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In this "Grand Challenges" paper, we review how the carbon isotopic composition of atmospheric CO2 has changed since the Industrial Revolution due to human activities and their influence on the natural carbon cycle, and we provide new estimates of possible future changes for a range of scenarios. Emissions of CO2 from fossil fuel combustion and land use change reduce the ratio of 13C/12C in atmospheric CO2 (δ13CO2). This is because 12C is preferentially assimilated during photosynthesis and δ13C in plant‐derived carbon in terrestrial ecosystems and fossil fuels is lower than atmospheric δ13CO2. Emissions of CO2 from fossil fuel combustion also reduce the ratio of 14C/C in atmospheric CO2 (Δ14CO2) because 14C is absent in million‐year‐old fossil fuels, which have been stored for much longer than the radioactive decay time of 14C. Atmospheric Δ14CO2 rapidly increased in the 1950s to 1960s because of 14C produced during nuclear bomb testing. The resulting trends in δ13C and Δ14C in atmospheric CO2 are influenced not only by these human emissions but also by natural carbon exchanges that mix carbon between the atmosphere and ocean and terrestrial ecosystems. This mixing caused Δ14CO2 to return toward preindustrial levels in the first few decades after the spike from nuclear testing. More recently, as the bomb 14C excess is now mostly well mixed with the decadally overturning carbon reservoirs, fossil fuel emissions have become the main factor driving further decreases in atmospheric Δ14CO2. For δ13CO2, in addition to exchanges between reservoirs, the extent to which 12C is preferentially assimilated during photosynthesis appears to have increased, slowing down the recent δ13CO2 trend slightly. A new compilation of ice core and flask δ13CO2 observations indicates that the decline in δ13CO2 since the preindustrial period is less than some prior estimates, which may have incorporated artifacts owing to offsets from different laboratories' measurements. Atmospheric observations of δ13CO2 have been used to investigate carbon fluxes and the functioning of plants, and they are used for comparison with δ13C in other materials such as tree rings. Atmospheric observations of Δ14CO2 have been used to quantify the rate of air‐sea gas exchange and ocean circulation, and the rate of net primary production and the turnover time of carbon in plant material and soils. Atmospheric observations of Δ14CO2 are also used for comparison with Δ14C in other materials in many fields such as archaeology, forensics, and physiology. Another major application is the assessment of regional emissions of CO2 from fossil fuel combustion using Δ14CO2 observations and models. In the future, δ13CO2 and Δ14CO2 will continue to change. The sign and magnitude of the changes are mainly determined by global fossil fuel emissions. We present here simulations of future δ13CO2 and Δ14CO2 for six scenarios based on the shared socioeconomic pathways (SSPs) from the 6th Coupled Model Intercomparison Project (CMIP6). Applications using atmospheric δ13CO2 and Δ14CO2 observations in carbon cycle science and many other fields will be affected by these future changes. We recommend an increased effort toward making coordinated measurements of δ13C and Δ14C across the Earth System and for further development of isotopic modeling and model‐data analysis tools. Key Points: Carbon isotopes, 14C and 13C, in atmospheric CO2 are changing in response to fossil fuel emissions and other human activitiesFuture simulations using different SSPs show continued changes in isotopic ratios that depend on fossil fuel emissions and, for 13C, BECCSApplications using atmospheric 14C and 13C in studies of the carbon cycle or other fields will be affected by future changes [ABSTRACT FROM AUTHOR]

The hypothesis that marine plankton ecosystems may effectively regulate climate by the production of dimethylsulphide (DMS) has attracted substantial research effort over recent years. This hypothesis suggests that DMS produced by marine ecosystems can affect cloud properties and hence the averaged irradiance experienced by the phytoplankton that produce DMS's precursor dimethylsulphoniopropionate (DMSP). This paper describes the use of a simple model to examine the effects of such a biogenic feedback on the ecosystem that initiates it. We compare the responses to perturbation of a simple marine nitrogen-phytoplankton-zooplankton (NPZ) ecosystem model with and without biogenic feedback. Our analysis of this heuristic model reveals that the addition of the feedback can increase the model's resilience to perturbation and hence stabilize the model ecosystem. This result suggests the hypothesis that DMS may play a role in stabilizing marine plankton ecosystem dynamics through its effect on the atmosphere. [ABSTRACT FROM AUTHOR]

Ocean alkalinity plays a fundamental role in the apportionment of CO2 between the atmosphere and the ocean. The primary driver of the ocean's vertical alkalinity distribution is the formation of calcium carbonate (CaCO3) by organisms at the ocean surface and its dissolution at depth. This so‐called "CaCO3 counterpump" is poorly constrained, however, both in terms of how much CaCO3 is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady‐state model of global ocean alkalinity using Ocean Circulation Inverse Model transport, biogeochemical cycling, and field‐tested calcite and aragonite dissolution kinetics. We find that limiting CaCO3 dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y−1 (from 73 m), but all converge to 1.0 Gt PIC y−1 export at 279 m, indicating that both high‐ and low‐export scenarios can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results demonstrate that dissolution is not a simple function of seawater CaCO3 saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the water column. Plain Language Summary: The acid neutralizing capacity, or alkalinity, in the ocean surface affects how much CO2 the ocean can absorb from the atmosphere. Alkalinity is lost from the surface ocean when marine organisms make calcium carbonate (CaCO3) shells; in contrast, alkalinity is returned to seawater when shells dissolve. Both processes are not well quantified. Here, we created a model to constrain how much CaCO3 sinks out of the surface ocean, and at what depth it dissolves. We find that CaCO3 must dissolve in the upper ocean where calcite and aragonite—the two most common forms of CaCO3—are thermodynamically oversaturated. We also find that a range of CaCO3 exports—from 1.1 to 1.8 Gt PIC y−1—can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results suggest that CaCO3 dissolution is not only controlled by seawater chemistry, and that other mechanisms, likely related to the biology and ecology of organisms that make CaCO3 shells, are important drivers of CaCO3 dissolution in the ocean. Key Points: Restricting calcium carbonate (CaCO3) dissolution to below saturation horizons cannot reproduce excess alkalinity observations in the upper oceanExports between 1.1 and 1.8 Gt PIC y−1 (from 73 m) can match GLODAP observations, but models converge to 1.0 Gt PIC y−1 export at 279 mIn addition to seawater CaCO3 saturation and mineral solubility, other mechanisms drive CaCO3 dissolution throughout the water column [ABSTRACT FROM AUTHOR]

Although zooplankton play a substantial role in the biological carbon pump and serve as a crucial link between primary producers and higher trophic level consumers, the skillful representation of zooplankton is not often a focus of ocean biogeochemical models. Systematic evaluations of zooplankton in models could improve their representation, but so far, ocean biogeochemical skill assessment of Earth system model (ESM) ensembles have not included zooplankton. Here we use a recently developed global, observationally based map of mesozooplankton biomass to assess the skill of mesozooplankton in six CMIP6 ESMs. We also employ a biome‐based assessment of the ability of these models to reproduce the observed relationship between mesozooplankton biomass and surface chlorophyll. The combined analysis found that most models were able to reasonably simulate the large regional variations in mesozooplankton biomass at the global scale. Additionally, three of the ESMs simulated a mesozooplankton‐chlorophyll relationship within the observational bounds, which we used as an emergent constraint on future mesozooplankton projections. We highlight where differences in model structure and parameters may give rise to varied mesozooplankton distributions under historic and future conditions, and the resultant wide ensemble spread in projected changes in mesozooplankton biomass. Despite differences, the strength of the mesozooplankton‐chlorophyll relationships across all models was related to the projected changes in mesozooplankton biomass globally and in regional biomes. These results suggest that improved observations of mesozooplankton and their relationship to chlorophyll will better constrain projections of climate change impacts on these important animals. Plain Language Summary: Zooplankton are marine animals that have a key role in transferring carbon from the atmosphere deeper into the ocean. They also serve as a crucial link in food chains between microscopic marine plants (phytoplankton) and predators like fish and whales. Researchers have created mathematical representations (models) of the linked processes of the oceans, atmosphere, and land, most of which include zooplankton. Yet how well these models represent zooplankton has not been adequately tested. We compared observations of zooplankton biomass to model estimates. We explored if these models reproduce the observed relationship between zooplankton biomass and chlorophyll concentration, which is useful for assessing how well the models represent predator‐prey relationships. Five of six models had similar patterns and comparable average biomasses across the global ocean as the observations. The historic relationship in three models fell within the observed relationship. The strength of the relationships across all models was related to how much zooplankton biomass will decrease with climate change. To improve the representation of zooplankton in models, we need better observations of the relationships between organisms. This would advance estimates of carbon transfer to the deep sea and carbon available to fish, and how they will change with climate change. Key Points: On the global scale, five of six models that include mesozooplankton perform moderately well with respect to the observationsWe identify an emergent constraint using the mesozooplankton versus chlorophyll relationship that can help constrain zooplankton projectionsMore attention needs to be paid to prey preferences, food web structure and temperature sensitivity in addition to existing key parameters [ABSTRACT FROM AUTHOR]

The deep ocean releases large amounts of old, pre‐industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO2 release is relevant to the global climate because its changes could alter atmospheric CO2 levels on long time scales, and also affects the present‐day potential of the Southern Ocean to take up anthropogenic CO2. Here, year‐round profiling float measurements show that this CO2 release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea‐ice edge. This band of high CO2 subsurface water coincides with the outcropping of the 27.8 kg m−3 isoneutral density surface that characterizes Indo‐Pacific Deep Water (IPDW). It has a potential partial pressure of CO2 exceeding current atmospheric CO2 levels (∆PCO2) by 175 ± 32 μatm. Ship‐based measurements reveal that IPDW exhibits a distinct ∆PCO2 maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2 decreases. Most of this vertical ∆PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high‐carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles. Key Points: Deep ocean CO2 release arises from high potential CO2 partial pressure in subsurface water between the Subantarctic Front and sea‐ice edgeMaximum subsurface potential CO2 is connected to the outcrop of upwelling Indo‐Pacific Deep Water (IPDW), rich in remineralized organic carbonHigh alkalinity and low temperature limit CO2 release from higher carbon‐content deep water outcropping south of IPDW [ABSTRACT FROM AUTHOR]

Carbonate chemistry of the Arctic Ocean seafloor and its vulnerability to ocean acidification remains poorly explored. This limits our ability to quantify how biogeochemical processes and bottom water conditions shape sedimentary carbonate chemistry, and to predict how climate change may affect such biogeochemical processes at the Arctic Ocean seafloor. Here, we employ an integrated data‐model assessment that explicitly resolves benthic pH and carbonate chemistry along a South—North transect in the Barents Sea. We identify the main drivers of observed carbonate dynamics and estimate benthic fluxes of dissolved inorganic carbon and alkalinity to the Arctic Ocean. We explore how bottom water conditions and in‐situ organic matter degradation shape these processes and show that organic matter transformation strongly impacts pH and carbonate saturation (Ω) variations. Aerobic organic matter degradation drives a negative pH shift (pH < 7.6) in the upper 2–5 cm, producing Ω < 1. This causes shallow carbonate dissolution, buffering porewater pH to around 8.0. Organic matter degradation via metal oxide (Mn/Fe) reduction pathways further increases pH and carbonate saturation state. At the northern stations, where Ω > 5 at around 10–25 cm, model simulations result in authigenic carbonate precipitation. Furthermore, benthic fluxes of dissolved inorganic carbon (12.5–59.5 μmol cm−2 yr−1) and alkalinity (11.3–63.2 μmol cm−2 yr−1) are 2–3‐fold greater in the northern sites due to greater carbonate dissolution. Our assessment is of significant relevance to predict how changes in the Arctic Ocean may shift carbon burial and pH buffering into the next century. Plain Language Summary: The Arctic Ocean seafloor is an important place for long‐term storage of carbon, locking it away from the atmosphere. However, such storage capacity is also vulnerable itself to climate change. Thus, we need to understand how carbon is recycled and packed away in the Arctic seafloor to be able to predict future shifts in Earth's climate. We use the Barents Sea to study carbon recycling in the seafloor and to make predictions of possible impacts of climate change. We use a model that computes chemical processes at the seafloor. This allows us to investigate how organic carbon recycling at the seafloor, and saturation state of bottom waters, can impact the dissolution of inorganic carbon, which then leads to the release of carbon back to Arctic Ocean waters. We also test how possible environmental conditions arising from climate change could impact carbon storage in the Arctic Ocean seafloor. We find important climate feedbacks, for example, that further acidification of bottom waters could reduce inorganic carbon storage, whereas greater deposition and recycling of organic carbon could help locking inorganic carbon within seafloor. These results can help scientists and policy makers finding ways of mitigating the effects of climate change in the future. Key Points: Changes in pH and carbonate saturation state of Barents Sea sediments are driven by organic matter degradation and bottom water chemistryMetabolic dissolution of CaCO3 in the top 5 cm depth buffers pH and saturation state, enabling carbonate precipitation in deeper sedimentsHigher benthic fluxes of alkalinity and dissolution of inorganic carbon from South to North reflect greater carbonate dissolution in the northern Barents Sea sites [ABSTRACT FROM AUTHOR]

The North Atlantic Ocean is a key region for carbon sequestration by the biological carbon pump (BCP). The quantity of organic carbon exported from the surface, the region and depth at which it is remineralized, and the subsequent timescale of ventilation (return of the remineralized carbon back into contact with the atmosphere), control the magnitude of BCP sequestration. Carbon stored in the ocean for >100 years is assumed to be sequestered for climate‐relevant timescales. We apply Lagrangian tracking to an ocean circulation and marine biogeochemistry model to determine the fate of North Atlantic organic carbon export. Organic carbon assumed to undergo remineralization at each of three vertical horizons (500, 1,000, and 2,000 m) is tracked to determine how much remains out of contact with the atmosphere for 100 years. The fraction that remains below the mixed layer for 100 years is defined as the sequestration efficiency (SEff) of remineralized exported carbon. For exported carbon remineralized at the 500, 1,000 and 2,000 m horizons, the SEff is 28%, 66% and 94%, respectively. Calculating the amount of carbon sequestered using depths ≤1,000 m, and not accounting for downstream ventilation, overestimates 100‐year carbon sequestration by at least 39%. This work has implications for the accuracy of future carbon sequestration estimates, which may be overstated, and for carbon management strategies (e.g., oceanic carbon dioxide removal and Blue Carbon schemes) that require long‐term sequestration to be successful. Plain Language Summary: The North Atlantic Ocean is a key region for carbon uptake and organic carbon storage in the interior ocean for climate‐relevant timescales (>100 years). Sinking organic carbon that reaches the interior ocean is respired there to form dissolved inorganic carbon. The fate of this carbon, that is, whether it remains in the ocean interior or returns to the surface and is ventilated to the atmosphere, depends on its physical circulation pathways. To investigate the efficiency of carbon storage in the North Atlantic Ocean we released virtual particles at three different starting depths (500, 1,000 and 2,000 m) into a global ocean model and tracked the pathways they followed for 100 years. 66% of virtual particles released at 1,000 m depth remained out of contact with the atmosphere for 100 years. Combining this pathway analysis with observation‐ and model‐derived geographical fields of sinking carbon flux, we estimate that the 100‐year North Atlantic carbon storage may be overestimated by 39% at 1,000 m because of this ventilation to the atmosphere. These findings have important implications for accurately estimating future ocean carbon storage. In particular, for carbon management strategies that require long‐term sequestration to be successful (e.g., oceanic carbon dioxide removal and Blue Carbon schemes). Key Points: Lagrangian simulations show that North Atlantic Ocean sequestration efficiency of remineralized exported carbon (REC) varies strongly by regionOnly 66% of North Atlantic REC at 1,000 m will remain out of contact with the atmosphere for 100 yearsFixed sequestration horizons ≤1,000 m can significantly overestimate long‐term carbon sequestration [ABSTRACT FROM AUTHOR]

Natural lakes and reservoirs are important yet not well‐constrained sources of greenhouse gasses to the atmosphere. In particular for N2O emissions, a huge variability is observed in the few, observation‐driven flux estimates that have been published so far. Recently, a process‐based, spatially explicit model has been used to estimate global N2O emissions from more than 6,000 reservoirs based on nitrogen (N) and phosphorous inflows and water residence time. Here we extend the model to a data set of 1.4 million standing water bodies comprising natural lakes and reservoirs. For validation, we normalized the simulated N2O emissions by the surface area of each water body and compared them against regional averages of N2O emission rates taken from the literature or estimated based on observed N2O concentrations. We estimate that natural lakes and reservoirs together emit 4.5 ± 2.9 Gmol N2O‐N year−1 globally. Our global‐scale estimate falls in the far lower end of existing, observation‐driven estimates. Natural lakes contribute only about half of this flux, although they contribute 91% of the total surface area of standing water bodies. Hence, the mean N2O emission rates per surface area are substantially lower for natural lakes than for reservoirs with 0.8 ± 0.5 versus 9.6 ± 6.0 mmol N·m−2·year−1, respectively. This finding can be explained by on average lower external N inputs to natural lakes. We conclude that upscaling‐based estimates, which do not distinguish natural lakes from reservoirs, are prone to important biases. Key Points: Global N2O emission from natural lakes and reservoirs estimated at 4.5 ± 2.9 Gmol N year‐1Natural lakes emit less N2O than reservoirsNorth America and Europe contribute nearly half of global N2O emission from natural lakes and reservoirs [ABSTRACT FROM AUTHOR]

We show that transport differences between two commonly used global chemical transport models, GEOS‐Chem and TM5, lead to systematic space‐time differences in modeled distributions of carbon dioxide and sulfur hexafluoride. The distribution of differences suggests inconsistencies between the transport simulated by the models, most likely due to the representation of vertical motion. We further demonstrate that these transport differences result in systematic differences in surface CO2 flux estimated by a collection of global atmospheric inverse models using TM5 and GEOS‐Chem and constrained by in situ and satellite observations. While the impact on inferred surface fluxes is most easily illustrated in the magnitude of the seasonal cycle of surface CO2 exchange, it is the annual carbon budgets that are particularly relevant for carbon cycle science and policy. We show that inverse model flux estimates for large zonal bands can have systematic biases of up to 1.7 PgC/year due to large‐scale transport uncertainty. These uncertainties will propagate directly into analysis of the annual meridional CO2 flux gradient between the tropics and northern midlatitudes, a key metric for understanding the location, and more importantly the processes, responsible for the annual global carbon sink. The research suggests that variability among transport models remains the largest source of uncertainty across global flux inversion systems and highlights the importance both of using model ensembles and of using independent constraints to evaluate simulated transport. Key Points: There are systematic differences in transport between two commonly used chemical transport models, TM5 and GEOS‐ChemThese systematic differences lead to significant and meaningful posterior flux uncertainties in atmospheric CO2 flux inversions [ABSTRACT FROM AUTHOR]

Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass-independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg. The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0), and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere-forest-soil interface. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are a geochemically relevant family of semivolatile compounds originating from fossil fuels, biomass burning, and their incomplete combustion, as well as biogenic sources. Even though PAHs are ubiquitous in the environment, there are no previous studies of their occurrence in the Southern Ocean and Antarctic atmosphere. Here we show the gas and aerosol phase PAHs concentrations obtained from three sampling cruises in the Southern Ocean (Weddell, Bellingshausen, and South Scotia Seas), and two sampling campaigns at Livingston Island (Southern Shetlands). This study shows an important variability of the atmospheric concentrations with higher concentrations in the South Scotia and northern Weddell Seas than in the Bellingshausen Sea. The assessment of the gas-particle partitioning of PAHs suggests that aerosol elemental carbon contribution is modest due to its low concentrations. Over the ocean, the atmospheric concentrations do not show a temperature dependence, which is consistent with an important role of long-range atmospheric transport of PAHs. Conversely, over land at Livingston Island, the PAHs gas phase concentrations increase when the temperature increases, consistently with the presence of local diffusive sources. The use of fugacity samplers allowed the determination of the air-soil and air-snow fugacity ratios of PAHs showing that there is a significant volatilization of lighter molecular weight PAHs from soil and snow during the austral summer. The higher volatilization, observed in correspondence of sites where the organic matter content in soil is higher, suggests that there may be a biogenic source of some PAHs. The volatilization of PAHs from soil and snow is sufficient to support the atmospheric occurrence of PAHs over land but may have a modest regional influence on the atmospheric occurrence of PAHs over the Southern Ocean. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a global and persistent contaminant, affecting human health primarily via marine fish consumption. Large anthropogenic releases of Hg to the atmosphere by mining and coal combustion have resulted in a significant perturbation to the biogeochemical cycling of Hg. The magnitude of this perturbation and the relative roles of the ocean and land as sinks for anthropogenic Hg remain unclear. Here we use a 3-D global ocean biogeochemical model to show that surface ocean Hg concentrations have increased fourfold over the last 600 years. We find that anthropogenic Hg enters the ocean's interior predominantly by absorption onto sinking organic matter particulates, which decompose and release Hg at a depth of 500-800 m, implying that the human perturbation is largest in subsurface waters of biologically productive regions. Our model simulation predicts that over the last six centuries half of emitted anthropogenic Hg has accumulated in the oceans and marine sediments. [ABSTRACT FROM AUTHOR]

We have estimated global surface fluxes of carbon dioxide for the period 1992-1996 using an inverse approach that sequentially considers four constraints: (1) atmospheric CO2, (2) ocean interior DIC (dissolved inorganic carbon) interpreted through an ocean interior inversion and surface ocean pCO2 (partial pressure of CO2), (3) annual prior fluxes for selected land regions, and (4) atmospheric model selection based on vertical transport skill. Estimated fluxes are monthly resolved for each of the 22 Transcom regions over land and ocean. The ocean constraint is particularly valuable, as it does not only add prior information about air-sea fluxes to the inversion problem but also preserves the regional variance-covariance structure from the underlying ocean interior inversion. It allows to constrain annual oceanic uptake of 1.8 PgCyr−1 to within 0.2 PgCyr−1, which implies a net annual land uptake of 1.3 (±0.3) PgCyr−1. Furthermore, it leads to a pronounced asymmetry in the seasonal pattern of global land uptake, which was not seen in previous atmosphere-only inversions. Tropical land is consistently estimated to be a source of carbon, though the source magnitude is reduced as more constraints are applied. With all four constraints, the inversion suggests a net tropical source of 1.1 (±0.9) PgCyr−1, which is comparable to global estimates of deforestation rates in tropical forests and therefore implies an annually balanced tropical land biosphere flux. This balance is not found, however, at the regional level: For the Amazonian region and after accounting for deforestation, we find a biospheric source of 0.6 (±0.5) PgCyr−1. This is at the upper range of estimates from bottom-up methods, which tend to identify the region as a sink. [ABSTRACT FROM AUTHOR]

Using in situ measurements, we find a semiannual oscillation (SAO) in the midtropospheric and surface C(X Chemistry transport models (2-D Caltech/JPL model, 3-D GEOS-Chem, and 3-D N40ZART-2) arc used to investigate possible sources for the SAO signal in the midtropospheric and surface CO2. From model sensitivity studies, it is revealed that the SAO signal in the midtropospheric CO2 originates mainly from surface CO2 with a small contribution from transport fields. It is also found that the source for the SAO signal in surface CO2 is mostly related to the CO2 exchange between the biosphere and the atmosphere. By comparing model CO2 with in situ CO2 measurements at the surface, we find that models are able to capture both annual and semiannual cycles well at the surface. Model simulations of the annual and semiannual cycles of CO2 in the iropical middle troposphere agree reasonably well with aircraft measurements [ABSTRACT FROM AUTHOR]

We propose the Breathing Earth System Simulator (BESS), an upscaling approach to quantify global gross primary productivity and evapotranspiration using MODIS with a spatial resolution of 1-5 km and a temporal resolution of 8 days. This effort is novel because it is the first system that harmonizes and utilizes MODIS Atmosphere and Land products on the same projection and spatial resolution over the global land. This enabled us to use the MODIS Atmosphere products to calculate atmospheric radiative transfer for visual and near infrared radiation wave bands. Then we coupled atmospheric and canopy radiative transfer processes, with models that computed leaf photosynthesis, stomatal conductance and transpiration on the sunlit and shaded portions of the vegetation and soil. At the annual time step, the mass and energy fluxes derived from BESS showed strong linear relations with measurements of solar irradiance (r2 = 0.95, relative bias: 8%), gross primary productivity (r2 = 0.86, relative bias: 5%) and evapotranspiration (r2 = 0.86, relative bias: 15%) in data from 33 flux towers that cover seven plant functional types across arctic to tropical climatic zones. A sensitivity analysis revealed that the gross primary productivity and evapotranspiration computed in BESS were most sensitive to leaf area index and solar irradiance, respectively. We quantified the mean global terrestrial estimates of gross primary productivity and evapotranpiration between 2001 and 2003 as 118 ± 26 PgC yr-1 and 500 ± 104 mm yr-1 (equivalent to 63,000 ± 13,100 km3 yr-1), respectively. BESS-derived gross primary productivity and evapotranspiration estimates were consistent with the estimates from independent machine-learning, data-driven products, but the process-oriented structure has the advantage of diagnosing sensitivity of mechanisms. The process-based BESS is able to offer gridded biophysical variables everywhere from local to the total global land scales with an 8-day interval over multiple years. [ABSTRACT FROM AUTHOR]

The flux of bioavailable Fe from mineral dust to the surface ocean is controlled not only by the processes in the atmosphere but also by the nature and source of the dust. In this study, we investigated how the nature of Fe minerals in the dust affects its potential Fe solubility (Fepsol) employing traditional and modern geochemical, mineralogical, and microscopic techniques. The chemical and mineralogical compositions, particularly Fe mineralogy, in soil samples as dust precursors collected from North African dust source regions were determined. The Fepsol was measured after 3 days of contact with sulfuric acid at pH 2 to simulate acid processes in the atmosphere. Fepsol of the soil dust samples were compared with calculated predictions of Fepsol based on the amount of individual Fe-bearing minerals present in the samples and Fe solubilities of corresponding standard minerals. The calculated Fepsol deviated significantly from the measured Fepsol of the soil dust samples. We attributed this to the variability in properties of Fe minerals (e.g., size of Fe oxides and heterogeneity of chemical compositions of clay minerals) in soil dusts in comparison to the standard minerals. There were, however, clear relationships between the degree of chemical weathering of North African soils and Fepsol. The Parker index and ratio of ascorbate plus dithionite Fe to total Fe ((FeA+FeD)/FeT) are positively and negatively correlated with Fepsol, respectively. In addition, the ratio of FeA/(FeA+FeD), which decreases with aging of the Fe oxides, was found to be positively correlated with Fepsol in the soil dusts. Overall, our results indicate that there is a significant regional variability in the chemical and Fe mineralogical compositions of dusts across North African sources, as a result of the differences in chemical weathering and aging of Fe oxides. Furthermore, the indices for these weathering processes can provide an estimate of the fraction of Fe which can be solubilized if acid processed in the atmosphere. [ABSTRACT FROM AUTHOR]

The contribution of West African savanna ecosystems to global greenhouse gas budgets is highly uncertain. In this study we quantified soil-atmosphere CH[sub4] and CO[sub2] fluxes in the southwest of Burkina Faso from June to September 2005 and from April to September 2006 at four different agricultural fields planted with sorghum (ii = 2), cotton, and peanut and at a natural savanna site with termite (Cubiterrnesfungifaber) mounds. During the rainy season both CH[sub4] uptake and CH[sub4] emission were observed in the savanna, which was on average a Cl4 source of 2.79 and 2.28 kg CH[sub4]-C ha[sup-1] a1 in 2005 and 2006, respectively. The crop sites were an average CH[sub4] sink of -0.67 and -0.70 kg CH[sub4]-C ha[sup-1] a[sup-1] in the 2 years, without significant seasonal variation. Mean annual soil respiration ranged between 3.86 and 5.82 t CO[sub2]-C ha1 a[sup-1] in the savanna and between 2.50 and 4.51 t CO[sub2]-C ha1 a[sup-1] at the crop sites. CH[sub4] emission from termite mounds was 2 orders of magnitude higher than soil CH[sub4] emissions, whereas termite CO[sub2] emissions were of the same order of magnitude as soil CO[sub2] emissions. Termite CH[sub4] and CO[sub2] release in the savanna contributed 8.8% and 0.4% to the total soil CH[sub4] and CO[sub2] emissions, respectively. At the crop sites, where termite mounds had been almost completely removed because of land use change, termite fluxes were insignificant. Mound density-based upscaling of termite CH[sub4] fluxes resulted in a global termite CH[sub4] source of 0.9 Tg a[sup-1], which corresponds to 0.15% of the total global CH[sub4] budget of 582 Tg a[sup-1], hence significantly lower than those obtained previously by biomass-based calculations. This study emphasizes that land use change, which is of high relevance in this region, has particularly affected soil CH[sub4] fluxes in the past and might still do so in the future. [ABSTRACT FROM AUTHOR]

We present a synthesis of available estimates of primary production, organic carbon burial, and lake-atmosphere carbon dioxide exchange data for large lakes of the world. All three fluxes showed significant relationships with latitude and related climate variables, with lower production, higher evasion of carbon dioxide, and higher burial efficiency at higher latitudes. There was no relationship between raw organic carbon mass accumulation rates and latitude. Our estimates suggest that an order of magnitude more carbon is lost to the atmosphere by evasion than is buried in sediments at a global scale, with total global production, evasion, and burial fluxes of approximately 250, 90, and 7 Tg C yr-1. Finally, the data suggest a trend from autotrophy in low-latitude large lakes to heterotrophy and increasing reliance on allochthonous carbon sources in lakes at higher latitudes. [ABSTRACT FROM AUTHOR]

We investigate the interannual variability in the flux of CO2 between the atmosphere and the Southern Ocean on the basis of hindcast simulations with a coupled physical-biogeochemical-ecological model with particular emphasis on the role of the Southern Annular Mode (SAM). The simulations are run under either pre-industrial or historical CO2 concentrations, permitting us to separately investigate natural, anthropogenic, and contemporary CO2 flux variability. We find large interannual variability (±0.19 PgC yr-1) in the contemporary air-sea CO2 flux from the Southern Ocean (<35°S). Forty-three percent of the contemporary air-sea CO2 flux variance is coherent with SAM, mostly driven by variations in the flux of natural CO2, for which SAM explains 48%. Positive phases of the SAM are associated with anomalous outgassing of natural CO2 at a rate of 0.1 PgC yr-1 per standard deviation of the SAM. In contrast, we find. an anomalous uptake of anthropogenic CO2 at a rate of 0.01 PgC yr-1 during positive phases of the SAM. This uptake of anthropogenic CO2 only slightly mitigates the outgassing of natural CO2, so that a positive SAM is associated with anomalous outgassing in contemporaneous times. The primary cause of the natural CO2 outgassing is anomalously high oceanic partial pressures of CO2 caused by elevated dissolved inorganic carbon (DIC) concentrations. These anomalies in DIC are primarily a result of the circulation changes associated with the southward shift and strengthening of the zonal winds during positive phases of the SAM. The secular, positive trend in the SAM has led to a reduction in the rate of increase of the uptake of CO2 by the Southern Ocean over the past 50 years. [ABSTRACT FROM AUTHOR]

Using appropriate clean methods for handling and preparation, 57 ice samples from the Devon Island Ice Cap, representing the period 134 to 15,800 years before present (BP), were analyzed for Pb, Sc, and Pb isotopes (206pb, 207Pb, 208Pb) using ICP-SMS. The greatest Pb concentrations were found in samples dating from the Younger Dryas. Despite the large range in Pb concentrations (from 2.2 to 181 pg#g-1), the Pb concentrations were proportional to those of Sc until 3100 BP when the Pb/Sc ratio exceeded by a factor of 2 the natural ‘background’ value (Pb/Sc = 6.3 ± 1.8) for the first time. The uniform ratio of Pb to Sc until 3100 BP is consistent with the hypothesis that soil dust particles derived from physical and chemical weathering dominate the inputs of Pb to the atmosphere, with the magnitude of these sources climate-dependent. Isotopic analyses of Pb further support this paradigm, with the average ratio of 206pb/207pb (1.230) and 208pb/206Pb (2.059) well within the range given for the Upper Continental Crust (UCC). The shift to higher Pb/Sc ratios and lower 206Pb/207Pb values starting at 3100 BP is consistent with historical records and other archival evidence of the onset of atmospheric Pb contamination caused by Pb mining and smelting in the Iberian Peninsula. Since that time, the Devon Island ice core records several other episodes of notable atmospheric Pb contamination, including those dating from Roman and medieval times, as well as the industrial period. The Pb, Sc, and Pb isotope data presented here represent the first chemical and isotopic record of the natural, ‘background’ atmospheric inputs to the Canadian Arctic, against which modern values may be compared. [ABSTRACT FROM AUTHOR]

The correlations between annual precipitation, evaporation, temperature, and annual carbon export (bicarbonate, dissolved organic carbon, and particulate organic carbon) are established for the Ohio, upper Mississippi, and Missouri watersheds and coefficients that predict C export on the basis of precipitation, evapotranspiration, and seasonal temperature are provided. Interannual variation in carbon export is controlled by the level of precipitation and evapotranspiration, the discharge to precipitation ratio (D:P), the concentration of the major carbon pool, the rating curve (discharge versus carbon concentration plots), and seasonal temperature. Within a watershed, precipitation has the strongest correlation with annual carbon export for all carbon pools. In the upper Mississippi and Ohio, levels of evapotranspiration can predict the majority of the residuals of precipitation versus carbon export. Bicarbonate demonstrates the largest response in yield to annual precipitation variation for each watershed. Rating curves, however, indicate that dissolved organic carbon and particulate organic carbon generally increase with increasing discharge, while bicarbonate decreases, causing a larger percentage response to precipitation for the organic carbon pools. Across watersheds the discharge to precipitation (D:P) ratio is the dominant determinant of how carbon yields respond to changes in precipitation, and watersheds with a high D:P ratio should demonstrate large changes in carbon yields with forecasted changes to precipitation. [ABSTRACT FROM AUTHOR]

Agricultural lime can be a source or a sink for CO2, depending on whether reaction occurs with strong acids or carbonic acid. Here we examine the impact of liming on global warming potential by comparing the sum of Ca2+ and Mg2+ to carbonate alkalinity in soil solutions beneath unmanaged vegetation versus limed row crops, and of streams and rivers in agricultural versus forested watersheds, mainly in southern Michigan. Soil solutions sampled by tension indicated that lime can act as either a source or a sink for CO2. However, infiltrating waters tended to indicate net CO2 uptake, as did tile drainage waters and streams draining agricultural watersheds. As nitrate concentrations increased in infiltrating waters, lime switched from a net CO2 sink to a source, implying nitrification as a major acidifying process. Dissolution of lime may sequester CO2 equal to roughly 25–50% of its C content, in contrast to the prevailing assumption that all of the carbon in lime becomes CO2. The ∼30 Tg/yr of agricultural lime applied in the United States could thus sequester up to 1.9 Tg C/yr, about 15% of the annual change in the U.S. CO2 emissions (12 Tg C/yr for 2002–2003). The implications of liming for atmospheric CO2 stabilization should be considered in strategies to mitigate global climate change. [ABSTRACT FROM AUTHOR]

A biogeochemical ocean general circulation model, driven with NCEP-RI and observed atmospheric CO2 history, is used to investigate and quantify the role that the Southern Annular Mode (SAM), identified as the leading mode of climate variability, has in driving interannual variability in Southern Ocean air-sea CO2 fluxes between 1980 and 2000. Our simulations show the Southern Ocean to be a region of decreased CO2 uptake during the positive SAM phase. The SAM induces changes in Southern Ocean CO2 uptake with a 2-month time lag explaining 42% of the variance in the total interannual variability in air-sea CO2 fluxes. Our analysis shows that the response of the Southern Ocean to the SAM is primarily governed by changes in ΔpCO2 (67%), and that this response is driven by changes in ocean physics that control the supply of nutrients to the upper ocean, primarily Dissolved Inorganic Carbon (DIC). The SAM is predicted to become stronger and more positive in response to climate change and our results suggest this will decrease the Southern Ocean CO2 uptake by 0.1PgC/yr per unit change in the SAM. [ABSTRACT FROM AUTHOR]

The growth of crops represents a sink of atmospheric CO2, whereas biomass is consumed by humans and housed animals, yielding respiratory sources of CO2. This process induces a lateral displacement of carbon and creates geographic patterns of CO2 sources and sinks at the surface of the globe. We estimated the global carbon flux harvested in croplands to be 1290 TgC/yr. Most of this carbon is transported into domestic trade, whereas a small fraction (13%) enters into international trade circuits. We then calculated the global patterns of CO2 fluxes associated with food and feedstuff trade, using country-based agricultural statistics and activity maps of human and housed animal population densities. The CO2 flux maps show regional dipoles of sources and sinks in Asia and North America. The effect of these fluxes on atmospheric CO2 was simulated using a global atmospheric transport model. The mean latitudinal CO2 gradients induced by the displacement of crop products are fairly small (≈0.2 ppm) compared with observations (4–5 ppm), indicating that this process has a only a small influence in explaining the latitudinal distribution of CO2 fluxes. On the other hand, the simulated longitudinal mean atmospheric CO2 gradients at northern midlatitudes (≈ up to 0.5 ppm) are comparable to the ones measured between atmospheric stations, suggesting that CO2 fluxes from crop products trade are an important component of continental- and regional-scale CO2 budgets. Thus they should be accounted for as prior information in regional inversions. [ABSTRACT FROM AUTHOR]

The spatiotemporal variability of upper ocean inorganic carbon parameters and air-sea CO2 exchange in the Indian Ocean was examined using inorganic carbon data collected as part of the World Ocean Circulation Experiment (WOCE) cruises in 1995. Multiple linear regression methods were used to interpolate and extrapolate the temporally and geographically limited inorganic carbon data set to the entire Indian Ocean basin using other climatological hydrographic and biogeochemical data. The spatiotemporal distributions of total carbon dioxide (TCO2), alkalinity, and seawater pCO2 were evaluated for the Indian Ocean and regions of interest including the Arabian Sea, Bay of Bengal, and 10°N-35°S zones. The Indian Ocean was a net source of CO2 to the atmosphere, and a net sea-to-air CO2 flux of +237 ± 132 Tg C yr-1 (+0.24 Pg C yr-1) was estimated. Regionally, the Arabian Sea, Bay of Bengal, and 10°N-10°S zones were perennial sources of CO2 to the atmosphere. In the 10°S-35°S zone, the CO2 sink or source status of the surface ocean shifts seasonally, although the region is a net oceanic sink of atmospheric CO2. [ABSTRACT FROM AUTHOR]

[1] We explored the climate impacts for two land-use change scenarios, aimed at mitigating the buildup of greenhouse gases in the atmosphere. Using the integrated assessment model IMAGE 2.2, we found that the large-scale implementation in the extratropics of either carbon-sequestration or modern-biomass plantations decreases the CO2 concentration with 70–80 ppmv by the year 2100 compared to a nonmitigation baseline. In a coupled land/atmosphere/ocean/sea-ice model this moderates global warming over the 21st century by 10%. However, the carbon-sequestration option raises the absorption of solar radiation due to a lower albedo compared to the scenario involving modem-biomass plantations (for biofuels production). The albedo-induced difference in global mean temperature is as large as the mitigation by CO2 changes in the two scenarios compared to the baseline. Further, an atmospheric circulation change in the carbon-plantation scenario weakens the supply of moisture from the oceans to North Africa and central Eurasia. In our model this decreases annual mean precipitation over North Africa by up to 10% and further increases summer temperatures over Eurasia. These findings lead us to conclude that other climate impacts than just CO2 changes have to be taken into account when discussing climate-change mitigation options that involve land-use changes. [ABSTRACT FROM AUTHOR]