic emissions to newly-formed carbonates again, largely due to little greenhouse effect within Tibetan plateau during winter. The renascent carbonates and organic matters are subsequently buried into Tibetan thickened crust and its adjacent foreland basins by the distinguished active tectonics, and thus are hardly back to atmosphere again. More importantly, the Tibetan geological carbon sink including the enhanced subsurface silicate chemical weathering during summer and chemical sedimentation during winter, as well as the tectonic burial, can be further artificially enhanced greatly by transforming Tibetan barren deserts to planting wetlands through building simple retaining dams at the specific valleys in Tibetan plateau. The global carbon neutrality is therefore achieved cheaply quickly by the enhanced Tibetan geological carbon sink, no matter how much anthropogenic emissions in the near future.

Keywords

Cheap carbon uptake, Enhanced Tibetan geological carbon sink, Global carbon neutrality, Tibetan plateau

Introduction

Numerous studies have revealed that rates of carbon uptake by well-known land carbon reservoirs such as African [1] and Amazonian [2] tropical forests, and ecosystem of Chinese mainland [3], as well as ocean carbon reservoirs [4,5] have remained constant or declined in recent decades [6], leading to severe consequences such as serious shortage of food and freshwater around the North Atlantic. Consequently, fast and cheap removal of massive anthropogenic emissions is an urgent need worldwide today. Several expensive approaches have been used to try to stabilize global atmospheric CO₂ concentrations [7]. For example, the CO₂ gas with high concentrations from condensed flue gas or other enriched sources have been subsequently injected into deep regions, particularly into deep oil reservoirs [7]. A large amount of silicate powder is scattered to croplands [8], grasslands and forests as an alternative approach to try to remove atmospheric CO₂ [7]. Unfortunately, these approaches normally require relatively high energy and infrastructure inputs every year [8]. And hence, much work has been done and much money has been spent [7,8], the global atmospheric CO₂ concentrations still remain relatively high growth rates. This means that these expensive approaches do not work well, largely due to the fact that it is still little-known what mechanisms are really responsible for rapidly net uptake of global atmospheric CO₂ [6,9,10], despite it has been well known for a long time that negative carbon-climate feedbacks stabilized Earth's long-term climate [11-13] and, in particular, atmospheric CO₂ levels [6,9,10].

In-depth understanding of the mechanisms controlling the historically changing processes of atmospheric CO₂ concentrations is, no doubt, critical to cheaply achieve global carbon neutrality in the near future. Numerous studies have previously revealed that during the Eocene, the atmospheric CO₂ concentrations were approximately 5 times that for today [14,15]. During the Eocene-Oligocene transition, the global atmospheric CO₂ concentrations plummeted quickly, leading to the formation of the Antarctic ice sheets [16,17]. However, it still remains controversial where the huge amounts of atmospheric CO₂ sink [6,14,18,19]. From a global mass balance perspective, it clearly suggests that at least one unknown carbon reservoir has continuously accommodated the huge amounts of atmospheric CO₂ since the late Eocene.

The persistent convergence between Indian and Asian continents has created growing Tibetan plateau (Figure 1). Gravity isostasy suggests that high topography corresponds to thickened crust, and vice versa. Therefore, the formative ages of the oldest thickened crust are generally regarded as the originally uplifting times of the proto plateau [20]. Previous studies have revealed that the oldest Tibetan thickened crust as a result of the continuous India-Asia collision [21] had been formed during the Eocene-Oligocene transition [20,22-23], corresponding to the formation of the Antarctic ice sheets exactly [16-17]. It has thus confirmed the previous view that the uplift of Tibetan plateau led to the global cooling and subsequent formation of the Antarctic ice sheets, largely due to intensive silicate chemical weathering [24-27]. Recent studies have further revealed that approximately 7 trillion tonnes of atmospheric CO₂ have been transformed to organic matters, carbonates and particularly carbonic magmas [28], lately trapped in the Tibetan thickened crust and its adjacent foreland basins since the Eocene-Oligocene transition by the uniquely flat subduction of Indian continent below Tibetan plateau [29-31], leading to the formation of glacial-interglacial climate [24-27]. The long-term imbalance between atmospheric inputs and outputs of CO₂ [14-17] could be perfectly accounted for by the fact that huge amounts of atmospheric CO₂ had been persistently accommodated by the growing Tibetan plateau [29-31], the only nascent carbon reservoir worldwide [30,31]. The formation of Tibetan plateau is thus an important global rather than regional event, driving the global climate change, as well as the fluctuation of global atmospheric CO₂ concentrations since the Eocene-Oligocene transition [30,31]. However, most Tibetan regions are barren deserts now (Figure 1) that are carbon sources rather than carbon sinks [31]. It is therefore a critical issue whether currently large anthropogenic emissions can be sequestered by the Tibetan geological processes within one year rather than one million years. Within a short-term time, traditional silicate chemical weathering plays a tiny role in the direct removal of atmospheric CO₂ outside of the unique Tibetan plateau, largely due to extremely low chemical reaction rates [32], so that the traditionally geological carbon sink is generally neglected [9]. However, the global carbon budgets have clearly shown that the land carbon sink is stably increased greatly [6,9-10]. It is still unknown where regions and what mechanisms are primarily responsible for the persistently enhanced land carbon sink [6,9-10], unfortunately. In addition, many studies have indicated that inland waters play an important role in the global carbon cycle [33]. Numerously inland salty lakes and/or wetlands occurs in Tibetan plateau and its northern neighboring regions (Figure 1). It is also unclear what contributions these inland salty lakes or wetlands make in the global carbon sink. These questions critical to the public worldwide are well addressed in this study. And it has been clearly illustrated how fast and cheaply the large amounts of anthropogenic emissions are transformed to massively renascent carbonates and relatively minor organic carbon matters that are later buried in the Tibetan thickened crust and its adjacent foreland basins by the uniquely Tibetan geological processes. Subsequently the cheapest approach is present here to rapidly stabilize global atmospheric CO₂ concentrations regardless of how much anthropogenic emissions in the near future.

Materials and Methods

Data acquisition and analysis

The entire Tibetan plateau and its adjacent regions including the Urumqi area in the northwest are research regions in this study (Figure 1). In order to quantitatively evaluate carbon uptake capacity of the critical regions (Figure 1) in the context of massive anthropogenic emissions today, abundantly observational multidisciplinary data were collected for comprehensively contrastive analyses (Table 1, Table 2, Figure 1 and Figure 2). Horizontal shortening rates between Indian and Asian continents (Figure 1) were from [34], indicating the deformative intensity of crust triggered off by the consecutive collision between Indian and Asian continents. From the Lhasa and Shiquan River regions in the south to the Urumqi and Xining regions in the north roughly, the crustal deformation is gradually decreasing [34] (Figure 1). Numerous observational data of precipitation and temperature were collected from the meteorological stations except Waliguan station within the studying regions (Table 1 and Figure 1). The Waliguan station recording atmospheric CO₂ concentrations professionally is situated on the top of Mt. Waliguan on the NE Tibetan plateau (Figure 1). The predominant wind directions of this station are from SW to NW (summer) and from ESE to NE (winter). The air sample collection and subsequent measurements of atmospheric CO₂ concentrations were reported in previous studies [35,36] in detail. Monthly mean data of atmospheric CO₂ concentrations in ppmv, rainfall in millimeters and temperature in degrees Celsius were listed in Table 1, respectively. Yearly mean rainfall data in millimeters were listed in Table 2. Additionally, in combination with the multidisciplinary data, comprehensive geological surveys have been performed to try to figure out the Tibetan carbon uptake mechanisms today.

Annual carbon uptake rates (in ppmv) were gotten by that the highest value in a year, the spring's value normally, subtracted the December's value. Bimonthly growth rates of atmospheric CO₂ concentrations (in ppmv) were acquired by that the year's highest value subtracted the February's data (Figure 2b). Annual growth rates (in ppmv) were acquired by the lowest value in a year, normally the summer's value, deducted the lowest one in the previous year (bottom to bottom) (Figure 2c). All of them were listed in the Table 2. Regional precipitations are sum of precipitation intensity of individual observatory stations (in millimeters) multiplied by precipitation areas (Figure 2b). Assuming all precipitation areas in each time in each observatory station were as same as one unit. This led to large uncertainties in estimations of the regional precipitations in three red spots in the Figure 2b. These large uncertainties will be greatly diminished by extra numerous precipitation data from newly-added meteorological stations within Tibetan plateau in the near future. The EXCEL software was used to make regression analyses between the regional precipitations and the bimonthly growth rates during spring (Figure 2b), as well as the July's precipitation recorded by the Lhasa station (Figure 1 and Table 1) and the annual growth rates (bottom to bottom) (Figure 2c), respectively.

In order to deeply reveal the relations between the variations of atmospheric CO₂ concentrations recorded by the Waliguan station, Tibetan plateau (Figure 1) and the changes of global atmospheric CO₂ levels, the annual growth rates (bottom to bottom) were projected into the evolution of interhemispheric CO₂ gradient and of CO₂ emissions from fossil fuel and cement [10] for a comprehensive comparison (Figure 2d).

Tibetan carbon sink estimations

Two completely different methods were used to evaluate the carbon uptake by Tibetan plateau in this study. The first approach is from [31]. The primary reason for this method is that large amounts of atmospheric CO₂ had been rapidly turned into massively secondary carbonates and organic carbon that are subsequently buried in the thickened crust and its adjacent foreland basins by the uniquely geological processes [29-31]. The carbon uptake can be therefore acquired by the direct measurements on the contents of the secondary carbonates and organic carbon in soils and rocks beneath the soils within Tibetan plateau and its adjacent foreland basins [31]. The calculated formula for this estimation is following:

F = ρ*S*f*( + C_inorg) + Q (1)

Where:

F = annual total atmospheric CO₂ uptake (in tonne);

ρ = average density (in tonnes per cubic meter);

S = average annual carbon burial rate (in millimeter per year), normally 1-2 millimeters per year due to horizontal shortening rates of 2 centimeters per year [34];

f = area of regions (in hectare);

C_org = buried organic carbon contents (in weight percent);

C_inorg = buried inorganic carbon contents (in weight percent);

Q = organic carbon over surface, roughly corresponding to the traditional ecosystem carbon sink (in tonnes per year).

The changing of CO₂ concentrations (in ppmv) was used as the second method that is relatively simple to estimate the carbon uptake by the entire Tibetan plateau and its adjacent regions in this study. The annual carbon uptake rates listed in the Table 2 roughly represent the net carbon uptake by these critical regions within one year. 1 ppmv = 7.782 gigatonnes atmospheric CO₂ [9].

Results and Discussion

Variability of atmospheric CO₂ concentrations within Tibetan plateau and its adjacent regions over the past 30 years

In general, within a year, the atmospheric CO₂ concentrations within Tibetan plateau rise during spring firstly, plummet largely during summer, and slightly decrease or roughly remain constant during winter (Figure 2a, Table 1 and Table 2). The large decline of atmospheric CO₂ levels is perfectly corresponding to July's heavy precipitation within the southern Tibetan plateau (Figure 2 and Table 1) where the horizontal shortening rates are the highest [34] (Figure 1). This region is therefore the most tectonically active silicate region due to the continuous collision between Indian and Asian continents (Figure 1). Whereas precipitation recorded by the Urumqi meteorological station, situated in the northern region outside of Tibetan plateau where the horizontal shortening rates are close to zero [34] and thus it is relatively tectonic stable (Figure 1), has little effect on the variations of atmospheric CO₂ concentrations (Figure 2a and Table 1). The regression analysis has further suggested that during spring, the bimonthly growth rates of atmospheric CO₂ concentrations are negatively correlated with Tibetan precipitations over the past 30 years (Figure 2b). For example, in the spring of 2012, the bimonthly growth rate was surprisingly as low as minus 0.39 ppmv (Table 2), largely due to heavy precipitation during this spring (Figure 2, Table 1 and Table 2). Moreover, the annual growth rates (bottom to bottom) were highly negatively correlated with the July's precipitation within southern Tibetan plateau where the tectonics are the most active (Figure 1, Figure 2, Table 1 and Table 2). For instance, in 2014, although the bimonthly growth rate was up to 3.59 ppmv (Table 2 and Figure 2) because of the cold, dry and windy weather during this spring (Table 1), the annual growth rate was incredible as low as minus 0.45 ppmv (Table 2 and Figure 2) and the annual carbon uptake rate was still up to 4.93 ppmv (Table 2), largely due to the July's heavy precipitation (Table 1). In 2015, although the bimonthly growth rate was as low as 1.71 ppmv due to relatively much water during this spring (Table 1, Table 2 and Figure 2), the annual growth rate was up to 3.00 ppmv, and the annual carbon uptake rate was only 1.2 ppmv, partially owing to the relatively less precipitation during July (Table 1, Table 2 and Figure 2). Furthermore, the annual growth rates were roughly corresponding to the changing of the interhemispheric CO₂ gradient [10] (Figure 2d). For example, in 2003, the annual growth rate was up to 6.09 ppmv (Table 2), roughly corresponding to the peak of the interhemispheric CO₂ gradient (Figure 2d). This suggested that the annual growth rates of atmospheric CO₂ concentrations in Tibetan plateau (Table 2) probably represented the evolution of global atmospheric CO₂ concentrations. That further implies that if we control the rise in the Tibetan atmospheric CO₂ concentrations in the near future, we could restrain the increasing in global atmospheric CO₂ concentrations quickly.

During spring, most of Tibetan plateau and its adjacently northern regions were cold and dry (Table 1) and strongly windy, resulting in strong physical weathering, as well as relatively weak silicate chemical weathering and photosynthesis owing to water shortage and relatively low temperature in spring (Table 1). Tibetan previously-formed organic-carbon-bearing soils disappeared partially (Figure 3), becoming sources of sand storms. Oxidization of soil organic carbon releases large amounts of CO₂ into atmosphere once again, leading to the formation of a carbon source in the case of water shortage particularly during spring (Table 1, Figure 2 and Figure 3). The bimonthly growth rates of atmospheric CO₂ concentrations (Table 2) are therefore controlled largely by the Tibetan surficial water.

Enhanced subsurface silicate chemical weathering

Intensive crust horizontal shortening triggered off by the India-Asian convergence in succession (Figure 1) resulted in the strongest deformation of the southern Tibetan silicates (e.g., Figure 3, Figure 4, Figure 5 and Figure 6). Abundant (micro) fractures therefore occurred in the southern Tibetan silicates normally (Figure 3, Figure 4, Figure 5 and Figure 6). Much freshwater is transferred to Tibetan plateau by heavy precipitation particularly during July (Table 1 and Figure 2a), leading to much surficial freshwater in locally cracked silicate regions (e.g., Figure 4 and Figure 5). Plants are thus actively growing up within the freshwater-enriched cracked silicate regions only during summer (e.g., Figure 4 and Figure 5). Through the most physiologically active plant roots within two months (Figure 4 and Figure 5), a large amount of freshwater and atmospheric CO₂ can be continuously sent to the Tibetan subsurface cracked silicate regions, subsequently transformed to high concentrations of organic and carbonic acids, respectively [37-40]. The local subsurface CO₂ concentrations can be up to 50,000 ppmv [38], as most of CO₂released by the plant roots and soils are subsequently trapped by the planting wetlands and subsurface clay minerals (Figure 4 and Figure 5), much better covers against the escape of CO₂ from the Tibetan water-enriched soils.

Whereas in the water-shortage silicate regions where the tectonics are also the most active (e.g., Figure 3), the plants are still the least physiologically active even during summer (e.g., Figure 3), largely due to the less precipitation. Little atmospheric CO₂ and water are therefore sent to the subsurface cracked silicate regions (e.g., Figure 3). The light-colored cracked silicates exposed in the air (e.g., Figure 3) have to experience the traditionally silicate chemical weathering, but its carbon uptake can be ignored completely because of little formation of secondary carbonates nor organic matters (e.g., Figure 3). These fractured silicates could remain stable in the air for a long time in the case of water-shortage (Figure 3).

Within the 3D space beneath the Tibetan planting wetlands where the tectonics are the most active (e.g., Figure 4, Figure 5, Figure 6 and Figure 7) rather than the 2D space where the conventional silicate chemical weathering takes normally place (e.g., Figure 3), the subsurface cracked silicates can chemically react completely with the large amounts of organic and carbonic acids with high concentrations exuded by the extremely physiological active plant roots in succession during summer (Table 1, Figure 4, Figure 5, Figure 6 and Figure 7). All weaknesses of the traditional silicate chemical weathering (e.g., Figure 3) are perfectly overcome, such as very short residence time of atmospheric CO₂ with very low concentrations for the silicate chemical weathering reactions (e.g., Figure 3), extremely small surface areas available for these chemical weathering reactions (e.g., Figure 3), conventionally higher pH values against the reactions, and relatively low temperature for the chemical reactions [32,40]. Consequently, within the most tectonically active silicate region rich in surficial freshwater triggered off by the heavy precipitation during summer (Figure 4, Figure 5, Figure 6 and Figure 7), the Tibetan subsurface silicate chemical weathering rates are at least 1 million times that for the traditional silicate chemical weathering (e.g., Figure 3) during summer [32]. Moreover, the newly-formed organic matters exuded by the extremely physiological active plant roots are normally trapped by the subsurface secondary clay minerals. These lead to a large decline of atmospheric CO₂ concentrations in a very short time, only 30 days, in the context of massive anthropogenic emissions (Table 1 and Figure 2). More importantly, the recent drillings in the tectonically active forest-wetlands have further revealed that the Tibetan subsurface silicates, up to 2,500 meters depth, are still subject to the intensive silicate chemical weathering (Figure 6). This clearly suggests that the huge amounts of the organic and carbonic acids with higher concentrations exuded by the most physiologically active Tibetan plant roots continuously are further transferred to much more depths to react chemically with the deeply cracked silicates completely (e.g., Figure 6) by groundwater along deep fractures or even large faults that are also results of the continuous India-Asia collision (Figure 7). Thus, the subsurface silicate chemical weathering is further enhanced greatly so that the Tibetan forest-wetlands (Figure 4) boast a bottomless appetite for the capturing of large amounts of atmospheric CO₂ (Figure 7). This is the primary reason why the annual growth rates of the Tibetan atmospheric CO₂ concentrations (bottom to bottom) are strongly negative correlated with the July's precipitations within the most tectonically active silicate region (Table 1, Table 2 and Figure 2). The unique Tibetan subsurface silicate chemical weathering thus plays an important role in the rapid transformation of the vastly anthropogenic emissions to massively secondary carbonates and organic matters trapped in the clay minerals normally.

Enhanced chemical sedimentation

During winter, Tibetan plateau becomes an icy plateau, largely owing to global warming [31,32,42,43], so that sunlight actually goes back to outer space by the massive snow or ice sheets at an altitude of over 4000 meters. This leads to a weak greenhouse effect in Tibetan plateau despite of relatively high atmospheric CO₂ concentrations during winter, typical negative feedbacks between the growing Tibetan plateau and global climate change during winter. Under relatively low temperature during winter (Table 1), particularly up to minus 40 degrees Celsius in the northern Tibetan plateau during night, large amounts of Tibetan plants rapidly die in the planting wetlands and massive carbonates fast crystallize from the numerously salty lakes in Tibetan plateau (Figure 1). Subsequently, massive carbonates and/or minor organic matters are quickly deposited in the bottom of the salty lakes (Figure 8) and/or planting wetlands (Figure 9), consuming extra large amounts of atmospheric CO₂ (Table 1 and Figure 2a). The inland waters within the central Asia including the salty lakes and planting wetlands (Figure 1) play an important role in the global carbon sink so that the Tibetan atmospheric CO₂ levels remain stable roughly or even decrease slightly in the context of much more anthropogenic emissions for heating during winter generally (Table 1 and Figure 2).

Tectonic burial

Tibetan plateau has been featured by the active tectonics triggered off the continuous collision between Indian and Asian continents (Figure 1, Figure 3, Figure 4, Figure 5 and Figure 6). The massively secondary carbonates and organic carbon (Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9) transformed fast from the huge anthropogenic emissions are further trapped in the Tibetan thickened crust and its adjacent foreland basins [29-31] by the active tectonics (Figure 10 and Figure 11). And subsequently, some of them will experience intensive decarbonization in the deep regions beneath Tibetan plateau by the persistently flat subduction of Indian continent beneath Tibetan plateau, releasing the carbonic magmas to the specific magma chambers in the thickened crust [28,29], and thus, resulting in the only nascent carbon reservoir worldwide [29-31]. Furthermore, the relics decarbonized in the deep regions are exhumated to shallow depth as little-weathered cracked silicates, subsequently undergoing the intensive carbonization such as the enhanced subsurface silicate chemical weathering (e.g., Figure 4, Figure 5 and Figure 6) once again, also as results of the persistently flat subduction of Indian continent beneath Tibetan plateau [29-31]. The large amounts of atmospheric CO₂ are therefore transferred persistently into the thickened crust. It is the major mechanism for large carbon uptake by the growing Tibetan plateau, leading to that global atmospheric CO₂ concentrations are never over 300 ppmv since the late Pleistocene [13]. Whereas within the regions outside of Tibetan plateau, the unique carbonization and decarbonization take never place due to the lacking of the distinguished active tectonics triggered off by the India-Asia collision in succession, and thus, large amounts of CO₂ are easily released into atmosphere once again. This is the primary reason why carbon sink saturation easily occurs in these regions outside of Tibetan plateau, such as African and Amazonian tropical forests [1,2], the Earth's largest tropical forests, and why the precipitation in the Urumqi area (Figure 1) has little impact on the variation of atmospheric CO₂ concentrations (Table 1, Table 2 and Figure 2a).

Implications

As mentioned above in detail, the carbon uptake rates of the well-known reservoirs have remained constant roughly or even decreased in recent decades [1-6], indicative of a positive carbon-climate feedback [4]. However, the terrestrial carbon sink is stably increased [6,9,10]. Based on the global carbon mass balance, these reduced sinks must be more than compensated for by an increase in the rates of unfamiliar carbon uptake. The annual carbon uptake rates listed in the Table 2 have clearly shown that over the past 30 years, the Tibetan plateau and its adjacent regions are always a large net carbon sink even in the context of huge anthropogenic emissions, distinguished from other well-known carbon reservoirs [1-6]. Satellite remote investigations in combination with field surveys have further confirmed that Tibetan barren regions are rapidly transforming to the planting wetlands [30-31,43-45], due in large part to global warming [30,31,42], clearly suggesting higher inputs of organic carbon to Tibetan regions in succession [46-48]. More importantly, as demonstrated above in detail, the chemical sedimentation during winter and subsurface silicate chemical weathering during summer, as well as the tectonic burial are greatly enhanced along with the increasing of the Tibetan planting wetlands. Recent space-borne measurements of atmospheric CO₂ [49,50], the independent investigations approved by the IPCC, have further confirmed the surveys in this study that the currently eastern Tibetan plateau fully covered by various plants is a large carbon sink. Multidiscipline pieces of evidence have therefore revealed that the Tibetan plateau and its adjacent regions do be such regions where the net carbon uptake has been only increased greatly worldwide, resulting in strongly negative carbon-climate feedbacks (Figure 12 and Figure 13). Such negative feedbacks between the growing Tibetan plateau and global climate changes are the major mechanism (Figure 12 and Figure 13) to stabilize atmospheric CO₂ concentrations in the future. For example, Yarlung Zangbo, one of the largest rivers in Tibetan plateau, is a barrier lake now (Figure 14) due to the landslides triggered off by coupled the active tectonics and heavy precipitation. A large amount of freshwater and atmospheric CO₂ is consecutively sent to the deeply silicate region beneath the barrier lake to enhance the Tibetan geological carbon sink including the enhanced subsurface silicate chemical weathering during summer (e.g., Figure 7) and chemical sedimentation during winter (e.g., Figure 9), as well as the tectonic burial (e.g., Figure 10 and Figure 11). Therefore, the Tibetan geological carbon sink contributes greatly the recent increase in the terrestrial carbon sink that is roughly estimated to be 10 billion tonnes of atmospheric CO₂ per year [9]. This estimation is approximately consistent with the calculations of carbon sink based on the changing of atmospheric CO₂ concentrations recorded by the Waliguan station (Table 1 and Table 2). Over the past 30 years, most annual carbon uptake rates are more than 2 ppmv (Table 2), corresponding to uptake of more than 10 billion tonnes of atmospheric CO₂ per year [9] by the entire Tibetan plateau and its adjacent regions. Consequently, compared with the traditional ecosystem carbon sink, such as the carbon sink in the African and Amazonian tropical forests [1,2], the Tibetan geological carbon sink is an extremely large size that can easily affect the variations of the global atmospheric CO₂ levels (e.g., Figure 2d). Therefore, the Tibetan geological carbon sink cannot be neglected in the near future. In addition, according to the method proposed by [31] firstly, within the southern Tibetan regions where the tectonics are the most active (Figure 1), approximately 30 and 100-150 tonnes of atmospheric CO₂ are estimated to be passively removed per year by the grass wetlands (e.g., Figures 5 and Figure 13b) and forest wetlands (e.g., Figure 4, Figure 10, Figure 11 and Figure 13c) per hectare, respectively. It should point out that the surface organic matters are not considered in this study, as some of them were rapidly buried by the active tectonics (e.g., Figure 10 and Figure 11) to easily yield double counting [31]. This means the model results here are underestimated slightly.

Although most Tibetan regions are barren lands because of freshwater-shortage (Figure 1, Figure 3 and Figure 13a), fossil studies [51,52] have further revealed that the barren Tibetan and its adjacent regions were fully covered by rain-forests or grass-wetlands during the previous warming periods. At that time, the entire Tibetan plateau was a greenly giant water-tower [30,31,43,51,52]. As demonstrated in the previous sections in detail, the carbon uptake by the tectonically active regions is strongly correlated with the surficial freshwater (Figure 2). These clearly indicate that the potential carbon uptake of the currently barren Tibetan regions where the tectonics are active (Figure 1, Figure 3 and Figure 13a) can be greatly improved rapidly by turning these tectonically active silicate lands poor in water (e.g., Figures 3 and Figure 13a) into artificial planting wetlands. Simple retaining dams constructed at the specific valleys can do this transformation rapidly. During summer, beneath the artificially planting wetlands within the tectonically active silicate regions, an extra huge amount of atmospheric CO₂ and freshwater are continuously sent to the much more depths through coupled the physiologically active plant roots and the active tectonics, subsequently chemically reacting with the deeply cracked silicates completely (e.g., Figure 7), regardless of whether there is heavy precipitation during summer. This implies that the currently negative correlation between the July's precipitation in southern Tibet and the annual growth rates (Figure 2) is probably replaced by a new one that the areas of the artificially planting wetlands within the most tectonically active silicate regions are negatively correlated with the annual growth rates of atmospheric CO₂ concentrations in the near future. During winter, massively newly-formed carbonates and organic matters are similarly deposited in the bottom of the artificially planting wetlands fast due to the strongly cold weather (e.g., Figure 9), also removing additionally huge amounts of atmospheric CO₂ directly. Moreover, physical weathering could be greatly diminished by the artificially planting wetlands, working against oxidization of the soil organic matter (e.g., Figure 2b). In addition, massively newly-formed carbonates and organic carbon in the artificial planting wetlands can be further buried in the thickened crust (e.g., Figure 10 and Figure 11). Therefore, the annual growth rates of the Tibetan atmospheric CO₂ concentrations will be largely diminished or even stopped completely by the vastly artificial planting wetlands within the most tectonically active silicate regions despite of currently massive anthropogenic emissions. Given that approximately 1 million square kilometers deserted regions, roughly 25% of the barren Tibetan and its adjacent regions (Figure 1), are rapidly transformed to artificially planting wetlands, net dozens of billion tonnes of atmospheric CO₂ could be directly removed per year by the Tibetan and its adjacent regions in the near future according to the relatively conservative estimations in this study.

The cost for the carbon uptake in this study is simply to construct the simple retaining dams at the specific valleys that can be used for several tens of years. These simple dams are mainly made up of local silicate rocks with minor cement and steel. Their sizes are generally 200 to 500 meters in length, 10 to 20 meters in height, and mean 200 meters in width. Excess water can easily across the simple dams to downstream. The total cost is approximately 1 million RBM for the constructure of one simple retaining dam that can be used for 30 years. The plateau internal is relatively flat so that a simple retaining dam can quickly transform 200 square kilometers barren deserts at least to artificial planting wetlands. As mentioned above in detail, approximately 30 tonnes of atmospheric CO₂ are removed per year per hectare by the grass wetlands (e.g., Figure 5 and Figure 13b). This means that it spends approximately 3 RMB to remove 60 tonnes atmospheric CO₂ per year. Therefore, the carbon uptake cost is less than 0.1 RMB/tonne/year here. Certainly, if local dominant species such as willows and pine trees are numerously planted in the artificial wetlands using the traditionally cheap approach, the carbon uptake cost will be even lower. Compared with the previously well-known approaches [7,8], this method is therefore the cheapest because of extremely low energy and infrastructure inputs every year. The global carbon neutrality is therefore achieved cheaply regardless of huge anthropogenic emissions in the near future. In addition, the mean global sea level hardly rises in the near future, as much freshwater is consecutively sent to Tibetan plateau and its adjacent inland regions from ocean [30,31,43], and thus cannot be back to ocean once again, partially due to these simply artificial retaining dams.

Conclusions

Although most Tibetan regions are barren deserts now, massively anthropogenic emissions are rapidly transformed to large amounts of secondary carbonates and organic carbon by a handful of Tibetan regions through the uniquely Tibetan geological carbon sink consisting of the enhanced subsurface silicate chemical weathering during summer and chemical sedimentation during winter, as well as the tectonic burial. The subsurface silicate chemical weathering is only working in the tectonically active silicate freshwater-enriched regions by coupled physiologically active plant roots and active tectonics. Whereas the chemical sedimentation occurs in the special setting of weak greenhouse effect with relatively high atmospheric CO₂ concentrations. The tectonic burial also occurs in the structurally unstable silicate regions rich in freshwater. Tibetan plateau boasts these unique features to become the only nascent carbon reservoir worldwide. The increasing in global atmospheric CO₂ concentrations is therefore regulated primarily by the Tibetan geological carbon sink. More importantly, the potential carbon uptake by the currently barren Tibetan regions where the tectonics are the most active could be greatly enhanced rapidly by turning these water-shortage silicate regions into artificially planting wetlands. The cheapest method for this transformation is simply to construct simple retaining dams at the specific valleys that can be used for several decades, as it requires extremely low energy and infrastructure inputs every year. If approximately 1 million square kilometers of Tibetan barren deserts are rapidly transformed to artificially planting wetlands within the tectonically active silicate regions, net dozens of gigatonnes of atmospheric CO₂ can be passively removed directly per year in the near future. The cost for the carbon uptake is as low as 0.1 RMB/tonne/year. Consequently, global carbon neutrality is easily achieved by the enhanced Tibetan geological carbon sink in the context of huge anthropogenic emissions.

Acknowledgments

We would really appreciate Dr. Yao Yang, Dr. Tingyuan Yuan, and Dr. Hongfei Liu for kind assistance during our field investigations.

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