Sink For Atmospheric CO2 Research Articles

Spatial 3D mapping of forest soil carbon stocks in Hesse, Germany

AbstractBackgroundForest soils are an important reservoir of organic carbon (OC) and a potential source or sink for atmospheric CO2. Prediction of OC stock changes under ongoing climatic and management changes requires a spatial explicit base. Knowledge of the vertical distribution of OC stocks is very important, since varying soil layers may be affected differently by environmental change.AimThree‐dimensional regionalization of OC stocks of the forest floor (FFC) and 5 cm depth increments of the mineral soil (SOC) of Hesse, Germany.MethodsDatasets of the second National Forest Soil Inventory (NFSI II) were used for parametrization of hierarchical generalized additive models (hGAM). Validation was performed by a 10 times repeated 10‐fold cross‐validation, and spatial model uncertainty was assessed.ResultsDepth‐dependent validation indicated that model performance was best between 15 and 60 cm (amount of variance explained ≈ 0.5). All covariates showed plausible partial effects. FFC stocks were predicted to be highest under coniferous forest with a high influence of N deposition. Climate and potential cation exchange capacity affected SOC stocks markedly, whereas soil class and parent material were most important for the depth distribution. Overall, average predicted OC stocks were between 78.0 and 92.5 t ha−1, amounting to 67.6 to 80.1 Mt for all forest soils of Hesse. Between 16% and 24% were stored in the forest floor. Sixty‐eight percent to 69% of predicted SOC stocks were stored in the upper 30 cm. Model uncertainty was highest at locations with high elevation or ground water influence.ConclusionsThis work provides the first spatial explicit database for OC stocks of forest soils in Hesse at an intermediate scale. Stocks can be assessed flexibly for varying depth from the forest floor down to 100 cm. Uncertainty analysis informs about locations, where the model results have to be handled with care.

Influence of the freshwater plume dynamics and the barrier layer thickness on the CO2 source and sink characteristics of the Bay of Bengal

The high influx of freshwater in the Bay of Bengal (BoB) region forms a freshwater plume at the surface. This plume's dispersal spreads a low-density layer of water on the surface, forming a barrier layer between the mixed layer depth and the isothermal layer. The low saline water reduces the sea surface partial pressure of carbon dioxide (pCO2), and the barrier layer thickness (BLT) inhibits the interaction between the subsurface and surface layers of BoB. In this study, we run a high resolution coupled physical-biogeochemical model to understand the effect of variability of freshwater plume spread (FPS) and the BLT on the seasonality of sea-surface pCO2. By comparing with observation data, we find that the model satisfactorily simulates the sea-surface pCO2 and the FPS for the BoB region. As the FPS begins to develop in the southwest monsoon season, the CO2 sink region also begins to form in the same manner. The region with sea surface salinity (SSS) less than 31 PSU was recognized as a freshwater plume. The region with SSS lower than 31 PSU showed under-saturated levels of pCO2. The model analysis showed that the CO2 sink spread exceeded the FPS in the winter monsoon season, covering almost 50% of the BoB region. This spread was attributed to the BLT, which is highest during the winter monsoon season. Based on the spatial CO2 flux, we find the Northern BoB a sink of atmospheric CO2 (−0.35±0.75 mol C m-2 yr-1). The Southwestern BoB acts as a source of CO2 (0.72±0.61 mol C m-2 yr-1) throughout the year and has the highest magnitude of flux, whereas the Southeastern BoB also acts as a weak source of CO2 (0.214±0.52 mol C m-2 yr-1).

Analysis of the Seasonal and Interannual Variations of Air‐Sea CO2 Flux in the Chukchi Sea Using a Coupled Ocean‐Sea Ice‐Biogeochemical Model

AbstractUnderstanding the responses of air‐sea carbon dioxide (CO2) flux to rapid environmental changes in the highly productive Chukchi Sea is of great significance for the climate prediction in the Arctic Ocean. Through analyzing the 1998–2015 hindcast simulation of a coupled ocean–sea ice–biogeochemical model, this study identifies that the key factors influencing oceanic carbon uptake in the Chukchi Sea vary seasonally. The effects of these seasonal carbon uptakes on the total annual carbon sink vary at different timescales. The model solution suggests that the Chukchi Sea acts as a perennial carbon sink of atmospheric CO2 with the annual mean influx of −3.73 ± 3.90 mmol m−2 d−1. Quantitative analyses reveal that the sea ice concentration predominates the annual cycle of air‐sea CO2 flux, while primary production and wind speed strengthen the capacity for carbon uptake during June–July (melt season) and October–December (freeze season), respectively. The total annual carbon sink shows an increasing trend during 1998–2015 with a multi‐year average of 9.22 ± 1.93 Tg C y−1. The carbon uptakes during June–July and August–September (open water season), which are respectively regulated by primary production and open water area at interannual scale, present significant long‐term increases and jointly determine the secular increase trend of the total annual carbon sink. Interannual CO2 absorption during the freeze season is sensitive to the sea ice concentration and wind conditions, presenting the greatest year‐to‐year variation but insignificant long‐term trend, and modulates the interannual amplitude of the total annual carbon sink.

Seasonal not annual precipitation drives 8-year variability of interannual net CO2 exchange in a salt marsh

Salt marshes are significant contributors to global “blue carbon” resources, and these habitats are sensitive to precipitation events due to periodically dry-wet alternation induced by tides. However, whether annual marsh-atmospheric CO2 flux responds more to annual or seasonal precipitation remains unclear. Here, eight years (2012-2019) of eddy covariance data were evaluated to determine typical CO2 budgets, and we assessed the effect of annual and seasonal precipitation on interannual net ecosystem CO2 exchange (NEE) in a salt marsh of the Yellow River Delta, China. The salt marsh was a sink for atmospheric CO2 in each of the eight study years, with an 8-year average NEE of -51.7 ± 9.7 g C m−2 a−1 varying from -8 to -85 g C m−2 a−1. Annual NEE was mainly regulated by precipitation levels during the early growth stage of plants, which modulated maximal plant biomass accumulation via water-salt transport. Besides, a spring precipitation distribution experiment showed that wetter early growth stage of plants enhanced carbon assimilation capacity in the salt marsh by decreasing soil salinity and promoting plant biomass accumulation. These findings suggest that soil water-salt conditions induced by precipitation during the onset of the growing season are crucial for interannual variations of NEE in salt marshes.

Decadal Dynamics of the CO2 System and Associated Ocean Acidification in Coastal Ecosystems of the North East Atlantic Ocean

Weekly and bi-monthly carbonate system parameters and ancillary data were collected from 2008 to 2020 in three coastal ecosystems of the southern Western English Channel (sWEC) (SOMLIT-pier and SOMLIT-offshore) and Bay of Brest (SOMLIT-Brest) located in the North East Atlantic Ocean. The main drivers of seasonal and interannual partial pressure of CO2 (pCO2) and dissolved inorganic carbon (DIC) variabilities were the net ecosystem production (NEP) and thermodynamics. Differences were observed between stations, with a higher biological influence on pCO2 and DIC in the near-shore ecosystems, driven by both benthic and pelagic communities. The impact of riverine inputs on DIC dynamics was more pronounced at SOMLIT-Brest (7%) than at SOMLIT-pier (3%) and SOMLIT-offshore (<1%). These three ecosystems acted as a weak source of CO2 to the atmosphere of 0.18 ± 0.10, 0.11 ± 0.12, and 0.39 ± 0.08 mol m–2 year–1, respectively. Interannually, air-sea CO2 fluxes (FCO2) variability was low at SOMLIT-offshore and SOMLIT-pier, whereas SOMLIT-Brest occasionally switched to weak annual sinks of atmospheric CO2, driven by enhanced spring NEP compared to annual means. Over the 2008–2018 period, monthly total alkalinity (TA) and DIC anomalies were characterized by significant positive trends (p-values < 0.001), from 0.49 ± 0.20 to 2.21 ± 0.39 μmol kg−1 year−1 for TA, and from 1.93 ± 0.28 to 2.98 ± 0.39 μmol kg–1 year–1 for DIC. These trends were associated with significant increases of calculated seawater pCO2, ranging from +2.95 ± 1.04 to 3.52 ± 0.47 μatm year–1, and strong reductions of calculated pHin situ, with a mean pHin situ decrease of 0.0028 year–1. This ocean acidification (OA) was driven by atmospheric CO2 forcing (57–66%), Sea surface temperature (SST) increase (31–37%), and changes in salinity (2–5%). Additional pHin situ data extended these observed trends to the 2008–2020 period and indicated an acceleration of OA, reflected by a mean pHin situ decrease of 0.0046 year–1 in the sWEC for that period. Further observations over the 1998–2020 period revealed that the climatic indices North Atlantic Oscillation (NAO) and Atlantic Multidecadal Variability (AMV) were linked to trends of SST, with cooling during 1998–2010 and warming during 2010–2020, which might have impacted OA trends at our coastal stations. These results suggested large temporal variability of OA in coastal ecosystems of the sWEC and underlined the necessity to maintain high-resolution and long-term observations of carbonate parameters in coastal ecosystems.

Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

High-latitude fjords and continental shelves are shown to be sinks for atmospheric CO2, yet large spatial-temporal variability and poor regional coverage of sea-air CO2 flux data, especially from fjord systems, makes it difficult to scale our knowledge on how they contribute to atmospheric carbon regulation. The magnitude and seasonal variability of atmosphere-sea CO2 flux was investigated in high-latitude northern Norwegian coastal areas over 2018 and 2019, including four fjords and one coastal bay. The aim was to assess the physical and biogeochemical factors controlling CO2 flux and partial pressure of CO2 in surface water via correlation to physical oceanographic and biological measurements. The results show that the study region acts as an overall atmospheric CO2 sink throughout the year, largely due to the strong undersaturation of CO2 relative to atmospheric concentrations. Wind speed exerted the strongest influence on the instantaneous rate of sea-air CO2 exchange, while exhibiting high variability. We concluded that the northernmost fjords (Altafjord and Porsangerfjord) showed stronger potential for instantaneous CO2 uptake due to higher wind speeds. We also found that fixation of CO2 was likely a significant factor controlling ΔpCO2 from April to June, which followed phenology of spring phytoplankton blooms at each location. Decreased ΔpCO2 and the resulting sea-air CO2 flux was observed in autumn due to a combined reduction of the mixed layer with entrain of high CO2 subsurface water, damped biological activity and higher surface water temperatures. This study provides the first measurements of atmospheric CO2 flux in these fjord systems and therefore an important new baseline for gaining a better understanding on how the northern Norwegian coast and characteristic fjord systems participate in atmosphere carbon regulation.

Sea Ice Suppression of CO2 Outgassing in the West Antarctic Peninsula: Implications For The Evolving Southern Ocean Carbon Sink

AbstractThe Southern Ocean plays an important role in the uptake of atmospheric CO2. In seasonally ice‐covered regions, estimates of air‐sea exchange remain uncertain in part because of a lack of observations outside the summer season. Here we present new estimates of air‐sea CO2 flux in the West Antarctic Peninsula (WAP) from an autonomous mooring on the continental shelf. In summer, the WAP is a sink for atmospheric CO2 followed by a slow return to atmospheric equilibrium in autumn and winter. Outgassing is almost entirely suppressed by ice cover from June through October, resulting in a modest net annual CO2 sink. Model projections indicate sea ice formation will occur later in the season in the coming decades potentially weakening the net oceanic CO2 sink. Interannual variability in the WAP is significant, highlighting the importance of sustained observations of air‐sea exchange in this rapidly changing region of the Southern Ocean.

Contribution of Biological Effects to Carbonate‐System Variations and the Air–Water CO2 Flux in Urbanized Bays in Japan

AbstractWe evaluated the contribution of net biological effects (photosynthesis + respiration + decomposition) to the carbonate parameters and air–water CO2 fluxes in Tokyo Bay, Ise Bay, and Osaka Bay in Japan. The carbonate parameters (fugacity of CO2, total alkalinity, and dissolved inorganic carbon) were measured mainly by cargo ships traveling between Japan and other countries. We used the measurement data from three inner bays and surrounding outer bays in Japan along with reference data from previous studies for complementary analysis. We found that (a) the inner and outer bays in this study were strong annual atmospheric CO2 sinks, (b) the annual biological effect on the air–water CO2 fluxes was about 6%–27% of the measured CO2 fluxes, and it affected the seasonal variation of the CO2 flux, and (c) the biological effect was largest in Tokyo Bay, and almost the same in Ise and Osaka Bays. The intensity of the biological effect corresponded mainly with nutrient concentrations, which seemed to be controlled by the wastewater treatment in urbanized areas around the bays. Our results suggest that labile carbon/nutrient ratio of wastewater should be a major consideration for evaluating the biological effect on the carbon cycle in urbanized bays, which will continue to expand globally.

Hydrology controls the carbon mass balance of a mountain lake in the eastern European Alps

AbstractLakes and streams in mountain regions are important contributors to carbon (C) fluxes. However, detailed carbon balances of the stream‐lake continuum are rare. Combining eddy covariance (EC) measurements of lake‐atmosphere net ecosystem CO2 exchange with measurements of fluvial C fluxes (dissolved organic C, DOC; particulate organic C, POC; dissolved inorganic C, DIC), and in‐lake sedimentation, we here present a detailed annual C balance of an oligotrophic clearwater lake in the eastern European Alps. The C flux into the lake was 1522 Mg C year−1 by DIC (93%), DOC (6%), and POC (0.7%). The largest C losses were fluvial exports (1595 Mg C year−1) of DIC (93%), DOC (6%), and POC (0.8%), while sedimentation accounted for 7.3 Mg C year−1. The residual of all fluvial and sedimentation fluxes revealed the lake as a net sink of atmospheric CO2 of 77 Mg C year−1. The EC measurement confirmed a small positive or negative contribution of atmospheric exchange to the lake C balance. In‐lake transfer among C pools was only significant for the flux from DIC to POC (8.4 Mg C year−1), which, following our model, is the transfer through primary production in summer. Fluvial DOC and DIC fluxes were controlled by discharge; POC retention and sedimentation depended on the meteorological season and in‐lake residence time. Following our findings, we conclude that hydrology acted as the most important control for the C balance of this clearwater mountain lake by controlling inflow, outflow, and sedimentation fluxes.

Calcium isotope evidence for early Archaean carbonates and subduction of oceanic crust

Continents are unique to Earth and played a role in coevolution of the atmosphere, hydrosphere, and biosphere. Debate exists, however, regarding continent formation and the onset of subduction-driven plate tectonics. We present Ca isotope and trace-element data from modern and ancient (4.0 to 2.8 Ga) granitoids and phase equilibrium models indicating that Ca isotope fractionations are dominantly controlled by geothermal gradients. The results require gradients of 500–750 °C/GPa, as found in modern (hot) subduction-zones and consistent with the operation of subduction throughout the Archaean. Two granitoids from the Nuvvuagittuq Supracrustal Belt, Canada, however, cannot be explained through magmatic processes. Their isotopic signatures were likely inherited from carbonate sediments. These samples (> 3.8 Ga) predate the oldest known carbonates preserved in the rock record and confirm that carbonate precipitation in Eoarchaean oceans provided an important sink for atmospheric CO2. Our results suggest that subduction-driven plate tectonic processes started prior to ~3.8 Ga.

Beyond biomass to carbon fluxes: application and evaluation of a comprehensive forest carbon monitoring system

Accurate quantification of forest carbon stocks and fluxes over regions is needed to monitor forest resources as they respond to changes in climate, disturbance and management, and also to evaluate contributions of forest sector to the regional and global carbon balances. In previous work we introduced a national forest carbon monitoring system (NFCMS) that combines forest inventory data, satellite remote sensing of stand biomass and forest disturbances, and an ecosystem carbon cycle model to assess contemporary forest carbon dynamics at a 30 m resolution. In this study, we evaluate the NFCMS estimates of biomass and carbon fluxes with available data products for the Pacific Northwest (PNW) region, and then analyze the regional carbon balance over the period 1986–2010. The biomass estimates have good agreements with evaluation datasets (eMapR, NBCD2000, and Hagen2005) at regional and forest type levels, and at spatial scales of 1 km2 and larger. Regionwide, PNW forests acted as a stable net sink for atmospheric CO2 (18.5 Tg C yr–1) within forestlands. However, harvesting activities removed significant amounts of carbon, equating to over 75% of annual net carbon sink, though only 25% of this (∼3.5 Tg C yr–1) is emitted to the atmosphere within 50 years. Wildfires contributed modestly to carbon emissions in most years, however, the severe fires of 2002 and 2006 released 16.6 and 7.1 Tg C, respectively. The study demonstrates the potential of the NFCMS framework to serve as a candidate measuring, reporting and verification system, informed by field and remotely sensed inventories, and tracking the carbon balance of the forest sector across the United States.

Sink or source? Methane and carbon dioxide emissions from cryoconite holes, subglacial sediments, and proglacial river runoff during intensive glacier melting on the Tibetan Plateau

High Mountain Asia glaciers are currently ignored in the estimation of global greenhouse gas budgets (e.g., methane (CH4) and carbon dioxide (CO2)). Similar to the Asian Water Tower and Third Pole, the Tibetan Plateau (TP) hosts the largest volume of glaciers outside the polar regions. These glaciers contain large reservoirs of organic carbon that can influence glacial ecosystems under rapid melting. However, no data exist on the current footprint of CH4 and CO2 from glaciers in the TP. Here, we report in situ observations of CH4 and CO2 fluxes for glacial cryoconite holes, subglacial sediments, and proglacial river runoff across the TP. Our results indicate that cryoconite holes and subglacial sediments can accelerate the export of greenhouse gasses during the melting season due to intensive glacier melting. However, to some extent, proglacial river runoff can be a significant sink of atmospheric CO2; this fact was not identified in previous studies. Our findings suggest that variations (source or sink) of greenhouse gasses from TP glacial basins should be considered in regional CH4 and CO2 budgets under climate warming.

Chemical weathering and carbon dioxide consumption in a small tropical river catchment, southwestern India

Studies done on small tropical west-flowing river catchments located in the Western Ghats in southwestern India have suggested very intense chemical weathering rates and associated CO2 consumption. Very less studies are reported from these catchments notwithstanding their importance as potential sinks of atmospheric CO2 at the global scale. A total of 156 samples were collected from a small river catchment in the southwestern India, the Payaswini–Chandragiri river Basin, during pre-monsoon, monsoon and post-monsoon seasons in 2016 and 2017, respectively. This river system comprises two small rivers originating at an elevation of 1350 m in the Western Ghats in peninsular India. The catchment area is dominated by biotite sillimanite gneiss. Sodium is the dominant cation, contributing ~ 50% of the total cations, whereas HCO3− contributes ~ 75% of total anions. The average anion concentration in the samples varied in the range HCO3− > Cl− > SO42− > NO3− > F−, whereas major cation concentration varied in the range Na+ > Ca2+ > Mg2+ > K+. The average silicate weathering rate (SWR) was 42 t km−2 y−1 in the year 2016 and 36 t km−2 y−1 in 2017. The average annual carbon dioxide consumption rate (CCR) due to silicate rock weathering was 9.6 × 105 mol km−2y−1 and 8.3 × 105 mol km−2 y−1 for 2016 and 2017, respectively. The CCR in the study area is higher than other large tropical river catchments like Amazon, Congo-Zaire, Orinoco, Parana and Indus because of its unique topography, hot and humid climate and intense rainfall.

Heterogeneous CO<sub>2</sub> and CH<sub>4</sub> content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

Abstract. Accelerated melting of the Greenland Ice Sheet has increased freshwater delivery to the Arctic Ocean and amplified the need to understand the impact of Greenland Ice Sheet meltwater on Arctic greenhouse gas budgets. We evaluate subglacial discharge from the Greenland Ice Sheet for carbon dioxide (CO2) and methane (CH4) concentrations and δ13C values and use geochemical models to evaluate subglacial CH4 and CO2 sources and sinks. We compare discharge from southwest (a sub-catchment of the Isunnguata Glacier, sub-Isunnguata, and the Russell Glacier) and southern Greenland (Kiattut Sermiat). Meltwater CH4 concentrations vary by orders of magnitude between sites and are saturated with respect to atmospheric concentrations at Kiattut Sermiat. In contrast, meltwaters from southwest sites are supersaturated, even though oxidation reduces CH4 concentrations by up to 50 % during periods of low discharge. CO2 concentrations range from supersaturated at sub-Isunnguata to undersaturated at Kiattut Sermiat. CO2 is consumed by mineral weathering throughout the melt season at all sites; however, differences in the magnitude of subglacial CO2 sources result in meltwaters that are either sources or sinks of atmospheric CO2. At the sub-Isunnguata site, the predominant source of CO2 is organic matter (OM) remineralization. However, multiple or heterogeneous subglacial CO2 sources maintain atmospheric CO2 concentrations at Russell but not at Kiattut Sermiat, where CO2 is undersaturated. These results highlight a previously unrecognized degree of heterogeneity in greenhouse gas dynamics under the Greenland Ice Sheet. Future work should constrain the extent and controls of heterogeneity to improve our understanding of the impact of Greenland Ice Sheet melt on Arctic greenhouse gas budgets, as well as the role of continental ice sheets in greenhouse gas variations over glacial–interglacial timescales.

Improved understanding of dynamic water and mass budgets of high‐alpine karst systems obtained from studying a well‐defined catchment area

AbstractLarge areas of Europe, especially in the Alps, are covered by carbonate rocks and in many alpine regions, karst springs are important sources for drinking water supply. Because of their high variability and heterogeneity, the understanding of the hydrogeological functioning of karst aquifers is of particular importance for their protection and utilisation. Climate change and heavy rainfall events are major challenges in managing alpine karst aquifers which possess an enormous potential for future drinking water supply. In this study, we present research from a high‐alpine karst system in the UNESCO Biosphere Reserve Großes Walsertal in Austria, which has a clearly defined catchment and is drained by only one spring system. Results show that (a) the investigated system is a highly dynamic karst aquifer with distinct reactions to rainfall events in discharge and electrical conductivity; (b) the estimated transient atmospheric CO2 sink is about 270 t/a; (c) the calculated carbonate rock denudation rate is between 23 and 47 mm/1000a and (d) the rainfall‐discharge behaviour and the internal flow dynamics can be successfully simulated using the modelling package KarstMod. The modelling results indicate the relevance of matrix storage in determining the discharge behaviour of the spring, particularly during low‐flow periods. This research and the consequent results can contribute and initiate a better understanding and management of alpine karst aquifers considering climate change with more heavy rainfall events and also longer dry periods.

Retrieval of Carbon Dioxide Using Cross-Track Infrared Sounder (CrIS) on S-NPP

The Cross-track Infrared Sounder (CrIS) aboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite is a spaceborne Fourier transform infrared spectrometer. The study aims to retrieve carbon dioxide (CO2) information (the CO2 profile and column-averaged dry-air mole fraction of XCO2) from June 2018 to December 2019 based on the The National Oceanic and Atmospheric Administration (NOAA)-Unique Combined Atmospheric Processing System (NUCAPS) Cloud-Cleared Radiances (CCRs) via the CrIS. The CCRs products for the CrIS with 2223 channels have been available since 22 May 2018. Characteristics of the CO2 weighting functions inform the choice of multiple channels that are around 15 μm in size that differ by latitude and season to maximize retrieval sensitivity to CO2 and minimize sensitivity to other interfering atmospheric parameters. CO2 was retrieved from these channels using an adopted nonlinear optimization algorithm. The temperature, water vapor, and ozone profiles used in the inversion process were gathered from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5). Validations of CO2 concentrations as retrieved from CrIS showed the following conclusions: (1) The relative error of the retrieved CO2 concentrations, as compared to Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) in situ aircraft measurements, was less than 0.5%, and the root mean square errors (RMSE) were less than 0.7 ppmv (with correlation coefficients of 0.56–0.86); (2) the retrieved XCO2 from June 2018 to December 2019 correlated well with the ground-based Total Carbon Column Observing Network (TCCON) observations, and the differences were within ±0.2 ppmv. Further analysis of the temporal and spatial distribution of the retrieved CO2 at 300 hPa demonstrated a strong seasonal variation of CO2 in 0–60° N in the Northern Hemisphere with the maximum values in June–August and larger amplitudes of seasonal variation in the northeast of Asia and northeastern part of North America. The variations likely occurred due to larger sinks of atmospheric CO2 that are dominated by CO2 uptake in the summer. In the Southern Hemisphere, the CO2 displayed high concentration anomalies in the latitudinal range of 30–60° S in September–November and December–February, which probably occurred due to the lofted smoke plumes from the strong fire seasons in South America and Southern Africa.

Carbon and Water Exchanges in a Mountain Meadow Ecosystem, Sierra Nevada, California

Ecosystem-atmosphere exchanges of carbon dioxide (CO2) and water vapor were investigated in a moist mountain meadow (Loney Meadow) at 1822 m MSL in the Sierra Nevada, California, USA. An eddy covariance (EC) tower was deployed for most of the snow-free period from May to September 2016. The meadow ecosystem progressed from a strong sink of CO2 in the peak of the growing season under saturated to wet soil conditions (−18.51 gC m−2 d−1) to a weak source (2.97 gC m−2 d−1) following a rapid decline in soil moisture as runoff decreased. The variability of Net Ecosystem Exchange (NEE) over diurnal, synoptic and seasonal timescales was dominated by Gross Primary Production (GPP) which ranged from 43 gC m−2 d−1 during the peak of the growing season to 19 gC m−2 d−1 during senescence. Ecosystem respiration was small in magnitude and variability compared to GPP. Approximations of annual NEE for the meadow ranged from −285 to −450 gC m−2 yr−1, which is high compared to grasslands, and more similar to mature wetland or forest ecosystems. At diurnal and synoptic scales, CO2 flux was driven most strongly by photosynthetically active radiation (PAR), while seasonally, the ecosystem was linked closely to changes in soil moisture. Light-use and water-use efficiencies of the meadow ecosystem were high compared with those found in most other ecosystems using comparable observations. These results suggest meadows have the potential to be large sinks of atmospheric CO2 and that their ability to do so is sensitive to water table height. This is important for understanding the future of carbon sequestration in mountain meadows in the context of changing hydroclimates and different land management decisions that impact meadow hydrology.

Multi-year eddy covariance measurements of net ecosystem exchange in tropical dry deciduous forest of India

Tropical deciduous forests are unique in terms of their geographical distribution, strong seasonality and their contribution to global carbon dynamics, but yet their carbon sequestration potential is poorly sampled. In the current study, we report the carbon balance of 65-year-old tropical dry deciduous forest in central India using the multi-year eddy covariance measurements from November 2011 to May 2019. Over the study period, the forest site was observed to be a net sink of atmospheric CO2 with a mean annual net ecosystem productivity (NEP) of 524 (± 40; ±1 SD across different years) g C m−2 yr−1 with a 233 (±15) day growing season. The NEP was partitioned into gross primary productivity (GPP) of 3358 (± 167) g C m−2 yr−1 and annual carbon loss due to respiration and decomposition (Reco) of 2834 (± 157) g C m−2 yr−1. The ecosystem showed a strong seasonality as a source of carbon during the leaf-off season (March to June), and as a carbon sink during the rest of the year with significant sequestration during the winter season (October to December). The intra-annual analysis suggested that CO2 flux is primarily controlled by canopy greenness with mean Reco/GPP ratio varying from 1.90 (± 0.12) to 0.79 (± 0.04) during leaf-off to leaf-on seasons. The monthly C fluxes in the growing season are found to be strongly correlated to the environmental variables rather than in the leaf-off season, while variability in monthly ecosystem respiration was better explained by air temperature during the leaf-off season than the growing season. Further, the inter-annual variability of NEP was mainly dependent on growing season length and mean annual temperature. The variations in annual GPP and Reco were directly dependent on the increase in mean annual temperature. The eddy covariance-based NEP was complemented by close independent biometric estimates, imparting confidence in our measurements. In conclusion, this tropical dry deciduous forest site is a substantial carbon sink and would add crucial information regarding carbon budgeting of tropical forests in global carbon balance models.

Surface CO2 system dynamics in the Gulf of Anadyr during the open water season

The Gulf of Anadyr (GA), one of the largest and most poorly investigated gulfs of the Bering Sea, is an important part of the Pacific Arctic. Drivers and mechanisms responsible for the spatial variations in surface seawater CO2 system parameters and associated air–sea CO2 fluxes were investigated during three oceanographic cruises conducted in the GA during the open water season of 1992, 2002, and 2011. It was shown that the surface waters of the GA were moderate to strong sinks for atmospheric CO2 during this season: mean air-sea CO2 fluxes for the investigated areas of the GA, for the three cruises, were −31, −15, and −30 mmol CO2 m−2 d−1, respectively. The summer air-sea CO2 flux was attributed to intensive phytoplankton bloom, and fall CO2 uptake coincided with a period of strong winds and seasonal enhanced primary production. The surface waters of the GA were supersaturated with respect to aragonite during the investigated period. Anadyr Strait, a region of high hydrodynamic activity, was the only area where CO2 efflux was observed. Surface waters of the strait were supersaturated with respect to CO2 and in quasi-equilibrium with respect to aragonite. We found that during the late summer/fall season, the Anadyr Current exported a well-mixed, relatively cold, salty, nutrient- and CO2-rich water through the western Anadyr Strait into the Chirikov Basin. Our research highlights the need for mesoscale studies of the western Bering Sea to produce the most accurate assessment of the regional CO2 budget.

Drivers of Air‐Sea CO2 Flux Seasonality and its Long‐Term Changes in the NASA‐GISS Model CMIP6 Submission

AbstractClimate change will affect both the mean state and seasonality of marine physical and biogeochemical properties, with important implications for the oceanic sink of atmospheric CO2. Here, we investigate the seasonal cycle of the air‐sea exchange of CO2 and pCO2,sw (surface seawater pCO2) and their long term changes using the CMIP6 submission of the NASA‐GISS modelE (GISS‐E2.1‐G). In comparison to the CMIP5 submission (GISS‐E2‐R), we find that on the global scale, the seasonal cycles of the CO2 flux and NPP have improved, while the seasonal cycles of dissolved inorganic carbon (DIC), alkalinity, and macronutrients have deteriorated. Moreover, for all ocean biogeochemistry fields, changes in skill between E2.1‐G and E2‐R display large regional variability. For E2.1‐G, we find similar modeled and observed CO2 flux seasonal cycles in the subtropical gyres, where seasonal anomalies of pCO2,sw and the flux are temperature‐driven, and the Southern Ocean, where anomalies are DIC‐driven. Biases in these seasonal cycles are largest in the subpolar and equatorial regions, driven by a combination of biases in temperature, DIC, alkalinity, and wind speed. When comparing the historical simulation to a simulation with an idealized increase in atmospheric pCO2, we find that the seasonal amplitudes of the CO2 flux and pCO2,sw generally increase. These changes are produced by increases in the sensitivity of pCO2,sw to its respective drivers. These findings are consistent with the notion that the seasonality of pCO2,sw is expected to increase due to the increase of atmospheric pCO2, with changes in the seasonality of temperature, DIC, and alkalinity having secondary influences.