Global Carbon Sink Research Articles

Evaluation and mapping soil organic carbon in seasonally frozen ground on the Tibetan Plateau

Seasonally frozen ground (SFG) significantly contributes to global carbon sinks. Global warming and anthropogenic-induced disturbances threaten the carbon storage capacity of SFG. Challenges in evaluating the SFG carbon storage potential include the lack of understanding of the control mechanisms of soil organic carbon (SOC) variations and timely spatial estimates of SOC. In this study, we investigated SOC stocks in SFG in the Tibet Autonomous Region, China, in 2020 and 2021. We employed partial least squares structural equation modeling (PLS-SEM) to explore the effect of complex processes (interacting roles of climate, plant physiology and phenology, freeze–thaw cycle, soil environment, and C inputs) on SOC and mapped SOC stocks in the topmost 30 cm. We identified four causal pathways: (1) an indirect pathway representing the effect of climate on plant physiology and phenology through changes in freeze–thaw cycles and soil environment, (2) an indirect pathway representing the effect of climate on C inputs through changes in freeze–thaw cycles, soil environment and plant physiology and phenology, (3) an indirect pathway representing the effect of climate on freeze–thaw cycles, and (4) an indirect pathway representing the effect of climate on the soil environment through changes in freeze–thaw cycles. C inputs exerted the greatest control on SOC. The effect of these factors decreased with increasing soil depth. We used PLS-SEM to generate maps of SOC stocks in SFG at a 500 m resolution with a moderate accuracy. The estimated mean SOC stocks in the 0–30 cm layer reached 6.87 kg m−2, with a 95% confidence interval ranging from 6.2 to 7.5 kg m−2. Our results indicated that it is critical to consider the depth dependence of the direct and indirect effects of environmental factors when assessing the control mechanisms of SOC variations. In this work, we also demonstrated that spatially explicit SOC estimates based on timely investigations are important for assessing C stocks against the background of considerable environmental changes across the Tibetan Plateau.

Blue carbon gain by plant invasion in saltmarsh overcompensated carbon loss by land reclamation

Saltmarsh is a major contributor to global blue carbon (C) sink. However, plant invasion and anthropic activities have greatly altered its distribution and its blue C function in the last decades. We used remote sensing and data synthesis to investigate how the four decades of plant invasion and land reclamation affect the spatiotemporal variation of blue C in Yancheng saltmarshes, the largest coastal ecosystem in eastern China. We revealed that the invasion of exotic cordgrass, Spartina alterniflora, greatly increased the regional blue C storage, mainly by the seaward expansion of cordgrass in mudflats and its high ecosystem C stock. The iconic Suaeda salsa “Red Beach” marshes shrank by 90%, mainly due to reed occupation by the native reed, Phragmites australis. Phragmites australis marsh area was greatly occupied by reclamation, but well offset by its intrusion into Suaeda salsa. The blue C gain by cordgrass invasion overcompensated C loss due to land reclamation and the loss of “Red Beach”. Our findings highlight that coastal blue C is greatly regulated by vegetation coverage, and the seaward cordgrass invasion can benefit the saltmarsh C sequestration. Such a similar process may apply in other regions of the world, with potentially large implications for blue C sinks.Graphical

Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

AbstractThis contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985–2018, using a combination of models and observation‐based products. The mean sea‐air CO2 flux from 1985 to 2018 is −1.6 ± 0.2 PgC yr−1 based on an ensemble of reconstructions of the history of sea surface pCO2 (pCO2 products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO2, which is estimated at −2.1 ± 0.3 PgC yr−1 by an ensemble of ocean biogeochemical models, and −2.4 ± 0.1 PgC yr−1 by two ocean circulation inverse models. The ocean also degasses about 0.65 ± 0.3 PgC yr−1 of terrestrially derived CO2, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO2 products reconstruct a trend in the ocean carbon sink of −0.61 ± 0.12 PgC yr−1 decade−1, while biogeochemical models and inverse models diagnose an anthropogenic CO2‐driven trend of −0.34 ± 0.06 and −0.41 ± 0.03 PgC yr−1 decade−1, respectively. This implies a climate‐forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate‐driven variability exceeding the CO2‐forced variability by 2–3 times. These results suggest that anthropogenic CO2 dominates the ocean CO2 sink, while climate‐driven variability is potentially large but highly uncertain and not consistently captured across different methods.

Limited driving of elevated CO2 on vegetation greening over global drylands

Drylands are the world’s largest biome and dominate the trends and interannual variability of global carbon sinks. Although a ‘greening’ trend of global drylands has been widely reported, large uncertainties remain in attributing its drivers. It is increasingly emphasized that elevated CO2 has greatly contributed to the vegetation greening over global drylands. Here we quantified the contributions of climate change, elevated CO2, and land use and land cover change (LULCC) on leaf area index (LAI) over drylands, using a process-based land surface model Noah-MP to investigate the drivers of vegetation change. The state-of-the-art model shows better performance in simulating the interannual variability of satellite-observed LAI over global drylands compared with that of the multi-model ensemble mean LAI from the TRENDY results. The area that LAI changes dominated by climate change (44.03%) is three times greater than that by CO2 (13.89%), and climate change also contributes most to the global drylands greening trend (55.07%). LULCC shows regional dominance over 13.35% of the global drylands, which is associated with afforestation, woody plant encroachment, and agricultural intensification. Our results imply that the vegetation greening area driven by elevated CO2 is much limited relative to the overwhelming climatic driving, which should be considered in predictions of trends and interannual variations of global carbon sinks.

Divergent response of leaf unfolding to climate warming in subtropical and temperate zones

The conservation of subtropical forest ecosystems is critical for stabilizing global vegetation productivity and land carbon sink. However, unlike temperate trees, the effects of climatic factors on the phenology of subtropical trees remain largely unclear. In this study, we examined the response of leaf unfolding to heat and chilling accumulation and drought by ridge regression using leaf unfolding data from 154 woody species across eastern China during 1963 and 2014 and compared the differences between subtropical and temperate zones. The results showed that the mean leaf unfolding dates for species in the subtropics were 20 days earlier than those in the temperate zone; similarly, heat accumulation for leaf unfolding in the subtropics was also significantly higher than that in the temperate zone, while chilling accumulation showed the opposite trend. Forcing temperature was still the primary driver of leaf unfolding in the both zones; nevertheless, there are more species that are not significantly affected by forcing temperature in the subtropics than in the temperate zones. Furthermore, more species showed delayed leaf unfolding due to reduced chilling in the subtropics, indicating that insufficient chilling caused by climate warming may have exerted a more noticeable effect on leaf unfolding in the subtropics than in the temperate zone. In addition, the average temperature sensitivity of leaf unfolding was higher in the subtropics than the temperate zone, which may have resulted from faster heat accumulation per 1 °C temperature increase, associated with more heat-accumulating hours due to the warmer climate in the subtropics. Moreover, a larger species-specific variation in the sensitivity of leaf unfolding to forcing temperature occurred in the subtropical zone than in the temperate zone. These differences in phenological responses among species may desynchronize ecological interactions and thereby threaten ecosystem function in the subtropics under continued warming.

Soil Respiration and Carbon Sequestration

An increase in the concentration of carbon dioxide in the atmosphere is a trigger for the activation of all processes of the carbon cycle, including soil respiration (SR), since it causes not only an increase in the greenhouse effect of the atmosphere, but also its fertilization. The consequence of fertilization is the tendency to increase the world’s net primary production of photosynthesis (NPP) and heterotrophic respiration of soils (RH). The increase in global terrestrial carbon sink was accompanied by an increase in CO2 in the atmosphere. The global increase in RH is due to global losses of soil organic carbon and is confirmed by models according to which the average residence time of organic carbon in the soil pool has decreased by 4.4 years over the past century. To assess the level of C sequestration in soils, it is necessary to determine the balance between the RH of the soil and the amount of new soil C-sink in the form of net biome production (NBP) – resistant to mineralization. Carbon sink into net ecosystem production (NEP) determines short-term unsustainable carbon sequestration.

Terrestrial Phosphorus Cycling: Responses to Climatic Change

Phosphorus (P) limits productivity in many ecosystems and has the potential to constrain the global carbon sink. The magnitude of these effects depends on how climate change and rising CO2 affect P cycling. Some effects are well established. First, P limitation often constrains CO2 fertilization, and rising CO2 often exacerbates P limitation. Second, P limitation and P constraints to CO2 fertilization are more common in warmer and wetter sites. Models that couple P cycling to vegetation generally capture these outcomes. However, due largely to differences between short-term and long-term dynamics, the patterns observed across climatic gradients do not necessarily indicate how climate change over years to decades will modify P limitation. These annual-to-decadal effects are not well understood. Furthermore, even for the well-understood patterns, much remains to be learned about the quantitative details, mechanisms, and drivers of variability. The interface between empirical and modeling work is particularly ripe for development.

Monitoring temporal changes in coastal mangroves to understand the impacts of climate change: Red Sea, Egypt

This paper focuses on the current state of mangroves in Egypt, analyses how climate change is affecting mangrove forests and suggests a conservation strategy to safeguard this valuable maritime resource. Mangroves in Egypt are primarily found in regions around the Red Sea, making them popular for ecotourism across the globe. A crucial breeding and feeding ground for many commercial fishery species, mangroves are a maritime habitat that is both economically and environmentally significant and a variety of living things call them home. Mangroves are one of the most threatened tropical and subtropical ecoregions in the world. These threats come from both natural and human sources, including oil spills, human waste runoff, herbicide use, and coastal development, all of which have the potential to harm the environment. Mangrove habitats are immediately impacted by salinity changes and rising sea levels brought on by climate change. This is unfortunate as mangroves are one of the most important global carbon sinks, and their loss due to climate change may accelerate sea level rise and ocean acidification, which will cause the extinction of numerous commercial fish species. A conservation strategy should be implemented to safeguard the mangroves around the Egyptian Red Sea by using mitigation techniques, creating marine protected zones near the mangroves, and consulting the owners of the mangrove stalks. Analysis of multi-temporal satellite imagery is one of the most significant methods to assist decision-makers and environmental planners in obtaining high-precision information about environmental and climatic changes. Analysis of satellite imagery is a valuable way to detect, monitor, assess and map the environmental, human, and natural activities that are threatening the growth of the mangrove forest ecosystem. Change detection analyses using satellite imagery were conducted to evaluate the effects of rapid coastal sustainable development, including human urban and tourism activities; the decline and destruction of large areas of the mangrove forest for animal feeding along camel trade roads; and the threat of natural impacts such as flash flooding, coastal and soil erosion or human impacts such as established desalination plants and other human activities along the coastal zone of the Egyptian Red Sea.

High rates of carbon burial linked to autochthonous production in artificial ponds

AbstractPonds are globally abundant and important to the global carbon cycle. Although ponds have large greenhouse gas emissions, they also sequester carbon in their sediments. Here, we studied organic carbon (OC) burial rates in 22 temperate experimental ponds with negligible watersheds, where carbon sequestration derives solely from autochthonous primary production. The ponds were built identically in 1964 and have since experienced different management strategies, allowing us to test how management actions influence burial rates. On average, the ponds accumulated 67.1 g OC m−2 yr−1 (range 38.3–113.6 g OC m−2 yr−1)—about double the global average burial rate for lakes and similar to global averages for wetlands. Carbon burial rates were higher in ponds with macrophytes, fish, and higher N : P loads. We contend that the global carbon sink for inland waters may be substantially underestimated due to the undervaluation of OC burial in natural and artificial ponds.

Ocean-based negative emissions technologies: a governance framework review

The model pathways of the Intergovernmental Panel for Climate Change (IPCC) for the timely achievement of global climate targets, especially the target of limiting global warming to 1.5°C compared to pre-industrial levels, suggest the need for safeguarding and enhancing the global carbon sink. Experts argue that the deployment of so-called negative emissions technologies for large-scale carbon dioxide removal holds potential for keeping the temperature in line with limits set by the Paris Agreement. Ocean-based negative emissions technologies (ONETs) intend to enhance carbon sequestration and storage in the ocean, e.g., by changing the ocean’s physical or biogeochemical properties. But in addition to these intended effects, ONETs may also cause unintentional impacts on the ocean’s condition and on related coastal and marine ecosystem services that are relevant for the attainment of a range of global policy goals. This article links potential direct and indirect, intentional and unintentional impacts of eight ONETs on the marine environment to the regulations and policy goals of international environmental agreements of the current global ocean governance regime. The results thereof outline a direct, implicit and indirect governance framework of ONETs. Hereby, a broader perspective of the concept of (global) ocean governance is adopted to outline a wider network that goes beyond the explicit regulation of ONETs within the realm of ocean governance. This first-order assessment derives gaps and challenges in the existing governance framework, as well as needs and opportunities for comprehensive governance of the technologies. It is determined that while the inclusion of ONETs in the global climate strategy may be deemed necessary for reaching net zero emission targets in the future, a range of potential trade-offs with other policy goals may need to be considered or dealt with when deploying ONETS for climate mitigation. Further, foresight-oriented and adaptive governance mechanisms appear imperative to bridge gaps resulting from extensive uncertainties and unknowns linked to ONET deployment in a changing ocean and. The identified ONET governance framework reiterates current challenges in ocean governance, for instance related to fragmentation, but also represents an opportunity for a synergistic and integrated approach to future governance.

Sensitivity of the Global Ocean Carbon Sink to the Ocean Skin in a Climate Model

AbstractThe ocean skin is composed of thin interfacial microlayers of temperature and mass of less than 1 mm where heat and chemical exchanges are controlled by molecular diffusion. It is characterized by a cooling of ∼−0.2 K and an increase in salinity of ∼0.1 g/kg (absolute salinity) relative to the water below. A surface observation‐based air‐sea CO2 flux estimate considering the variation of the CO2 concentration in these microlayers has been shown to lead to an increase in the global ocean sink of the anthropogenic CO2 by +0.4 PgC yr−1 (15% of the global sink). This study analyzes this effect in more details using a 15‐year (2000–2014) simulation from an Earth System Model (ESM) that incorporates a physical representation of the ocean surface layers (diurnal warm layer and rain lenses) and microlayers. Results show that considering the microlayers increases the simulated global ocean carbon sink by +0.26 to +0.37 PgC yr−1 depending on assumptions on the chemical equilibrium. This is indeed about 15% of the global sink (2.04 PgC yr−1) simulated by the ESM. However, enabling the ocean skin adjustment to feedback on ocean carbon concentrations reduces this increase to only +0.13 (±0.09) PgC y−1. Coupled models underestimate the ocean carbon sink by ∼5% if the ocean skin effect is not included.

CO2 Consumption Rates in the Glacierized Himalayan Headwaters: The Importance of Sulfuric and Nitric Acid‐Mediated Chemical Weathering Reactions in Geologic Carbon Cycle

AbstractSilicate and carbonate weathering reactions consume atmospheric CO2 depending on the type of weathering agents, namely carbonic (H2CO3), sulfuric (H2SO4), and nitric acids (HNO3), and have potential climate implications. However, the importance of HNO3 in weathering processes in the Himalayan glacierized basins has not been examined yet but is critical to better constrain the concomitant short (<103 years) and long‐term (>106 years) variability in the carbon cycle as it can drive negative feedback to a climate. By analyzing time‐series hydro‐geochemical data of proglacial meltwater in the Ganga headwaters of Central Himalaya, we demonstrate that the weathering rate of carbonate minerals is increased 1.06 times when the role of HNO3 is considered together with H2CO3 and H2SO4 in comparison to the role of H2CO3 and H2SO4. However, we also observe that the CO2 drawdown rate decreases 1.13 times and 1.06 times when the role of all three acids is considered in silicate and carbonate weathering reactions, respectively, compared to the CO2 drawdown rates linked to the role of H2CO3 and H2SO4. Moreover, the involvement of HNO3 in chemical weathering can reduce the inorganic global carbon sink by releasing CO2 into the ocean‐atmosphere system. We conclude that HNO3‐mediated chemical weathering reactions are important processes that alter the geologic carbon cycle of high‐altitude glacierized Himalayan catchments as well as on a global scale.

Interannual variability of spring and summer monsoon growing season carbon exchange at a semiarid savanna over nearly two decades

Eddy covariance measurements of land-atmosphere energy, carbon, and water exchange now span multiple decades at some sites, supporting an improved understanding of flux interannual variability (IAV) and its ecophysiological and physical controls. Most eddy covariance IAV studies have focused on temperate forest ecosystems, where carbon fluxes are large and flux records are longest – but also where IAV is much lower than in dryland regions, which have been identified as an essential driver of the trend and variability in the global terrestrial carbon sink. In this study, we leveraged 19 years of continuous micrometeorological measurements at the AmeriFlux US-SRM mesquite savanna site in southern Arizona, USA to quantify the IAV, trends, and drivers of carbon fluxes during the distinct spring and summer growing seasons. We also assessed the ability of modern satellite and land surface models to capture the IAV of seasonal water and carbon fluxes. Annual net ecosystem production (NEP) was small and highly variable (23 +/- 64 gC m − 2 yr−1). Precipitation and associated measures of water availability determined most of the variability in NEP, largely through their influence on annual and seasonal gross ecosystem productivity (GEP) as opposed to ecosystem respiration (ER). Root-zone soil moisture captured between 73% (spring) and 85% (summer) of GEP variability and between 73% (spring) and 58% (summer) of ER variability. Throughout the study period, soil moisture and greenness increased with associated increases in GEP, ER and NEP. These trends were strongly influenced by very productive and wet summer growing seasons during the last two years, which were characterized by abundant understory grass cover. Typically, less than half of the variability in growing season GEP and evapotranspiration was captured by satellite-based estimates and land surface model simulations with local site forcing and calibration, highlighting the ongoing utility of long-term datasets to support careful model testing and improvement.

Modeling the impact of urbanization and climate changes on terrestrial vegetation productivity in China by a neighborhood substitution analysis

Vegetation plays a vital role in global carbon sink in terrestrial ecosystems and could be affected by climate changes and human activities. In the current context of rapid urban expansion, understanding the impacts of urbanization and climate changes on net primary productivity (NPP) is helpful to sequester more atmospheric carbon and achieve carbon neutrality. We explored the terrestrial spatio-temporal NPP dynamics in China during the years 2000–2010 and 2010–2020, respectively, using the Carnegie-Ames-Stanford Approach (CASA) model based on multi-source remote sensing data. We then proposed a neighborhood substitution model to isolate the effects of urbanization and climate changes on NPP and examined the driving forces for the NPP update. The results revealed that while urbanization was likely to reduce NPP, averagely by 48.57 Tg C and 50.13 Tg C in the two stages, climate changes improved NPP by 97.71 Tg C and 92.46 Tg C, respectively, indicating that the climate changes offset the reduced vegetation productivity from rapid urbanization. The results highlighted that the rapidly urbanizing process reduced vegetation productivity due to the lost vegetated land and degraded vegetation productivity. In addition, the residual effect other than urbanization and the climate changes also played a part on the degraded vegetation carbon sequestration, reducing NPP by an average of 4.29 Tg C year−1 and 3.94 Tg C year−1 in the two stages, which could be related to other human activities. We recommend protecting vegetation cover and making informed land use plan as means to improve carbon sequestration in the context of rapid urban expansion and climate changes.

Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities.

Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities.

Underestimated Interannual Variability of Terrestrial Vegetation Production by Terrestrial Ecosystem Models

AbstractVegetation gross primary production (GPP) is the largest terrestrial carbon flux and plays an important role in regulating the carbon sink. Current terrestrial ecosystem models (TEMs) are indispensable tools for evaluating and predicting GPP. However, to which degree the TEMs can capture the interannual variability (IAV) of GPP remains unclear. With large data sets of remote sensing, in situ observations, and predictions of TEMs at a global scale, this study found that the current TEMs substantially underestimate the GPP IAV in comparison to observations at global flux towers. Our results also showed the larger underestimations of IAV in GPP at nonforest ecosystem types than forest types, especially in arid and semiarid grassland and shrubland. One cause of the underestimation is that the IAV in GPP predicted by models is strongly dependent on canopy structure, that is, leaf area index (LAI), and the models underestimate the changes of canopy physiology responding to climate change. On the other hand, the simulated interannual variations of LAI are much less than the observed. Our results highlight the importance of improving TEMs by precisely characterizing the contribution of canopy physiological changes on the IAV in GPP and of clarifying the reason for the underestimated IAV in LAI. With these efforts, it may be possible to accurately predict the IAV in GPP and the stability of the global carbon sink in the context of global climate change.

High Resolution Fourier Transform Spectrometer for Ground-Based Verification of Greenhouse Gases Satellites

Satellite remote sensing is currently the best monitoring means to obtain global carbon source and sink data. The United States, Japan, China and other countries are vigorously developing spaceborne detection technology. However, the important factors that restrict the application of greenhouse gas satellite remote sensing technology include the limited accuracy of data products. How to improve the retrieval level of greenhouse gas payloads is a problem that needs to be solved urgently. One effective way to improve data quality is to carry out satellite ground synchronous authenticity verification and system error correction. This paper mainly aims at the shortcomings of the existing TCCON and the portable verification equipment EM27/SUN, and develops a High-Resolution Fourier Transform Spectrometer (HRFTS) based on dynamic collimation technology. Through the gas absorption method and the band scanning method of the hyperspectral monochromatic light source, the instrument’s absorption spectrum measurement capability and the Instrument Line Shape (ILS) are demonstrated. The instrument’s spectral resolution is consistent with the on-orbit greenhouse gas satellite load, reaching 0.26 cm−1. For the interference data obtained by the spectrometer, spectral restoration processing, data quality control and inversion algorithm optimization were carried out to solve the problems of baseline correction, spectral fine registration, and environmental parameter profile reconstruction, and cross comparison experiments with EM27/SUN were carried out simultaneously. Finally, for the gases monitoring instrument (GMI) of the GF5-02 satellite launched on 7 September 2021, the first satellite ground synchronization verification experiment with high space-time matching was carried out. The results showed that the CO2 column concentration deviation of the satellite ground synchronization inversion was about 1.5 ppm, and the CH4 column concentration deviation was about 11.3 ppb, which verified the on-orbit detection accuracy of the GMI, and laid a foundation for the subsequent satellite inversion algorithm optimization and systematic error correction.

Estimation of global karst carbon sink from 1950s to 2050s using response surface methodology

ABSTRACT For the estimation of global karst carbon sink, a few conventional methods usually require the parameters that are difficult to measure, resulting in the big cost. Moreover, under the constraints of incomplete and timeliness issues in the collection of data over a large region, it has remained a challenge for these methods to study global karst carbon sink. Therefore, this paper proposes estimating the global karst carbon sink, and analyzing the suitability of the response surface methodology and the fluctuating variation of karst carbon sink in global karst regions from 1951 to 2050. This paper shows that the proposed method can reduce the time of numerical calculation and is suitable for application in global weathering models; The global karst carbon sink in the future changes not only displays an upward trend but also exposures its fluctuating trend largely. This fluctuation is probably due to global warming.

Diagnosing destabilization risk in global land carbon sinks.

Global net land carbon uptake or net biome production (NBP) has increased during recent decades1. Whether its temporal variability and autocorrelation have changed during this period, however, remains elusive, even though an increase in both could indicate an increased potential for a destabilized carbon sink2,3. Here, we investigate the trends and controls of net terrestrial carbon uptake and its temporal variability and autocorrelation from 1981 to 2018 using two atmospheric-inversion models, the amplitude of the seasonal cycle of atmospheric CO2 concentration derived from nine monitoring stations distributed across the Pacific Ocean and dynamic global vegetation models. We find that annual NBP and its interdecadal variability increased globally whereas temporal autocorrelation decreased. We observe a separation of regions characterized by increasingly variable NBP, associated with warm regions and increasingly variable temperatures, lower and weaker positivetrends in NBP and regions where NBP became stronger and less variable. Plant species richness presented a concave-down parabolic spatial relationship with NBP and its variability at the global scale whereas nitrogen deposition generally increased NBP. Increasing temperature and its increasing variability appear as the most important drivers of declining and increasingly variable NBP. Our results show increasing variability of NBP regionally that can be mostly attributed to climate change and that may point to destabilization of the coupled carbon-climate system.

Global and northern-high-latitude net ecosystem production in the 21st century from CMIP6 experiments

Abstract. Climate warming is accelerating the changes in the global terrestrial ecosystems and particularly those in the northern high latitudes (NHLs; poleward of 50∘ N) and rendering the land–atmosphere carbon exchange highly uncertain. The Coupled Model Intercomparison Project Phase 6 (CMIP6) employs the most updated climate models to estimate terrestrial ecosystem carbon dynamics driven by a new set of socioeconomic and climate change pathways. By analyzing the future (2015–2100) carbon fluxes estimated by 10 CMIP6 models, we quantitatively evaluated the projected magnitudes, trends, and uncertainties in the global and NHL carbon fluxes under four scenarios plus the role of NHLs in the global terrestrial ecosystem carbon dynamics. Overall, the models suggest that the global and NHL terrestrial ecosystems will be consistent carbon sinks in the future, and the magnitude of the carbon sinks is projected to be larger under scenarios with higher radiative forcing. By the end of this century, the models on average estimate the NHL net ecosystem productivity (NEP) as 0.54 ± 0.77, 1.01 ± 0.98, 0.97 ± 1.62, and 1.05 ± 1.83 Pg C yr−1 under SSP126, SSP245, SSP370, and SSP585, respectively. The uncertainties are not substantially reduced compared with earlier results, e.g., the Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP). Although NHLs contribute a small fraction of the global carbon sink (∼ 13 %), the relative uncertainties in NHL NEP are much larger than the global level. Our results provide insights into future carbon flux evolutions under future scenarios and highlight the urgent need to constrain the large uncertainties associated with model projections for making better climate mitigation strategies.