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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 ability to reliably estimate CO2 fluxes from current in situ atmospheric CO2 measurements and future satellite CO2 measurements is dependent on transport model performance at synoptic and shorter timescales. The TransCom continuous experiment was designed to evaluate the performance of forward transport model simulations at hourly, daily, and synoptic timescales, and we focus on the latter two in this paper. Twenty-five transport models or model variants submitted hourly time series of nine predetermined tracers (seven for CO2) at 280 locations. We extracted synoptic-scale variability from daily averaged CO2 time series using a digital filter and analyzed the results by comparing them to atmospheric measurements at 35 locations. The correlations between modeled and observed synoptic CO2 variabilities were almost always largest with zero time lag and statistically significant for most models and most locations. Generally, the model results using diurnally varying land fluxes were closer to the observations compared to those obtained using monthly mean or daily average fluxes, and winter was often better simulated than summer. Model results at higher spatial resolution compared better with observations, mostly because these models were able to sample closer to the measurement site location. The amplitude and correlation of model-data variability is strongly model and season dependent. Overall similarity in modeled synoptic CO2 variability suggests that the first-order transport mechanisms are fairly well parameterized in the models, and no clear distinction was found between the meteorological analyses in capturing the synoptic-scale dynamics. [ABSTRACT FROM AUTHOR]

This paper reports on a basin-wide inventory of anthropogenic CO2 in the East (Japan) Sea determined from high-quality alkalinity, chlorofluorocarbon, and nutrient data collected during a summertime survey in 1999 and total dissolved inorganic carbon data calculated from pH and alkalinity measurements. The data set comprises measurements from 203 hydrographic stations and covers most of the East Sea with the exception of the northwestern boundary region. Anthropogenic CO2 concentrations are estimated by separating this value from total dissolved inorganic carbon using a tracer- based (chlorofluorocarbon) separation technique. Wintertime surface CFC-12 data collected in regions of deep water formation off Vladivostok, Russia, improve the accuracy of estimates of anthropogenic CO2 concentrations by providing improved air- sea CO2 disequilibrium values for intermediate and deep waters. Our calculation yields a total anthropogenic CO2 inventory in the East Sea of 0.40 ± 0.06 petagrams of carbon as of 1999. Anthropogenic CO2 has already reached the bottom of the East Sea, largely owing to the effective transport of anthropogenic CO2 from the surface to the ocean interior via deep water formation in the waters off Vladivostok. The highest specific column inventory (vertically integrated inventory per square meter) of anthropogenic CO2 of 80 mol C m-2 is found in the Japan Basin (40°N-44°N). Comparison of this inventory with those for other major basins of the same latitude band reveal that the East Sea values are much higher than the inventory for the Pacific Ocean (20-30 mol C m-2) and are similar to the inventory for the North Atlantic (66-72 mol C m-2). The substantial accumulation of anthropogenic CO2 in the East Sea during the industrial era has caused the aragonite and calcite saturation horizons to move upward by 80-220 m and 500-700 m, respectively. These upward movements are approximately 5 times greater than those found in the North Pacific. Both the large accumulation of anthropogenic CO2 and its significant impact on carbonate chemistry in the East Sea suggest that this sea is an important site for monitoring the future impact of the oceanic invasion of anthropogenic CO2. [ABSTRACT FROM AUTHOR]

Multiyear estimates of organic matter (OM) export based primarily on oxygen and dissolved inorganic carbon surface layer budgets applied basin‐wide for the Pacific, Atlantic, and S. Indian Oceans yield an inter‐basin range from 1 to 3 mol C/m2/yr with a global mean of 2.0 mol C/m2/yr (8.5 Gt C/yr). OM export rates per area in the Pacific and Atlantic oceans are twice than that in the Indian Ocean. The supply of nutrients from the Southern Ocean can potentially support ∼70% of the observed OM export in the ocean based on observed surface current velocities and PO4 distributions. Horizontal flux of PO4 and dissolved organic phosphorous in the surface layer can support ∼50%, 20%, and 15% of observed OM export in the Pacific, S. Indian and Atlantic oceans, respectively, with the remainder being supplied vertically from the subsurface. Potential utilization of unused surface PO4 in the subtropical gyre yields ∼0.1 mol C/m2/yr increase in OM export in the Pacific and Atlantic oceans but a ∼0.8 mol C/m2/yr increase in the S. Indian ocean suggesting that stronger nutrient limitation contributes to lower export rates observed in the Indian ocean. Plain Language Summary: A small fraction of the organic matter (OM) produced photosynthetically by phytoplankton in the surface ocean is transferred (exported) to deeper portions of the ocean. The rate at which the OM is exported to the subsurface ocean has a significant impact on the ocean's concentrations of dissolved carbon dioxide and oxygen gases. New estimates of the OM export rate per unit area for the Atlantic, Pacific, and Indian oceans are presented based on surface layer oxygen and carbon dioxide budgets. The OM export rate is similar for the Atlantic and Pacific oceans but twice the rate in the Indian ocean. Most of the dissolved nutrients needed by phytoplankton to produce OM originate on the surface of the Southern Ocean (south of 50°S) and are transported via circulation to all the ocean basins where they are consumed by phytoplankton. Between ∼15 and 50% of the nutrients are supplied via surface currents, whereas the remainder is supplied via mixing from the subsurface layers of the ocean. Key Points: Oxygen and inorganic carbon budgets yield organic matter (OM) export rates for the major ocean basins that range from 1 to 3 mol C/m2/yrDissolved nutrients originating in the Southern Ocean can support ∼70% of the global OM exportSurface transport of dissolved nutrients support 15%–50% of export in the ocean basins with the rest supplied vertically from depth [ABSTRACT FROM AUTHOR]

The oceans are acidifying in response to the oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere, yet the global‐scale progression of this acidification has been poorly documented so far by observations. Here, we fill this gap and use an updated version of the in situ and satellite observation‐based product OceanSODA‐ETHZ to determine the trends and drivers of the surface ocean aragonite saturation state (Ωar) and pH = –log([H+]) (total scale) over the last four decades (1982–2021). In the global mean, Ωar and pH declined at rates of −0.071 ± 0.006 decade−1 and −0.0166 ± 0.0010 decade−1, respectively, with the errors of the trends largely reflecting the uncertainties in the reconstructed pH and Ωar fields. These global mean trends are driven primarily by the increase in surface ocean concentration of dissolved inorganic carbon (DIC) in response to the uptake of anthropogenic CO2, but moderated by changes in natural DIC. Surface warming enhances the decrease in pH, accounting for ∼15% of the global trend. The long‐term trends vary substantially across regions and also differ distinctly between pH and Ωar. The highest trends in pH are found in the high latitudes, while Ωar decreases the fastest in the low latitudes. These regional differences are primarily a consequence of regional differences in the ability of the surface ocean to take up and buffer the anthropogenic CO2. Substantial El Niño‐driven interannual variability is superimposed on these trends, with Ωar showing greater variability than pH, resulting in substantially longer time of emergence for Ωar. Plain Language Summary: As the ocean takes up human‐made carbon dioxide from the atmosphere, it becomes more acidic, that is, its pH is dropping and so is its saturation state of seawater (Ωar) with respect to aragonite, a type of carbonate mineral. These chemical changes, generally referred to as "Ocean Acidification," are harming marine organisms. Here, we use an observation‐based data set to investigate the trends and drivers of these two important metrics of acidification in the global surface ocean over the last four decades (1982–2021). Our results confirm that pH and Ωar have been declining across the global ocean and that these trends are predominantly driven by the increase in the surface ocean concentration of dissolved inorganic carbon resulting from the accumulation of human‐made CO2. We can thus unambiguously demonstrate that it is the human activities‐driven emissions of CO2 that is causing these trends in ocean acidification (OA). We also show that the observed ocean warming enhances the decline in pH. Our study provides, for the first time, a global observation‐based quantification of the progression and driving factors of OA, which will help to better understand the impact of OA on marine life. Key Points: From 1982 through 2021, surface ocean Ωar and pH declined at −0.071 ± 0.006 and −0.0166 ± 0.0010 per decade, respectivelyThe trends vary spatially, with Ωar decreasing most strongly in the low latitudes, while pH decreases the most in the high latitudesThe trends are predominantly caused by rising atmospheric CO2, with ocean warming enhancing the pH trend [ABSTRACT FROM AUTHOR]

Inland waters are important sources of the greenhouse gasses (GHGs) carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) to the atmosphere. In the framework of the second phase of the REgional Carbon Cycle Assessment and Processes (RECCAP‐2) initiative, we synthesize existing estimates of GHG emissions from streams, rivers, lakes and reservoirs, and homogenize them with regard to underlying global maps of water surface area distribution and the effects of seasonal ice cover. We then produce regionalized estimates of GHG emissions over 10 extensive land regions. According to our synthesis, inland water GHG emissions have a global warming potential of an equivalent emission of 13.5 (9.9–20.1) and 8.3 (5.7–12.7) Pg CO2‐eq. yr−1 at a 20 and 100 years horizon (GWP20 and GWP100), respectively. Contributions of CO2 dominate GWP100, with rivers being the largest emitter. For GWP20, lakes and rivers are equally important emitters, and the warming potential of CH4 is more important than that of CO2. Contributions from N2O are about two orders of magnitude lower. Normalized to the area of RECCAP‐2 regions, S‐America and SE‐Asia show the highest emission rates, dominated by riverine CO2 emissions. Key Points: We synthesized estimates of river, lake and reservoir emissions of CO2, CH4, and N2O for 10 world regions and globallyWe re‐estimate global inland water emission of 5.5 (3.5–9.1) Pg CO2 yr−1, 100 (82–135) Tg CH4 yr−1, and 322 (248–590) Gg N2O yr−1At 100 or 20 years horizon, CO2 or CH4 dominate global warming potential of emissions, respectively [ABSTRACT FROM AUTHOR]

Inland waters are important emitters of the greenhouse gasses (GHGs) carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere. In the framework of the 2nd phase of the REgional Carbon Cycle Assessment and Processes (RECCAP‐2) initiative, we review the state of the art in estimating inland water GHG budgets at global scale, which has substantially advanced since the first phase of RECCAP nearly 10 years ago. The development of increasingly sophisticated upscaling techniques, including statistical prediction and process‐based models, allows for spatially explicit estimates that are needed for regionalized assessments of continental GHG budgets such as those established for RECCAP. A few recent estimates also resolve the seasonal and/or interannual variability in inland water GHG emissions. Nonetheless, the global‐scale assessment of inland water emissions remains challenging because of limited spatial and temporal coverage of observations and persisting uncertainties in the abundance and distribution of inland water surface areas. To decrease these uncertainties, more empirical work on the contributions of hot‐spots and hot‐moments to overall inland water GHG emissions is particularly needed. Key Points: We explore the state‐of‐the‐art in inland water greenhouse gas emissions, discussing existing estimates and underlying methodologiesDevelopment of models increasingly allows for assessment of spatial and temporal variability of emission fluxesThere is a persisting need for observations that capture hot‐spots and hot‐moments in emissions, including from small water bodies [ABSTRACT FROM AUTHOR]

Fixed nitrogen limits primary productivity in most areas of the surface ocean. Nitrite oxidation is the main source of nitrate, the most abundant form of inorganic fixed nitrogen. Even though known as an aerobic process, nitrite oxidation is not always stimulated by increased oxygen concentration, and nitrite oxidation occurs in layers of oxygen minimum zones (OMZs) where oxygen is not detectable. Nitrite‐oxidizing bacteria, known since their original isolation as aerobes, were also detected in these layers. Whether and how nitrite oxidation is occurring in the anoxic seawater is debated. Here, we reassess recent advances in marine nitrite oxidation in OMZ regions using previous work and new data sets we collected in two Pacific OMZs. We analyze the complex relationship between nitrite oxidation and oxygen. We discuss potential mechanisms explaining nitrite oxidation in different layers of OMZs based on recent findings and propose future directions to resolve the controversial question of apparently anaerobic nitrite oxidation in anoxic layers. Plain Language Summary: No organism can survive without nitrogen. The scarcity of fixed nitrogen in large surface areas of the ocean limits the capacity of phytoplankton to fix carbon dioxide. Oxygen minimum zones (OMZs) are critical oceanic regions for marine nitrogen inventory because the full spectrum of oxygen conditions in OMZs allows the retention and the loss of fixed nitrogen. The anoxic layer in OMZs was thought to host only anaerobic nitrogen loss processes. However, nitrite oxidation, conventionally known as an aerobic process that retains fixed nitrogen, was also detected in the anoxic zone. We revisit the complex relationship between nitrite oxidation and oxygen, and present mechanisms explaining nitrite oxidation in different layers of the ocean. We also chart a way forward for deciphering the mystery of anaerobic nitrite oxidation. Key Points: The detection of nitrite oxidation in anoxic seawater suggests the necessity to reevaluate our understanding of the nitrogen budgetDifferent mechanisms explaining nitrite oxidation in different oceanic layers are discussedThe complex relationship between nitrite oxidation and O2 indicates the importance of rigorous O2 control during experimental manipulations [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is an influential greenhouse gas (GHG) and an unregulated ozone‐depleting substance. The extent to which N2O emissions from natural forest soils have been enhanced by high atmospheric nitrogen (N) deposition in China during past decades is unclear; however, assessing land‐related N2O emissions for 2060 national "carbon neutrality" goal is urgent. In this study, we proposed a "gray box" conceptual model and deduced a linear relationship between soil N2O emissions and N deposition on a large scale. On this basis, we created a "process‐augmented data‐driven" approach using which we combined experimental N addition data from 38 Chinese forests with N deposition observation data to estimate regional soil N2O emissions. We found that the N2O emission budget of eastern China forest soils averaged 0.24 ± 0.07 TgN yr−1 from 1996 to 2015; of this, ∼36% (0.087 TgN yr−1) was directly induced by N deposition. Soil N2O emission rates (RN2O) fluctuated slightly after 2006, and the RN2Os of different ecoregions were significantly different (p < 0.001). Soil factors and N‐deposition‐related factors dominated the spatial variation of RN2O. Our findings provide country‐specific and ecoregion‐specific emission factors for national GHG inventories in China. Our approach bridges the gap between site‐level experiments and demand for regional N2O emissions, and is applicable for estimating N2O emissions in other countries/regions. Meanwhile, more N addition experiments are needed to comprehensively understand N cycling processes and extend the predictive capability of the approach. Plain Language Summary: Nitrous oxide (N2O) is a greenhouse gas that is around 300 times more powerful than carbon dioxide. As many countries and regions are aiming to achieve zero net carbon emission ("carbon neutrality") by the mid‐21st century, we need to evaluate the emissions of nitrous oxide to build an accurate picture of the current situation. However, it is difficult to estimate the emission of N2O from natural soils because the spatial variation is high. Inspired by the linkage between elemental nitrogen (N) that enters an ecosystem and N that leaves the ecosystem, we proposed a conceptual model and deduced a linear relationship between N deposition and soil N2O emissions. On the basis, we used experimental N addition data from various sites to quantify the linear relationship. Then, the N2O emission and its variation was estimated with dynamic N deposition data. In eastern China forests where the N deposition rate is exceptionally high, around 0.24 million tons of N2O‐N was emitted annually from 1996 to 2015, around 36% of which was directly induced by N deposition. From another perspective, soil N2O emissions in this region are expected to decline owing to pollution control practices and decreasing N deposition. Key Points: Eastern China forest soils emitted 0.24 TgN2O‐N yr−1 from 1996 to 2015N deposition directly induced ∼36% of emitted N2O from eastern China forest soilsDirect contribution of N deposition to soil N2O emissions increased from tropical to boreal forests [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]

Observations and climate models indicate that changes in the seasonal amplitude of sea surface carbon dioxide partial pressure (A‐pCO2) are underway and driven primarily by anthropogenic carbon (Cant) accumulation in the ocean. This occurs because pCO2 is more responsive to seasonal changes in physics (including warming) and biology in an ocean that contains more Cant. A‐pCO2 changes have the potential to alter annual ocean carbon uptake and contribute to the overall marine carbon cycle feedback. Using the GFDL ESM2M Large Ensemble and a novel analysis framework, we quantify the influence of Cant accumulation on pCO2 seasonal cycles and sea‐air CO2 fluxes. Specifically, we reconstruct alternative evolutions of the contemporary ocean state in which the sensitivity of pCO2 to seasonal thermal and biophysical variation is fixed at preindustrial levels, however the background, mean‐state pCO2 fully responds to anthropogenic forcing. We find near‐global A‐pCO2 increases of >100% by 2100, under RCP8.5 forcing, with rising Cant accounting for ∼100% of thermal and ∼50% of nonthermal pCO2 component amplitude changes. The other ∼50% of nonthermal pCO2 component changes are attributed to modeled changes in ocean physics and biology caused by climate change. Cant‐induced A‐pCO2 changes cause an 8.1 ± 0.4% (ensemble mean ± 1σ) increase in ocean carbon uptake by 2100. The is because greater wintertime wind speeds enhance the impact of wintertime pCO2 changes, which work to increase the ocean carbon sink. Thus, the seasonal ocean carbon cycle feedback works in opposition to the larger, mean‐state feedback that reduces ocean carbon uptake by ∼60%. Plain Language Summary: Using simulations of an Earth System Model, we isolate different factors contributing to future changes in the surface ocean carbon dioxide partial pressure (pCO2). We examine how the seasonal cycle of pCO2, and the associated sea‐air exchange of CO2, responds to changes in the ocean's temperature, circulation, biology, and chemistry. We find that the pCO2 seasonal cycle is significantly amplified across the global ocean (by ∼100% on average). This occurs because pCO2 is more responsive to seasonal changes in temperature as well as biological and physical (biophysical) processes in a future ocean that contains more anthropogenic carbon (Cant). The increased temperature sensitivity is almost exclusively due to added Cant. The increased biophysical sensitivity is equally due to added Cant and changes in ocean physics and biology caused by climate change. Seasonal wind speed variation systematically enhances the impact of altered pCO2 seasonal cycles during wintertime, causing an 8% increase in the ocean carbon sink strength by the year 2100. Within the evaluated model, this indicates that the seasonal ocean carbon cycle feedback works in opposition to the larger, mean‐state ocean carbon cycle feedback, which may cause up to a ∼60% reduction in the ocean sink by the year 2100. Key Points: Ocean anthropogenic carbon (Cant) accumulation enhances the sensitivity of ocean carbon dioxide partial pressure (pCO2) to seasonal changes in ocean physics and biologyCant‐induced changes in the pCO2 seasonal cycle under RCP8.5 forcing increase cumulative ocean carbon uptake by 8% in ESM2MThe net increase in cumulative ocean carbon uptake is driven by the interaction of wintertime changes in pCO2 and strong winter winds [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]

The oceanic absorption of anthropogenic carbon dioxide (CO2) is expected to continue in the following centuries, but the processes driving these changes remain uncertain. We studied these processes in a simulation of future changes in global climate and the carbon cycle following the RCP8.5 high emission scenario. The simulation shows increasing oceanic uptake of anthropogenic CO2 peaking towards the year 2080 and then slowing down but remaining significant in the period up to the year 2300. These multi-century changes in uptake are dominated by changes in sea-air CO2 fluxes in the tropical and southern oceans. In the tropics, reductions in upwelling and vertical gradients of dissolved carbon will reduce the vertical advection of carbon-rich thermocline waters, suppressing natural outgassing of CO2. In the Southern Ocean, the upwelling of waters with relatively low dissolved carbon keeps the surface carbon relatively low, enhancing the uptake of CO2 in the next centuries. The slowdown in CO2 uptake in the subsequent centuries is caused by the decrease in CO2 solubility and storage capacity in the ocean due to ocean warming and changes in carbon chemistry. A collapse of the Atlantic Meridional Overturning Circulation (AMOC) predicted for the next century causes a substantial reduction in the uptake of anthropogenic CO2. In sum, predicting multi-century changes in the global carbon cycle depends on future changes in carbon chemistry along with changes in oceanic and atmospheric circulations in the Southern and tropical oceans, together with a potential collapse of the AMOC. [ABSTRACT FROM AUTHOR]

The Amazon River strongly modifies the biogeochemistry of the Western Tropical Atlantic (WTA). To disentangle the different mechanisms driving these modifications, we conduct a series of modeling experiments with a high‐resolution regional ocean model (ROMS) coupled to a biogeochemical/ecological model (BEC) that we augmented to include Diatom‐Diazotroph‐Assemblages (DDAs). In our model, the Amazon River increases net primary production (NPP) in the WTA by almost 10%, exceeding the stimulation expected from the supplied inorganic nitrogen and phosphorus by a factor of two. This amplification is fueled by new nitrogen stemming from DDA‐driven N2 fixation in the plume region, supported, in part, by the consumption of riverine dissolved organic phosphorus. The vertical export of organic carbon is enhanced by a shift of the phytoplankton community toward diatoms induced by the large amount of Si(OH)4 delivered by the Amazon. These changes in NPP and export production induce a strong uptake of atmospheric CO2. In contrast, the remineralization of the river‐delivered terrestrial organic matter leads to a release of CO2 over the WTA, which is partially offset by a net uptake induced by the riverine dissolved inorganic carbon and alkalinity. Overall, the Amazon reduces the strong outgassing of the WTA in our simulations by more than 50%. Our study demonstrates how rivers modify the marine biological pump and the air‐sea CO2 fluxes in the downstream ocean through a myriad of cascading effects, highlighting the need to fully consider the land‐ocean aquatic continuum in the modeling of the Earth System. Key Points: The Amazon River stimulates the biological pump in the WTA beyond a strict utilization of the riverine nutrientsDiatom‐diazotrophic assemblages, benefiting from the river supplied phosphorus, provide the required extra nitrogenThe Amazon‐driven stimulation of the biological pump induces a strong uptake of CO2 from the atmosphere [ABSTRACT FROM AUTHOR]

Cover: In Lovenduski et al. [doi ], model-estimated trends in the sea-air flux of (first column) natural, (second column) anthropogenic, and (third column) contemporary CO2 over (first row) the simulated period (1958-2007) and (second row) the observed period (1981-2007). Units are mol C m-2 yr-2, and positive indicates CO2 outgassing trend. Only those trends with significance = 95% are shown. Black lines mark the edges of the model biomes used the study. From south to north, these biomes are the Southern Ocean seasonally ice covered biome (SO-ICE) and the Southern Ocean subpolar seasonally stratified biome (SO-SPSS). See pp. 416-426. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N2O cycling in an ODZ, we analyzed N2O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N2O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 ± 20.5 nM at the coast. Above the σθ = 25.0 kg/m3 isopycnal, Keeling plot analysis revealed two sources to the near‐surface N2O maximum, with different δ15N2Oα and δ15N2Oβ values, but each with a site preference (SP) of 6‰–8‰. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%–26% nitrification (bacterial or archeal), and 74%–83% denitrification (or nitrifier‐denitrification). Below the σθ = 25.0 kg/m3 isopycnal, box model analysis indicated that the observed 46‰–50‰ SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N2O production by denitrification, and may indicate instead a transient net consumption of N2O. Furthermore, time‐dependent model results indicated that while δ15N2Oα and δ18O‐N2O reflect both N2O production and consumption in the anoxic core of the ODZ, δ15N2Oβ predominantly reflects N2O sources. Finally, we infer that the high (N2O) observed at some stations derive from a set of conditions supporting high rates of N2O production that have not been previously encountered in this region. Plain Language Summary: Each molecule of nitrous oxide emitted to the atmosphere has the greenhouse gas potential of 265 molecules of carbon dioxide—in other words, if carbon dioxide were the currency of climate change, then nitrous oxide would be the $300 bill. But the mechanisms and rates by which nitrous oxide is produced in the ocean remain poorly constrained, especially in regions of the ocean with little to no oxygen. In the eastern tropical North Pacific oxygen‐deficient zone, we measured higher nitrous oxide concentrations near the surface than had previously been found in the region. Using the site‐specific isotope ratios of nitrogen and oxygen in nitrous oxide, we identified two potential sources contributing to these near‐surface accumulations of N2O. In the oxygen‐deficient waters below the near surface maximum, we found that low concentrations of nitrous oxide result from consumption of nitrous oxide, some of which is transported in from elsewhere, and not a balanced cycle of local production and consumption. Key Points: In 2018, nitrous oxide concentrations were observed in the eastern tropical North Pacific exceeding prior measurements in this regionThe site‐specific isotope ratios of nitrous oxide indicate two distinct sources to these high near‐surface concentrationsIn core anoxic depths, nitrous oxide may be both produced and consumed, but this cycling is likely not in steady state [ABSTRACT FROM AUTHOR]

Drylands occupy nearly 40% of the land surface and comprise a globally significant carbon reservoir. Dryland‐atmosphere carbon exchange may regulate interannual variability in atmospheric CO2. Quantifying soil respiration rates in these environments is often complicated by the presence of calcium carbonates, which are a common feature of dryland soils. We show with high‐precision O2 measurements in a laboratory potted soil experiment that respiration rates after watering were similar in control and carbonate treatment soils. However, CO2 concentrations were up to 72% lower in the carbonate treatment soil because CO2 was initially consumed during calcite dissolution. Subsequently, CO2 concentrations were over 166% greater in the carbonate treatment soil as respiration slowed and calcite precipitated, releasing CO2. Elevated δ13C values of soil CO2 (>6‰ higher in the treatment than control) confirm that observed differences were due to calcite dissolution. These findings demonstrate that calcite dissolution and precipitation can occur rapidly enough to affect soil gas compositions and that changes in soil CO2 are not always directly related to changes in soil respiration rates. Studies of local soil respiration rates and carbon exchange are likely to be influenced by dissolution and precipitation of calcium carbonates in soils. We estimate that one fifth of global soil respiration occurs in soils that contain some amount of soil carbonate, underscoring the need to account for its obscuring effects when trying to quantify soil respiration and net ecosystem exchange on a regional or global scale. Plain Language Summary: Carbon dioxide can be removed from the atmosphere by plants and stored in soils. Respiration in soils can later return it to the atmosphere. Measuring how much carbon dioxide is returned to the atmosphere is important for soils in water‐limited locations, because they may control how much carbon is stored in terrestrial environments from year‐to‐year. This measurement is complicated because the dissolution and formation of calcite, a mineral commonly found in arid soils, can also change the amount of soil carbon dioxide. We conducted a laboratory experiment with two soils, one containing calcite and one without calcite to study how calcite affects soil carbon dioxide. Our results show that calcite effects can be identified by simultaneously measuring the amount of carbon dioxide and oxygen present in soils. After a simulated rainfall, calcite dissolves and temporarily limits the amount of carbon dioxide returned to the atmosphere. However, this storage is only temporary, because carbon dioxide is gradually released back to the atmosphere as calcite reforms when the soil dries out. This work illustrates that calcite formation and dissolution can occur rapidly within soils. It also highlights the need to understand calcite behavior to accurately measure soil carbon fluxes. Key Points: Soil calcite dissolution and precipitation can mask or inflate the actual respiration‐driven CO2 response to precipitationHigh‐precision measurements of soil CO2 and O2 enable indirect monitoring of soil calcite dissolution and precipitationCalcite dissolution and formation is rapid enough to affect soil CO2 δ13C values, suggesting semiclosed system behavior of soil gasses [ABSTRACT FROM AUTHOR]

Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0–250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond. Plain Language Summary: Iron is a key micronutrient for organisms living in the upper ocean, and thus, its availability is one of the key factors controlling the removal of carbon dioxide via phytoplankton growth in much of the global ocean. Until very recently, measurements of internal iron cycling were scarce. This includes estimates of how much iron leaves the surface ocean via sinking particles. Due to the lack of observations, models struggle to reproduce observed patterns in global surface iron distributions. For the first time, we constrain the rate of iron loss from the upper ocean along three basin‐wide transects and bring together all preexisting estimates to determine the timescales on which different forms of iron are retained in the upper ocean. Overall, our findings suggest that iron cycles more rapidly between the surface and the subsurface ocean than previously estimated, and we encourage the modeling community to utilize the wealth of data presented here to explore the global consequences of these findings. Key Points: The majority of the residence times determined for total Fe in the upper 250 m fall between 10 and 100 daysExport‐based results suggest that dissolved Fe cycles sub‐annually to annually in the upper ocean, typically faster than previously thought [ABSTRACT FROM AUTHOR]

Peat soils drained for agriculture and forestry are important sources of carbon dioxide and nitrous oxide. Rewetting effectively reduces these emissions. However, rewetting also increases methane emissions from the soil and, on forestry‐drained peatlands, decreases the carbon storage of trees. To analyze the effect of peatland rewetting on the climate, we built radiative forcing scenarios for tropical peat soils, temperate and boreal agricultural peat soils, and temperate and boreal forestry‐drained peat soils. The effect of tree and wood product carbon storage in boreal forestry‐drained peatlands was also estimated as a case study for Finland. Rewetting of tropical peat soils resulted in immediate cooling. In temperate and boreal agricultural peat soils, the warming effect of methane emissions offsets a major part of the cooling for the first decades after rewetting. In temperate and boreal forestry‐drained peat soils, the effect of rewetting was mostly warming for the first decades. In addition, the decrease in tree and wood product carbon storage further delayed the onset of the cooling effect for decades. Global rewetting resulted in increasing climate cooling, reaching −70 mW (m2 Earth)−1 in 100 years. Tropical peat soils (9.6 million ha) accounted for approximately two thirds and temperate and boreal agricultural peat soils (13.0 million ha) for one third of the cooling. Forestry‐drained peat soils (10.6 million ha) had a negligible effect. We conclude that peatland rewetting is beneficial and important for mitigating climate change, but abandoning tree stands may instead be the best option concerning forestry‐drained peatlands. Key Points: Rewetting of tropical peat soils resulted in immediate coolingIn temperate and boreal agricultural peat soils, methane emissions offset a major part of the cooling for the first decadesAbandoning tree stands may be more beneficial than rewetting in temperate and boreal forestry‐drained peatlands [ABSTRACT FROM AUTHOR]

Multiple aquatic ecosystems (pond, lake, river, lagoon, and ocean) on the Arctic Coastal Plain near Utqiaġvik, Alaska, USA, were visited to determine their relative atmospheric CO2 flux and how this may have changed over time. The nearshore coastal waters and large freshwater lakes were small sinks of atmospheric CO2, whereas smaller waterbodies were substantial sources. pCO2 was linked to dissolved organic carbon concentrations across broad spatial and temporal scales, with greater concentrations found in smaller freshwater systems (i.e., ponds and rivers). On a day‐to‐day basis, water temperatures appeared to be the strongest driver of pCO2 levels in tundra ponds, where warmer temperatures likely stimulated microbial mineralization of carbon in both aquatic and hydrologically linked terrestrial environments. Large rainfall events, which may lead to inflow of carbon‐rich groundwater into these ponds, also were associated with increased daily average pCO2. Based on comparison to historical data, we estimate that CO2 concentrations in tundra ponds have increased more than 1.8 times over the past 40 years. Quantifying CO2 flux from these abundant aquatic ecosystems on the Arctic Coastal Plain and elsewhere in the high northern latitudes will likely have important implications for furthering understanding of landscape‐level and nearshore carbon dynamics in the Arctic. Key Points: Nearshore coastal marine waters and large freshwater lakes were small sinks of atmospheric CO2, whereas smaller waterbodies with tighter linkages to the terrestrial landscape (i.e., ponds or rivers) were substantial sourcesAquatic pCO2 was primarily linked to dissolved organic carbon (DOC) concentrations, water temperatures, and large rainfall eventsWe estimate that pCO2 concentrations in tundra ponds have nearly doubled over the past 40 years [ABSTRACT FROM AUTHOR]

The contemporary air‐sea flux of CO2 is investigated by the use of an air‐sea flux equation, with particular attention to the uncertainties in global values and their origin with respect to that equation. In particular, uncertainties deriving from the transfer velocity and from sparse upper ocean sampling are investigated. Eight formulations of air‐sea gas transfer velocity are used to evaluate the combined standard uncertainty resulting from several sources of error. Depending on expert opinion, a standard uncertainty in transfer velocity of either ~5% or ~10% can be argued and that will contribute a proportional error in air‐sea flux. The limited sampling of upper ocean fCO2 is readily apparent in the Surface Ocean CO2 Atlas databases. The effect of sparse sampling on the calculated fluxes was investigated by a bootstrap method, that is, treating each ship cruise to an oceanic region as a random episode and creating 10 synthetic data sets by randomly selecting episodes with replacement. Convincing values of global net air‐sea flux can only be achieved using upper ocean data collected over several decades but referenced to a standard year. The global annual referenced values are robust to sparse sampling, but seasonal and regional values exhibit more sampling uncertainty. Additional uncertainties are related to thermal and haline effects and to aspects of air‐sea gas exchange not captured by standard models. An estimate of global net CO2 exchange referenced to 2010 of −3.0 ± 0.6 Pg C/year is proposed, where the uncertainty derives primarily from uncertainty in the transfer velocity. Plain Language Summary: The oceanic carbon sink reduces the rate of accumulation of CO2 in the atmosphere but is also responsible for the acidification of the ocean. One method of estimating the size of the oceanic carbon sink depends on a calculation of upward and downward flows of CO2 at the sea surface. This study revisits this calculation using updated knowledge of the transfer processes at the sea surface and the results of a large international collaborative effort (Surface Ocean CO2 Atlas) to collect and compile measurements of CO2 in the upper ocean. Greater sampling of the oceans improves estimates, but direct calculation in each year is not practical. Instead, we calculate fluxes in a recent year (2010) using upper ocean measurements of CO2 over many years. The remaining uncertainty is dominated by limited knowledge of the efficiency of stirring of gas across the sea surface, the air‐sea transfer velocity. The study suggests a relatively large downward flow of CO2 into the ocean compared to previous applications of this method and other methods to estimate the oceanic carbon sink. Increased knowledge is rewarded by reduced uncertainty in the net global flux; that flux is estimated at −3.0 ± 0.6 Pg C/year. Further understanding of transfer velocities and better sampling may reduce the uncertainty in the future. Key Points: Increased understanding of air‐sea gas transfer processes and better sampling of the upper ocean enables higher confidence in calculations of air‐sea CO2 fluxesThe calculations imply a relatively large global net air‐to‐sea flux of −3.0 Pg C/year (referenced to 2010)This flux is known within 0.6 Pg C/year, where uncertainty in air‐sea transfer velocity is the largest contribution to the combined uncertainty [ABSTRACT FROM AUTHOR]

Land models are often used to simulate terrestrial responses to future environmental changes, but these models are not commonly evaluated with data from experimental manipulations. Results from experimental manipulations can identify and evaluate model assumptions that are consistent with appropriate ecosystem responses to future environmental change. We conducted simulations using three coupled carbon‐nitrogen versions of the Community Land Model (CLM, versions 4, 4.5, and—the newly developed—5), and compared the simulated response to nitrogen (N) and atmospheric carbon dioxide (CO2) enrichment with meta‐analyses of observations from similar experimental manipulations. In control simulations, successive versions of CLM showed a poleward increase in gross primary productivity and an overall bias reduction, compared to FLUXNET‐MTE observations. Simulations with N and CO2 enrichment demonstrate that CLM transitioned from a model that exhibited strong nitrogen limitation of the terrestrial carbon cycle (CLM4) to a model that showed greater responsiveness to elevated concentrations of CO2 in the atmosphere (CLM5). Overall, CLM5 simulations showed better agreement with observed ecosystem responses to experimental N and CO2 enrichment than previous versions of the model. These simulations also exposed shortcomings in structural assumptions and parameterizations. Specifically, no version of CLM captures changes in plant physiology, allocation, and nutrient uptake that are likely important aspects of terrestrial ecosystems' responses to environmental change. These highlight priority areas that should be addressed in future model developments. Moving forward, incorporating results from experimental manipulations into model benchmarking tools that are used to evaluate model performance will help increase confidence in terrestrial carbon cycle projections. Plain Language Summary: How do changes in the availability of nitrogen in soils or carbon dioxide in the atmosphere affect the amount of carbon that can be stored on land? Answering this question is critical, but it remains difficult for land models that are used to make climate change projections—in part because of limited understanding in how terrestrial ecosystems will respond to environmental change. Experimental manipulations that increase the availability of nitrogen or carbon dioxide, however, provide insights into how ecosystems are likely to respond to changes in resource availability. We expect that models should exhibit similar responses to those observed in the real world. Our results show that over the course of model development later versions of the Community Land Model do a better job of simulating the global carbon cycle and capturing appropriate ecosystem responses to nitrogen and carbon dioxide enrichment. This improves our confidence in the future carbon cycle projections made by more recent versions of the Community Land Model. Our results also identify assumptions in the model that are not well supported by observations and can help to prioritize future model developments. Key Points: Experimental manipulations provide critical insights into ecosystem responses to environmental change that can evaluate land modelsParametric and structural changes to the Community Land Model version 5 improve the simulated response to environmental changeModel assumptions related to nutrient acquisition strategies and trade‐offs between carbon and nitrogen limitation deserve further attention [ABSTRACT FROM AUTHOR]

The Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen‐deplete waters, N2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N2 fixation in suboxic (<20 μmol/kg O2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N·L−1·day−1) were also observed in suboxic waters near volcanic islands where N2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N2 fixation rates were highest near the continent, exceeding 500 μmol N·m−2·day−1 at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen‐replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of Nr. Despite high rates at some inshore stations, regional N2 fixation appears insufficient to compensate for Nr loss locally as observed previously in the Eastern Tropical South Pacific ODZ. Plain Language Summary: Nitrogen is an essential component of life's building blocks. Lack of nitrogen can limit growth of phytoplankton, the photosynthetic microbes at the base of most oceanic food webs. Phytoplankton in the surface ocean facilitate the removal of carbon dioxide, a strong greenhouse gas, from the atmosphere by converting it to organic forms that can aggregate and sink. Therefore, understanding the factors that control the ocean's nitrogen reservoir is important to predicting Earth's future climate. Previous research has suggested that nitrogen inputs and losses may occur in close spatial proximity because nitrogen loss processes result in a nitrogen deficit, which is thought to favor dinitrogen (N2) fixation—the microbe‐mediated conversion of unreactive N2 gas to a bioavailable form. Here, we measured rates of N2 fixation in an important region of nitrogen loss. We found that rates of N2 fixation in the region were low, suggesting that nitrogen input and removal processes do not occur in close spatial proximity here. Nevertheless, high rates of N2 fixation were observed near shore and volcanic islands, suggesting that N2 fixation can occur here but was restricted. This study contributes to our understanding of N2 fixation in a globally important region likely to change as Earth warms. Key Points: Dark, oxygen‐deplete waters in the region harbor N2 fixers, but their activity is patchy and limited by organic carbonN2 fixation rates were high in inshore euphotic waters, suggesting that coastal zones harbor more N2 fixation than previously thoughtN2 fixation occurs in the region, but rates appear too low to compensate for reactive N losses on a local scale [ABSTRACT FROM AUTHOR]

The Arctic Ocean (AO) and its associated marginal seas have recently experienced rapid climate and environmental changes, most notably sea‐ice area (SIA) loss and warming potentially impacting its uptake of carbon dioxide (CO2). We used the state‐of‐the‐art ECCO2‐Darwin coupled ocean‐biogeochemistry model to simulate the 2006–2013 period and investigate the impact of changing SIA on the CO2 uptake of the AO. We find that the mean annual CO2 sink of the AO is 153 ± 14 TgC a−1 and the CO2 sink decreased at a rate of 3.6 TgC a−1 even though SIA decreased by 8 × 104 km2 a−1 over the same period. Extreme SIA loss in 2007 resulted in a 185.4 TgC CO2 sink, an increase ∼20% over the 2006–2013 mean. In contrast, extreme SIA loss of 2012 resulted in a CO2 sink of the AO of only 146.3 TgC due to two main factors: (1) increased both wind speed and stratification in the Eastern Siberian Sea absorbing less CO2 and (2) decreased primary production and area of air‐sea gas exchange in the Chukchi and Nordic Seas. Our model captures a trend of decreasing CO2 sink in most of the Chukchi Sea during fall but does not show the changes in winter CO2 sink in the Nordic and Barents Seas as previous independent studies have suggested. Our results indicate that future AO‐atmosphere CO2 exchange will be determined by complex interplay of SIA and other environmental drivers. Plain Language Summary: The seasonal process of melting and formation of sea‐ice area (SIA) in the Arctic Ocean (AO) is a key factor to control the physical, chemical, and biological processes of the upper ocean. We used a numerical model to understand the potential consequences of two drastic events of SIA reduction in both 2007 and 2012. Our model simulations show that the biological and chemical responses of the AO were different when we compared 2007 versus 2012. In 2007, the severe reduction of SIA was followed by a large uptake of carbon dioxide (CO2, as also expected from theory) due to the increase in ice‐free area and to the enhanced photosynthetic activity. However, in 2012, the response was different due to several factors: (1) increased wind speed and decreased primary production in the Eastern Siberian Sea in the summer causing less CO2 uptake and (2) the increase in the SIA in the fall in the Chukchi sea and in the summer in the Nordic Seas (close to Greenland) inhibiting the air‐sea gas CO2 exchange. All these processes, when combined together, yielded a reduced CO2 sink of the AO in 2012 when compared to 2007. Key Points: Changes in the Arctic Ocean CO2 sink are mainly controlled by sea‐ice extent, but changes in wind speed and SST can contribute tooModel results suggest that the events of sea‐ice loss in 2007 and 2012 triggered two different responses of the Arctic Ocean CO2 sinkDuring the 2006‐2013 period in the Arctic Ocean, the simulated CO2 sink and sea‐ice area show opposite trends [ABSTRACT FROM AUTHOR]

The boreal biome is characterized by extremely dense and complex fluvial networks that are closely coupled to land. Reconstructing the role that these fluvial networks play in regional carbon (C) budgets requires identifying landscape and environmental drivers of riverine C that operate at the whole network scale and that can be applied across landscapes. Here we explore drivers of CO2, CH4, and dissolved organic (DOC) and inorganic C (DIC) across 190 streams and rivers spanning 8 Strahler orders over an area of 500,000 km2 of heterogeneous boreal landscape in Québec, focusing on those drivers that can be readily obtained from remote sensing data. Each C species (except DIC) could be modeled as a function of a proximal network‐scale property, such as flow distance or elevation, but adding regional structure to these models greatly improved predictions. These modeled regional effects were similar for DOC, CO2, and CH4 and were strongly related to average regional soil organic content and especially to NPP, the latter integrating regional differences in climate and other environmental factors. These results suggest that there may be regional C baselines determined by a combination of landscape and climate features, which simultaneously influence the average CO2, DOC, and CH4 within fluvial networks, albeit through different underlying mechanisms and with varying degrees of influence on each C species. The latter two C species appear to be more sensitive to regional differences in soil, NPP, and climate than CO2 or DIC, and therefore more likely to shift under future scenarios of change in northern landscapes. Key Points: identified readily obtained remote sensing data to predict CO2, CH4, and DOC in boreal river networksLocally, CO2 was mainly driven by downstream distance while CH4 and DOC by elevationCO2, CH4, and DOC covary at the regional level suggesting there are baselines of carbon related to climate [ABSTRACT FROM AUTHOR]

Deserts comprise a large portion of the Earth's land area, yet their role in the fluxes and cycles of greenhouse gases is poorly known and their likely response to climate change largely unexplored. We report a reconnaissance investigation of the concentrations and fluxes of CO2, CH4, and N2O along two elevation (climate) gradients in the southwestern United States. In‐soil concentrations of CO2 increased with elevation (up to 5,000 ppm). Concentrations of CH4 declined with depth in all soils (to less than 1 ppm), but the rates of decrease with depth increased with elevation. In contrast, concentrations and depth trends of N2O varied erratically. Soils were net CO2 sources (0 to >1,500 kg CO2·ha−1·year−1), and net CH4 sinks (0.2 to >3 kg CH4·ha−1·year−1). The small and variable N2O fluxes were inconsistent with the trends in soil N δ15N values, which decreased by 5‰ to 6‰ over about 1,000 m of elevation. The high soil N δ15N values (up to nearly 17‰ at the lowest elevation) indicate that there is a soil N loss mechanism that is highly depleted in 15N, and gaseous losses—either NH3 or N2O/N2—are suspected of driving these values. In summary, there appears to be a strong climate control on both soil CO2 and CH4 concentrations and to a lesser degree on calculated fluxes. The soil N trace gas concentrations indicate that deserts can be either small sources or sinks of N2O and that there may be significant consumption of arid soil N2O. Plain Language Summary: We conducted the first comprehensive study of how greenhouse gas production and consumption varies with climate and season in the deserts of California and Nevada. Desert soils are significant consumers of atmospheric methane, an important greenhouse gas. Soils are both small producers and small consumers of nitrous oxide. Given the small nitrous oxide emission rates, there must be an unidentified gaseous loss of nitrogen occurring in desert soils. Finally, the carbon dioxide emission rates, coupled with the amounts of organic carbon in the soils, suggest that deserts—like many ecosystems—will lose organic carbon due to global warming and will in turn act to accentuate greenhouse gases in the atmosphere. Key Points: We present measurements on the soil carbon dioxide, methane, and nitrous oxide concentrations and fluxes in the Great Basin and Mojave Deserts of California and NevadaThe desert is a significant methane sinkThere appear to be fairly large, but unidentified, gaseous losses of N from desert soils [ABSTRACT FROM AUTHOR]

To improve estimates of agricultural greenhouse gas emissions in sub‐Saharan Africa, we measured over six individual periods of 25–29 days fluxes of methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) with subdaily time resolution from dung patches of different quality (C/N ratio: 23–41) and quantity (0.5 and 1.0 kg) on a Kenyan rangeland during dry and wet seasons. Methane emissions peaked following dung application, whereas N2O and CO2 fluxes from dung patches were similar to fluxes from rangeland soils receiving no N additions. Greenhouse gas emissions scaled linearly with dung quantity during both seasons. Dung with a low (23) C/N ratio produced up to 10 times more CH4 than dung with a high (41) C/N ratio. Overall, CH4 emission factors (EFs) ranged from 0.001 to 0.042%, lower than those derived in temperate regions. Cumulative CO2 and N2O emissions were similar for all treatments across the different seasons. The N2O EF ranged from 0 to 0.01%, less than 1% of the Intergovernmental Panel on Climate Change Tier 1 default EF (2%) for N2O emissions from dung and urine patches, likely because of the low dung N content (9.7–16.5 g N kg−1 dry matter). However, these results were consistent with the updated cattle dung EF (0.2%) developed for Kenya in 2016/2017 (EF database ID# 422665). In view of the wide range of climates, soils, and management practices across sub‐Saharan Africa, development of robust GHG EFs from dung patches for SSA requires additional studies. Plain Language Summary: With regard to the agricultural sector, livestock production systems are the dominant greenhouse gas (GHG) source. A significant part of emissions due to livestock production is linked to GHG emissions from dung patches on rangelands. While this source is rather well constrained for countries with developed economies, little is known about GHG emissions from dung patches in developing countries, specifically for countries in Sub‐Saharan Africa. Based on own measurements and an extensive literature review we show that GHG emissions from dung patches are likely highly overestimated as poor feed quality and differences in environmental conditions strongly limit GHG emissions. Our work calls for a revision of emission estimates from this important GHG source for developing countries. Key Points: N2O fluxes from dung patches on rangeland in Kenya are only 1% of current IPCC estimateLow emissions are very likely due to low N content of the dung, which results from poor feedsEmission factor for dung patches needs to be revised, specifically for livestock systems in developing countries [ABSTRACT FROM AUTHOR]

The North Atlantic Ocean plays a major role in climate change not the least due to its importance in CO2 uptake and thus natural carbon sequestration. The CO2 concentration in its surface waters, which determines the ocean's CO2 sink/source function, varies on seasonal and interannual time scales and is mainly driven by air‐sea gas exchange, temperature variability, and biological production/respiration. The variability in stable carbon isotope signatures can provide further insight and help to improve the understanding of the controls of the surface ocean carbon system. In this work, a cavity ringdown spectrometer was coupled to a classical, equilibrator‐based pCO2 system on a voluntary observing ship line that regularly sails across the subpolar North Atlantic between North America and Europe. From 2012 to 2014, a 3‐year time series of underway surface δ13C(CO2) data was obtained along with continuous measurements of temperature, salinity, and fCO2. We perform a decomposition of thermal and nonthermal drivers of fCO2 and δ13C(CO2). The direct measurement of the surface ocean δ13C(CO2) allows us to estimate the mass flux and also the stable carbon isotope fractionation during air‐sea gas exchange. While the CO2 mass flow was in the range of 1–2 mol CO2·m−2·year−1 on the shelves and 2.5–3.5 mol CO2·m−2·year−1 in the open ocean, the isotope signature of this CO2 flux with respect to the sea surface ranged from −2.6 ± 1.4‰ on the shelves to −6.6 ± 0.9‰ in the western and −4.5 ± 0.9‰ in the eastern part of the open ocean section. Key Points: Three years of continuous underway measurements of stable carbon isotopes in sea surface CO2 were performed in the North Atlantic OceanA cavity ringdown spectrometer was successfully implemented and operated autonomously on a commercial vesselThe isotope signature of air‐sea gas exchange was determined [ABSTRACT FROM AUTHOR]

Abstract: We have analyzed decade‐long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable‐slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process‐based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10‐year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space‐to‐time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales. [ABSTRACT FROM AUTHOR]

Abstract: The biogeochemistry of the ocean exerts a strong influence on the climate by modulating atmospheric greenhouse gases. In turn, ocean biogeochemistry depends on numerous physical and biological processes that change over space and time. Accurately simulating these processes is fundamental for accurately simulating the ocean's role within the climate. However, our simulation of these processes is often simplistic, despite a growing understanding of underlying biological dynamics. Here we explore how new parameterizations of biological processes affect simulated biogeochemical properties in a global ocean model. We combine 6 different physical realizations with 6 different biogeochemical parameterizations (36 unique ocean states). The biogeochemical parameterizations, all previously published, aim to more accurately represent the response of ocean biology to changing physical conditions. We make three major findings. First, oxygen, carbon, alkalinity, and phosphate fields are more sensitive to changes in the ocean's physical state. Only nitrate is more sensitive to changes in biological processes, and we suggest that assessment protocols for ocean biogeochemical models formally include the marine nitrogen cycle to assess their performance. Second, we show that dynamic variations in the production, remineralization, and stoichiometry of organic matter in response to changing environmental conditions benefit the simulation of ocean biogeochemistry. Third, dynamic biological functioning reduces the sensitivity of biogeochemical properties to physical change. Carbon and nitrogen inventories were 50% and 20% less sensitive to physical changes, respectively, in simulations that incorporated dynamic biological functioning. These results highlight the importance of a dynamic biology for ocean properties and climate. [ABSTRACT FROM AUTHOR]

Abstract: Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here we construct such a budget for eastern North America using historical data, empirical models, remote sensing algorithms, and process‐based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they, respectively, make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters, and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling. [ABSTRACT FROM AUTHOR]

The remineralization depth of particulate organic carbon (POC) fluxes exported from the surface ocean exerts a major control over atmospheric CO₂ levels. According to a long-held paradigm most of the POC exported to depth is associated with large particles. However, recent lines of evidence suggest that slow-sinking POC (SSPOC) may be an important contributor to this flux. Here we assess the circumstances under which this occurs. Our study uses samples collected using the Marine Snow Catcher throughout the Atlantic Ocean, from high latitudes to midlatitudes. We find median SSPOC concentrations of 5.5 μg L−1, 13 times smaller than suspended POC concentrations and 75 times higher than median fast-sinking POC (FSPOC) concentrations (0.07 μg L−1). Export fluxes of SSPOC generally exceed FSPOC flux, with the exception being during a spring bloom sampled in the Southern Ocean. In the Southern Ocean SSPOC fluxes often increase with depth relative to FSPOC flux, likely due to midwater fragmentation of FSPOC, a process which may contribute to shallow mineralization of POC and hence to reduced carbon storage. Biogeochemical models do not generally reproduce this behavior, meaning that they likely overestimate long-term ocean carbon storage. [ABSTRACT FROM AUTHOR]

Our knowledge of fundamental drivers of the temperature sensitivity ( Q10) of soil carbon dioxide (CO2) release is crucial for improving the predictability of soil carbon dynamics in Earth System Models. However, patterns and determinants of Q10 over a broad geographic scale are not fully understood, especially in alpine ecosystems. Here we addressed this issue by incubating surface soils (0-10 cm) obtained from 156 sites across Tibetan alpine grasslands. Q10 was estimated from the dynamics of the soil CO2 release rate under varying temperatures of 5-25°C. Structure equation modeling was performed to evaluate the relative importance of substrate, environmental, and microbial properties in regulating the soil CO2 release rate and Q10. Our results indicated that steppe soils had significantly lower CO2 release rates but higher Q10 than meadow soils. The combination of substrate properties and environmental variables could predict 52% of the variation in soil CO2 release rate across all grassland sites and explained 37% and 58% of the variation in Q10 across the steppe and meadow sites, respectively. Of these, precipitation was the best predictor of soil CO2 release rate. Basal microbial respiration rate ( B) was the most important predictor of Q10 in steppe soils, whereas soil pH outweighed B as the major regulator in meadow soils. These results demonstrate that carbon quality and environmental variables coregulate Q10 across alpine ecosystems, implying that modelers can rely on the 'carbon-quality temperature' hypothesis for estimating apparent temperature sensitivities, but relevant environmental factors, especially soil pH, should be considered in higher-productivity alpine regions. [ABSTRACT FROM AUTHOR]

Six profiling floats equipped with nitrate and oxygen sensors were deployed at Ocean Station P in the Gulf of Alaska. The resulting six calendar years and 10 float years of nitrate and oxygen data were used to determine an average annual cycle for net community production (NCP) in the top 35 m of the water column. NCP became positive in February as soon as the mixing activity in the surface layer began to weaken, but nearly 3 months before the traditionally defined mixed layer began to shoal from its winter time maximum. NCP displayed two maxima, one toward the end of May and another in August with a summertime minimum in June corresponding to the historical peak in mesozooplankton biomass. The average annual NCP was determined to be 1.5 ± 0.6 mol C m−2 yr−1 using nitrate and 1.5 ± 0.7 mol C m−2 yr−1 using oxygen. The results from oxygen data proved to be quite sensitive to the gas exchange model used as well as the accuracy of the oxygen measurement. Gas exchange models optimized for carbon dioxide flux generally ignore transport due to gas exchange through the injection of bubbles, and these models yield NCP values that are two to three time higher than the nitrate-based estimates. If nitrate and oxygen NCP rates are assumed to be related by the Redfield model, we show that the oxygen gas exchange model can be optimized by tuning the exchange terms to reproduce the nitrate NCP annual cycle. [ABSTRACT FROM AUTHOR]

The extended multilinear regression method is used to determine the uptake and storage of anthropogenic carbon in the Atlantic Ocean based on repeat occupations of four cruises from 1989 to 2014 (A16, A20, A22, and A10), with an emphasis on the 2003-2014 period. The results show a significant increase in basin-wide anthropogenic carbon storage in the North Atlantic, which absorbed 4.4 ± 0.9 Pg C decade−1 from 2003 to 2014 compared to 1.9 ± 0.4 Pg C decade−1 for the 1989-2003 period. This decadal variability is attributed to changing ventilation patterns associated with the North Atlantic Oscillation and increasing release of anthropogenic carbon into the atmosphere. There are small changes in the uptake rate of CO2 in the South Atlantic for these time periods (3.7 ± 0.8 Pg C decade−1 versus 3.2 ± 0.7 Pg C decade−1). Several eddies are identified containing ~20% more anthropogenic carbon than the surrounding waters in the South Atlantic demonstrating the importance of eddies in transporting anthropogenic carbon. The uptake of carbon results in a decrease in pH of ~0.0021 ± 0.0007 year−1 for surface waters during the last 10 years, in line with the atmospheric increase in CO2. [ABSTRACT FROM AUTHOR]

Quantification of biogenic carbon fluxes from agricultural lands is needed to generate comprehensive bottom-up estimates of net carbon exchange for global and regional carbon monitoring. We estimated global agricultural carbon fluxes associated with annual crop net primary production (NPP), harvested biomass, and consumption of biomass by humans and livestock. These estimates were combined for a single estimate of net carbon exchange and spatially distributed to 0.05° resolution using Moderate Resolution Imaging Spectroradiometer satellite land cover data. Global crop NPP in 2011 was estimated at 5.25 ± 0.46 Pg C yr−1, of which 2.05 ± 0.05 Pg C yr−1 was harvested and 0.54 Pg C yr−1 was collected from crop residues for livestock fodder. Total livestock feed intake in 2011 was 2.42 ± 0.21 Pg C yr−1, of which 2.31 ± 0.21 Pg C yr−1 was emitted as CO2, 0.07 ± 0.01 Pg C yr−1 was emitted as CH4, and 0.04 Pg C yr−1 was contained within milk and egg production. Livestock grazed an estimated 1.27 Pg C yr−1 in 2011, which constituted 52.4% of total feed intake. Global human food intake was 0.57 ± 0.03 Pg C yr−1 in 2011, the majority of which was respired as CO2. Completed global cropland carbon budgets accounted for the ultimate use of approximately 80% of harvested biomass. The spatial distribution of these fluxes may be used for global carbon monitoring, estimation of regional uncertainty, and for use as input to Earth system models. [ABSTRACT FROM AUTHOR]

The cyanobacterium Trichodesmium is responsible for a significant proportion of the annual 'new' nitrogen introduced into the global ocean. Despite being arguably the best studied marine diazotroph, the factors controlling the distribution and growth of Trichodesmium remain a subject of debate, with sea surface temperature, the partial pressure of CO2, and nutrients including iron (Fe) and phosphorus (P), all suggested to be important. Synthesizing data from seven cruises collectively spanning large temporal and spatial scales across the Atlantic Ocean, including two previously unreported studies crossing the largely undersampled South Atlantic gyre, we assessed the relationship between proposed environmental drivers and both community N2 fixation rates and the distribution of Trichodesmium. Simple linear regression analysis would suggest no relationship between any of the sampled environmental variables and N2 fixation rates. However, considering the concentrations of iron and phosphorus together within a simplified resource-ratio framework, illustrated using an idealized numerical model, indicates the combined effects these nutrients have on Trichodesmium and broader diazotroph biogeography, alongside the reciprocal maintenance of different biogeographic provinces of the (sub)tropical Atlantic in states of Fe or P oligotrophy by diazotrophy. The qualitative principles of the resource-ratio framework are argued to be consistent with both the previously described North-South Atlantic contrast in Trichodesmium abundance and the presence and consequence of a substantial non- Trichodesmium diazotrophic community in the western South Atlantic subtropical gyre. A comprehensive, observation-based explanation of the interactions between Trichodesmium and the wider diazotrophic community with iron and phosphorus in the Atlantic Ocean is thus revealed. [ABSTRACT FROM AUTHOR]

While it is widely recognized that peatlands are important in the global carbon cycle, there is limited information on belowground gas production in tropical peatlands. We measured pore water methane (CH4) and carbon dioxide (CO2) concentrations and δ13C isotopic composition and CH4 and CO2 production rates in peat incubations from the Changuinola wetland in Panama. Our most striking finding was that CH4 was depleted in 13C (−94‰ in pore water and produced at −107‰ in incubated peat) relative to CH4 found in most temperate and northern wetlands, potentially impacting the accuracy of approaches that use carbon isotopes to constrain global mass balance estimates. Fractionation factors between CH4 and CO2 showed that hydrogenotrophic methanogenesis was the dominant CH4 production pathway, with up to 100% of the CH4 produced via this route. Far more CO2 than CH4 (7 to 100X) was measured in pore water, due in part to loss of CH4 through ebullition or oxidation and to the production of CO2 from pathways other than methanogenesis. We analyzed data on 58 wetlands from the literature to determine the dominant factors influencing the relative proportions of CH4 produced by hydrogenotrophic and acetoclastic methanogenesis and found that a combination of environmental parameters including pH, vegetation type, nutrient status, and latitude are correlated to the dominant methanogenic pathway. Methane production pathways in tropical peatlands do not correlate with these variables in the same way as their more northerly counterparts and thus may be differently affected by climate change. [ABSTRACT FROM AUTHOR]

Tropical wetlands are not included in Earth system models, despite being an important source of methane (CH4) and contributing a large fraction of carbon dioxide (CO2) emissions from land use, land use change, and forestry in the tropics. This review identifies a remarkable lack of data on the carbon balance and gas fluxes from undisturbed tropical wetlands, which limits the ability of global change models to make accurate predictions about future climate. We show that the available data on in situ carbon gas fluxes in undisturbed forested tropical wetlands indicate marked spatial and temporal variability in CO2 and CH4 emissions, with exceptionally large fluxes in Southeast Asia and the Neotropics. By upscaling short-term measurements, we calculate that approximately 90 ± 77 Tg CH4 year−1 and 4540 ± 1480 Tg CO2 year−1 are released from tropical wetlands globally. CH4 fluxes are greater from mineral than organic soils, whereas CO2 fluxes do not differ between soil types. The high CO2 and CH4 emissions are mirrored by high rates of net primary productivity and litter decay. Net ecosystem productivity was estimated to be greater in peat-forming wetlands than on mineral soils, but the available data are insufficient to construct reliable carbon balances or estimate gas fluxes at regional scales. We conclude that there is an urgent need for systematic data on carbon dynamics in tropical wetlands to provide a robust understanding of how they differ from well-studied northern wetlands and allow incorporation of tropical wetlands into global climate change models. [ABSTRACT FROM AUTHOR]

We determined the historical change in soil organic carbon (SOC) stocks from long-term field trials that represent major soil types and climatic conditions of northern China. Soil carbon and general circulation models were validated using these field trial data sets. We then applied these models to predict future change in SOC stocks to 2100 using two net primary production (NPP) scenarios (i.e., current NPP or 1% year−1 NPP increase). The conversion rate of plant residues to SOC was higher in single-cropping sites than in double-cropping sites. The prediction of future SOC sequestration potential indicated that these soils will be a net source of carbon dioxide (CO2) under no fertilizer inputs. Even when inorganic nutrients were applied, the additional carbon input from increased plant residues could not meet the depletion of SOC in parts of northern China. Manure or straw application could however improve the SOC sequestration potential at all sites. The SOC sequestration potential in northern China was estimated to be −4.3 to 18.2 t C ha−1 by 2100. The effect of projected climate change on the annual rate of SOC change did not differ significantly between climate scenarios. The average annual rate of SOC change under current and increased NPP scenarios (at 850 ppm CO2) was approximately 0.136 t C ha−1 yr−1 in northern China. These findings highlight the need to maintain, and where possible increase, organic carbon inputs into these farming systems which are rapidly becoming inorganic fertilizer intensive. [ABSTRACT FROM AUTHOR]

Growth of the Phoenix metropolitan area led to road cut or house platform exposures of the internal bedrock material of surrounding semiarid mountain ranges. Similar exposures in the surrounding Sonoran and Mojave Deserts reveal the presence of sedimentary calcium carbonate infilling the preexisting fracture matrix of the bedrock. Field surveys at 31 sites with bedrock fractures filled with carbonate, referred to as BFFC in the following text, reveal an average of 0.079 ± 0.036 mTC/m2 stored in the upper 2 m of analyzed bedrock exposures. Backscattered electron microscopy images indicate the presence of carbonate at the micron scale, not included in this estimate of carbon storage. Pilot radiocarbon and Sr isotope analyses suggest that one of the surveyed BFFC veins was flushed into the bedrock from a nonbedrock source during the wetter last glacial period in the late Pleistocene. [ABSTRACT FROM AUTHOR]

The equatorial Pacific is a dynamic region that plays an important role in the global carbon cycle. This region is the largest oceanic source of carbon dioxide (CO2) to the atmosphere, which varies interannually dependent on the El Niño-Southern Oscillation (ENSO) and other climatic and oceanic drivers. We present high-resolution observations of surface ocean CO2 partial pressure ( pCO2) at four fixed locations in the Niño 3.4 area with data sets encompassing 10 ENSO warm and cold events from 1997 to 2011. The mooring observations confirm that ENSO controls much of the interannual variability in surface seawater pCO2 with values ranging from 315 to 578 µatm. The mooring time series also capture the temporal variability necessary to make the first estimates of long-term pH trends in the equatorial Pacific, which suggests that the combination of ocean acidification and decadal variability creates conditions for high rates of pH change since the beginning of the mooring record. Anthropogenic CO2 increases play a dominant role in significant observed seawater pCO2 trends of +2.3 to +3.3 µatm yr−1 and pH trends of −0.0018 to −0.0026 yr−1 across the full time series in this region. However, increased upwelling driven by increased trade winds, a shallower thermocline, and increased frequency of La Niña events also contribute an average of 40% of the observed trends since 1998. These trends are higher than previous estimates based on underway observations and suggest that the equatorial Pacific is contributing a greater amount of CO2 to the atmospheric CO2 inventory over the last decade. [ABSTRACT FROM AUTHOR]

Multiple analyses of dissolved organic carbon (DOC) from pore waters were conducted to define the processes that govern carbon balance in peatlands: (1) source, reactivity, and transport of DOC with respect to vegetation, peat, and age of carbon substrate, (2) reactivity of DOC with respect to molecular size, and (3) lability to photoxidation of surficial DOC. We found that surface organic production fuels heterotrophic respiration at depth in advection-dominated peatlands, especially in fens. Fen DOC was Δ14C enriched relative to the surrounding fen peat, and fen respiration products were similar to this enriched DOC indicating that DOC was the main microbial substrate. Bog DOC was more variable showing either enrichment in ∆14C at depth or ∆14C values that follow peat values. This variability in bogs is probably controlled by the relative importance of vertical transport of labile carbon substrates within the peat profile versus DOC production from bog peat. These results extended our set of observations to 10 years at one bog-fen pair and add two additional bog-fen pairs to our series of observations. Anaerobic incubations of peat, rinsed free of residual DOC, produced DOC and respiration products that were strikingly similar to the peat values in a bog and two fens. This result demonstrated conclusively that downward advection is the process responsible for the presence of modern DOC found at depth in the peat column. Fen DOC has lower C/N values and up to twice as much LMW (<1 kDa) DOC as bogs due to differences in organic inputs and greater microbial processing. Fluorescence irradiation experiments showed that fen DOC is more photolabile than bog DOC. [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]

The 14C/C abundance in CO2(Δ14CO2) promises to provide useful constraints on regional fossil fuel emissions and atmospheric transport through the large gradients introduced by anthropogenic activity. The currently sparse atmospheric Δ14CO2 monitoring network can potentially be augmented by using plant biomass as an integrated sample of the atmospheric Δ14CO2. But the interpretation of such an integrated sample requires knowledge about the day-to-day CO2 uptake of the sampled plants. We investigate here the required detail in daily plant growth variations needed to accurately interpret regional fossil fuel emissions from annual plant samples. We use a crop growth model driven by daily meteorology to reproduce daily fixation of Δ14CO2 in maize and wheat plants in the Netherlands in 2008. When comparing the integrated Δ14CO2 simulated with this detailed model to the values obtained when using simpler proxies for daily plant growth (such as radiation and temperature), we find differences that can exceed the reported measurement precision of Δ14CO2(∼2‰). Furthermore, we show that even in the absence of any spatial differences in fossil fuel emissions, differences in regional weather can induce plant growth variations that result in spatial gradients of up to 3.5‰ in plant samples. These gradients are even larger when interpreting separate plant organs (leaves, stems, roots, or fruits), as they each develop during different time periods. Not accounting for these growth-induced differences in Δ14CO2 in plant samples would introduce a substantial bias (1.5-2 ppm) when estimating the fraction of atmospheric CO2 variations resulting from nearby fossil fuel emissions. [ABSTRACT FROM AUTHOR]

Flooding land for water reservoir creation has many environmental impacts including the production of the greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4). To assess processes governing GHG emissions from the flooding of terrestrial carbon, three experimental reservoirs were constructed in upland boreal forest areas of differing carbon stores as part of the Flooded Upland Dynamics Experiment (FLUDEX). We calculated process-based GHG budgets for these reservoirs over 5 years following the onset of flooding. Stable isotopic budgets of carbon were necessary to separate community respiration ( CR), which produces CO2, from net primary production ( NPP), which consumes CO2, and to separate CH4 production from CH4 consumption via oxidation. NPP removed up to 44% of the CO2 produced from CR. CR and NPP exhibited different year-after-year trends. CH4 flux to the atmosphere increased about twofold over 3 years, yet isotopic budgets showed CH4 production in flooded soils increased nearly tenfold. CH4 oxidation near the flooded soil-water interface greatly decreased the CH4 flux from the water column to the atmosphere. Ebullition was the most important conduit of CH4 to the atmosphere after 3 years. Although CH4 production increased with time, the total GHG flux, in CO2 equivalents, declined. Contrary to expectations, neither CR nor total GHG fluxes were directly related to the quantity of organic carbon flooded. Instead, these reservoirs produced a strikingly similar amount of CO2 equivalents over 5 years. [ABSTRACT FROM AUTHOR]

Rates of net community production (NCP) and air-sea CO2 flux in me Northeast Pacific subarctic, transition zone and subtropical regions (22°N-50°N, 145°W-152°W) were determined on a cruise in August-September 2008 by continuous measurement of surface values of the ratio of dissolved oxygen to argon (O2/Ar) and the partial pressure of CO2 (pCO2). These estimates were compared with simultaneous measurements of sea surface temperature (SST), chlorophyll-a (chl-a), flow cytometry, and discrete surface nutrient concentrations. NCP and CO2 influx were greatest in the subarctic (45°N-50°N, 25.8 ± 4.6 and 4.1 ± 0.9 mmol C m-2 d-1) and northern transition zone (40°N-45°N, 17.1 ± 4.4 and 2.1 ± 0.5 mmol C m-2 d-1), with mean NCP ∼6-8× greater than mean CO2 invasion (error estimates reflect 1 σ confidence intervals). Contrastingly, the southern transition zone (32°N-40°N) and subtropics (22°N-32°N) had lower mean NCP (5.4 ± 1.8 and 8.1 ± 2.1 mmol C m-2 d-1, respectively) and mean CO2 efflux (3.0 ± 0.5 and 0.1 ± 0.0 mmol C m-2 d-1, respectively). In the subarctic and transition zone, NCP was highly correlated with surface chl-a and CO2 influx, indicating strong coupling between the biological pump and CO2 uptake. Meridional trends in our NCP estimates in the transition zone and subtropics were similar to those for integrated summertime NCP along the cruise track determined using an upper ocean climatological carbon budget. [ABSTRACT FROM AUTHOR]

A mooring at 6°S, 10°W has been equipped with an oxygen optode and a Carioca sensor for monitoring hourly oxygen (O2) and the fugacity of CO2 (fCO2) at 1.5 m depth since June 2006. Due to biofouling, the oxygen time series lasted between 72 days in 2008 to 159 days in 2009. Using an alkalinity-salinity relationship determined for the area, dissolved inorganic carbon (DIC) is calculated. Short-term and long-term net community productions (NCP) are calculated using a mass balance approach for DIC and O2 under the assumption of no mixing conditions. The mooring site is always supersaturated in oxygen except in 2007 when depleted oxygen waters are observed from June to September, during the upwelling season. Averaging all the short time events, NCP calculated from the rates of changes of O2 and DIC leads to a NCP of 16.6 ± 6.1 mmol C m-2d-1, ranging from 14.7 mmol C m-2d-1 in 2008 to 17.4 mmol C m-2d-1 in 2006. The mean daily oxygen biological production rate determined over the whole time series shows a significant year to year variability with 7.5 mmol C m-2d-1 in 2008 to 15.6 mmol C m-2d-1 in 2009. A photosynthetic quotient ranging between 1.0 to 1.3 has been determined when both carbon and oxygen NCP values are available. [ABSTRACT FROM AUTHOR]

Observations from November 2006 in the southwestern Ross Sea indicate that stratification developed in a localized fashion, proximal to upper ocean fronts. These regions were hotspots for biological productivity, exhibiting greater drawdown of CO2 and accumulation of oxygen, indicative of enhanced photosynthesis and air-sea gas exchange. While the effect of stratification is clear, the reasons for its development was not; air temperatures were unseasonably cold, sea-ice melt and sea surface warming were not significant. By comparing a one-dimensional mixed layer model with two-dimensional numerical simulations that include horizontal density gradients characteristic of the region, it is shown that Ekman advection is critical to structuring early season stratification. Where fronts are forced by winds that oppose the surface frontal current, Ekman advection displaces lighter water over dense. As biological productivity is light limited in the Ross Sea, and thus sensitive to the depth of the mixed layer, Ekman restratification plays an important role in determining the spatial distribution and development of the annual phytoplankton bloom in the region. The presence of fronts is therefore of first-order importance to the restratification and bloom dynamics of the Ross Sea in the early spring. [ABSTRACT FROM AUTHOR]

Lakes and reservoirs typically emit CO2 to the atmosphere as a result of respiration in excess of primary production. For reservoirs, this respiration is often assumed to be primarily of benthic origin, associated with drowned vegetation and soils. This study took measurements from a young hydroelectric reservoir (Eastmain-1) in boreal Canada and nearby natural lakes to compare the rates of water column respiration and hypolimnetic CO2 accumulation during a stratified period. Reservoir water column respiration rates were significantly higher than those in natural lakes, with surface means of 61.4 ± 4.6 mg C m-3 d-1 in Eastmain-1 and 40.2 ± 3 mg C m-3 d-1 in lakes and profundal means of 58.1 ± 11 mg C m-3 d-1 in Eastmain-1 and 17.6 ± 7.5 mg C m-3 d-1 in lakes. Reservoir hypolimnetic CO2 accumulation rates were also higher than those in natural lakes, with means of 78.8 ± 16 mg C m-3 d-1 in Eastmain-1 and 19.8 ± 2.7 mg C m-3 d-1 in lakes. Our findings further indicate that the relative contribution of benthic respiration (estimated as the difference between hypolimnetic CO2 accumulation and water column respiration) toward total net CO2 production during stratified periods was similar in both aquatic systems (23% in lakes, 27% in Eastmain-1). By identifying the major role of the water column in the reservoir's CO2 production, this study calls for a significant paradigm shift away from the common assumption that reservoir CO2 emissions are predominantly the result of decaying vegetation in the benthic environment. [ABSTRACT FROM AUTHOR]