Ocean CO2 Flux Research Articles

Riverine influence on the tropical Atlantic Ocean biogeochemistry

Abstract. We assess the role of riverine inputs of N, Si, Fe, organic and inorganic C in the tropical Atlantic Ocean using a global ocean biogeochemistry model. We use a standard model scenario and three sensitivity tests to investigate the role of total river nutrient and carbon inputs, as well as the western (South American) and eastern (African) river inputs on the tropical Atlantic Ocean biogeochemistry, between 20° S–20° N and 70° W–20° E. Increased nutrient availability from river inputs in this area (compared to a sensitivity scenario without river nutrient inputs, NO_RIVER) leads to an increase in primary production (PP) and export production (EP), mainly in the coastal ocean area (modeled ocean area with bathymetry <200 m). Model results suggest an enhanced N-fixation by diazotrophs on the tropical Atlantic mainly in open ocean areas. The increased rate of N-fixation in the TODAY scenario is proportional to the increase in PP and EP relative to the NO_RIVER scenario, and may support up to 14% of the coastal ocean export production. Inputs from South American rivers have an impact in coastal PP and EP two times higher than those from African rivers. On the other hand, results suggest that the contribution of African and South American rivers to the total increase in open ocean PP and EP is similar. Considering the amount of delivered nutrients (2–3 times less nutrients and carbon inputs by African rivers) one concludes that African riverine inputs may have a larger impact on the whole tropical Atlantic Ocean biogeochemistry. This is probably due to a combination of nutrient trapping in upwelling areas off the large rivers' outflows and shallow mixed layers in the eastern tropical Atlantic, concomitantly to the differences in delivered nutrient ratios leading to alleviation in limitation conditions, mainly for diatoms. When river inputs are added to the model, we estimate a modest decrease in open ocean sea-air CO2 fluxes (−5.2 Tg C a−1) and an increase in coastal ocean CO2 fluxes, mainly provoked by the remineralization of riverine organic matter delivered by the South American rivers.

Biosphere model simulations of interannual variability in terrestrial 13C/12C exchange

Previous studies suggest that a large part of the variability in the atmospheric ratio of 13CO2/12CO2originates from carbon exchange with the terrestrial biosphere rather than with the oceans. Since this variability is used to quantitatively partition the total carbon sink, we here investigate the contribution of interannual variability (IAV) in biospheric exchange to the observed atmospheric 13C variations. We use the Simple Biosphere ‐ Carnegie‐Ames‐Stanford Approach biogeochemical model, including a detailed isotopic fractionation scheme, separate 12C and 13C biogeochemical pools, and satellite‐observed fire disturbances. This model of 12CO2 and 13CO2 thus also produces return fluxes of 13CO2from its differently aged pools, contributing to the so‐called disequilibrium flux. Our simulated terrestrial 13C budget closely resembles previously published model results for plant discrimination and disequilibrium fluxes and similarly suggests that variations in C3 discrimination and year‐to‐year variations in C3and C4 productivity are the main drivers of their IAV. But the year‐to‐year variability in the isotopic disequilibrium flux is much lower (1σ=±1.5 PgC ‰ yr−1) than required (±12.5 PgC ‰ yr−1) to match atmospheric observations, under the common assumption of low variability in net ocean CO2 fluxes. This contrasts with earlier published results. It is currently unclear how to increase IAV in these drivers suggesting that SiBCASA still misses processes that enhance variability in plant discrimination and relative C3/C4productivity. Alternatively, 13C budget terms other than terrestrial disequilibrium fluxes, including possibly the atmospheric growth rate, must have significantly different IAV in order to close the atmospheric 13C budget on a year‐to‐year basis.

Sea–air CO<sub>2</sub> fluxes in the Southern Ocean for the period 1990–2009

Abstract. The Southern Ocean (44–75° S) plays a critical role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Different approaches have been used to estimate sea–air CO2 fluxes in this region: synthesis of surface ocean observations, ocean biogeochemical models, and atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Southern Ocean sea–air CO2 fluxes between 1990–2009. Using all models and inversions (26), the integrated median annual sea–air CO2 flux of −0.42 ± 0.07 Pg C yr−1 for the 44–75° S region, is consistent with the −0.27 ± 0.13 Pg C yr−1 calculated using surface observations. The circumpolar region south of 58° S has a small net annual flux (model and inversion median: −0.04 ± 0.07 Pg C yr−1 and observations: +0.04 ± 0.02 Pg C yr−1), with most of the net annual flux located in the 44 to 58° S circumpolar band (model and inversion median: −0.36 ± 0.09 Pg C yr−1 and observations: −0.35 ± 0.09 Pg C yr−1). Seasonally, in the 44–58° S region, the median of 5 ocean biogeochemical models captures the observed sea–air CO2 flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in the net flux. South of 58° S, neither atmospheric inversions nor ocean biogeochemical models reproduce the phase and amplitude of the observed seasonal sea–air CO2 flux, particularly in the Austral Winter. Importantly, no individual atmospheric inversion or ocean biogeochemical model is capable of reproducing both the observed annual mean uptake and the observed seasonal cycle. This raises concerns about projecting future changes in Southern Ocean CO2 fluxes. The median interannual variability from atmospheric inversions and ocean biogeochemical models is substantial in the Southern Ocean; up to 25% of the annual mean flux, with 25% of this interannual variability attributed to the region south of 58° S. Resolving long-term trends is difficult due to the large interannual variability and short time frame (1990–2009) of this study; this is particularly evident from the large spread in trends from inversions and ocean biogeochemical models. Nevertheless, in the period 1990–2009 ocean biogeochemical models do show increasing oceanic uptake consistent with the expected increase of −0.05 Pg C yr−1 decade−1. In contrast, atmospheric inversions suggest little change in the strength of the CO2 sink broadly consistent with the results of Le Quéré et al. (2007).

Analysis of a 39-year continuous atmospheric CO<sub>2</sub> record from Baring Head, New Zealand

Abstract. We present an analysis of a 39-year record of continuous atmospheric CO2 observations made at Baring Head, New Zealand, filtered for steady background CO2 mole fractions during southerly wind conditions. We discuss relationships between variability in the filtered CO2 time series and regional to global carbon cycling. Baring Head is well situated to sample air that has been isolated from terrestrial influences over the Southern Ocean, and experiences extended episodes of strong southerly winds with low CO2 variability. The filtered Baring Head CO2 record reveals an average seasonal cycle with amplitude of 0.95 ppm that is 13% smaller and 3 weeks earlier in phase than that at the South Pole. Seasonal variations in a given year are sensitive to the timing and magnitude of the combined influences of Southern Ocean CO2 fluxes and terrestrial fluxes from both hemispheres. The amplitude of the seasonal cycle varies throughout the record, but we find no significant long-term seasonal changes with respect to the South Pole. Interannual variations in CO2 growth rate in the Baring Head record closely match the El Niño-Southern Oscillation, reflecting the global reach of CO2 mole fraction anomalies associated with this cycle. We use atmospheric transport model results to investigate contributions to seasonal and annual-mean components of the observed CO2 record. Long-term trends in mean gradients between Baring Head and other stations are predominately due to increases in Northern Hemisphere fossil-fuel burning and Southern Ocean CO2 uptake, for which there remains a wide range of future estimates. We find that the postulated recent reduction in the efficiency of Southern Ocean anthropogenic CO2 uptake, as a result of increased zonal winds, is too small to be detectable as significant differences in atmospheric CO2 between mid to high latitude Southern Hemisphere observing stations.

Analysis of seasonality and annual mean distribution of atmospheric potential oxygen (APO) in the Pacific region

We present a data set of atmospheric potential oxygen (APO = O2 + 1.1 × CO2) based on the atmospheric O2/N2 and CO2measurements of flask samples collected at two monitoring stations in Japan and on commercial cargo ships sailing between Japan and U.S./Canada and Australia/New Zealand. Since APO is invariant with respect to the terrestrial biotic exchange, its variation mainly reflects the spatiotemporal distribution in the air‐to‐sea gas exchange. From the observed APO for the years 2002–2008, we find: (1) elevated annual mean values near the equator, (2) elevated annual mean values and large seasonal amplitudes in the northwestern North Pacific, and (3) a deep trough of low annual mean values at latitudes 20–40°N in the Western Pacific. In addition, latitudinal distributions in the timing of the observed seasonal maximum and minimum show asymmetric patterns across the equator. Comparing these observations with a series of simulated APO generated in the NIES99 atmospheric transport model driven by a set of climatological oceanic O2 and CO2 flux fields, we find a good agreement except for the observed deep trough at the midlatitude. Simulations with different transport mechanisms and fluxes reveal that the seasonal covariation between oceanic O2 flux and atmospheric transport contribute significantly to the observed APO variations in the northern North Pacific; also the seasonal variation in the meridional transport affects the latitudinal difference in the seasonal cycle. The observed latitudinal gradient of the annual mean APO in the Southern Hemisphere is better reproduced by the model based on the recently revised ocean CO2 flux distribution than that based on the previous CO2 flux distribution. The observed APO trough at 36°N in the Western Pacific is about 10 per meg lower than the simulation with the more recent pCO2 data, suggesting the existence of additional APO sinks in that latitudinal region. Indeed, a model simulation performed with an additional ocean O2 sink flux of about 30 Tmol yr−1 within the region (30–50°N, 120–180°E) reproduces considerably well the observed APO trough.

Spatial distribution of pCO2, ΔO2/Ar and dimethylsulfide (DMS) in polynya waters and the sea ice zone of the Amundsen Sea, Antarctica

We report the first simultaneous measurements of surface water pCO2, biological oxygen saturation (ΔO2/Ar) and dimethylsulfide (DMS) concentrations in polynya waters and the sea ice zone of the Amundsen Sea, Antarctica. Across our survey region, we observed large spatial variability in surface water gas concentrations, tied to strong gradients in hydrography and phytoplankton biomass. Variability in sea surface temperature and salinity was attributed to the interacting effects of surface ocean circulation and heat fluxes, sea ice melt and the upwelling of relatively warm, saline and nutrient-rich modified circumpolar deep water. Phytoplankton biomass ranged from <1μgL−1 to ∼40μgL−1 chlorophyll a (Chl a), with the highest values observed in regions of shallow mixed layer depths. Phytoplankton assemblages were dominated by the colonial haptophyte Phaeocystis antarctica at most sampling stations, with lesser abundances of diatoms found throughout the polynya and sea ice zone. Cryptophytes were abundant at a few stations along the continental shelf break. Across the open polynya waters and sea ice zone, ΔO2/Ar ranged from ∼−40% to 40% (mean 8.6%), pCO2 ranged from 80 to 530μatm (mean 250μatm) and DMS concentrations varied from <1nM to ∼350nM. Strong gradients in gas concentrations were observed over short (i.e. <10km) spatial scales. The distribution of pCO2 and ΔO2/Ar across our survey transects reflected the balance between deep water entrainment and net community production, with a significant imprint of air–sea exchange. Chl a concentrations were significantly correlated to both pCO2 and ΔO2/Ar, and the slope of the ΔO2/Ar vs. pCO2 relationship was consistent with photosynthetic stoichiometry (∼1.25molO2 evolved per mol DIC removed). DMS and Chl a concentrations were also correlated in surface waters, but the spatial distribution of DMS was often uncoupled from pCO2 and ΔO2/Ar, likely due to complex microbial cycling processes. Sea surface temperature and salinity were related to surface gas concentrations through their effects on mixed layer depth and as tracers of upwelling. During the time of our cruise, the Amundsen Sea acted as an overall net sink for CO2 (mean sea–air flux=−15.9mmolm−2d−1) and a DMS source (mean sea–air flux=23.1μmolm−2d−1). Sea–air CO2 and DMS fluxes were more than 2-fold higher in open polynya waters relative to the overall cruise means. Simple calculations suggest that the Amundsen Sea contributes ∼5% of total Southern Ocean CO2 fluxes, and ∼1% of DMS fluxes.

Pacific climate variability and the possible impact on global surface CO2 flux.

BackgroundClimate variability modifies both oceanic and terrestrial surface CO2 flux. Using observed/assimilated data sets, earlier studies have shown that tropical oceanic climate variability has strong impacts on the land surface temperature and soil moisture, and that there is a negative correlation between the oceanic and terrestrial CO2 fluxes. However, these data sets only cover less than the most recent 20 years and are insufficient for identifying decadal and longer periodic variabilities. To investigate possible impacts of interannual to interdecadal climate variability on CO2 flux exchange, the last 125 years of an earth system model (ESM) control run are examined.ResultsGlobal integration of the terrestrial CO2 flux anomaly shows variation much greater in amplitude and longer in periodic timescale than the oceanic flux. The terrestrial CO2 flux anomaly correlates negatively with the oceanic flux in some periods, but positively in others, as the periodic timescale is different between the two variables. To determine the spatial pattern of the variability, a series of composite analyses are performed. The results show that the oceanic CO2 flux variability peaks when the eastern tropical Pacific has a large sea surface temperature anomaly (SSTA). By contrast, the terrestrial CO2 flux variability peaks when the SSTA appears in the central tropical Pacific. The former pattern of variability resembles the ENSO-mode and the latter the ENSO-modoki1.ConclusionsOur results imply that the oceanic and terrestrial CO2 flux anomalies may correlate either positively or negatively depending on the relative phase of these two modes in the tropical Pacific.

The relationship of the phase and amplitude of the annual cycle of CO2 to phenological events

Background: A key current issue in studies of global CO2 is the cause of the recent pronounced changes in the timing (phase) and amplitude of its annual cycle. Aim: To use a sensitivity analysis in order to identify the most influential parameters in a new four-box diffusion model of global CO2 transport. Methods: A new diffusion model of global atmospheric transport is developed, implemented by optimisation and used to assess the effects of oceanic, terrestrial and anthropogenic CO2 fluxes on the annual CO2 cycle. As the terrestrial phenology of living organisms in the temperate zones is identified as the central process of concern an improved phenological model for the temperate zones is proposed and used in up-scaling from plant to planet. Results: The diffusion model is found to be able to mimic many of the features observed by the GLOBALVIEW-CO2 network, including the global trend, latitudinal gradient, and annual waveform. Conclusions: It is concluded that, out of five rival hypotheses considered at the outset, the springtime phenology of terrestrial biomes is the dominant controller of recent changes to the annual CO2 cycle.

Evaluating a 3-D transport model of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; using ground-based, aircraft, and space-borne data

Abstract. We evaluate the GEOS-Chem atmospheric transport model (v8-02-01) of CO2 over 2003–2006, driven by GEOS-4 and GEOS-5 meteorology from the NASA Goddard Global Modeling and Assimilation Office, using surface, aircraft and space-borne concentration measurements of CO2. We use an established ensemble Kalman Filter to estimate a posteriori biospheric+biomass burning (BS + BB) and oceanic (OC) CO2 fluxes from 22 geographical regions, following the TransCom-3 protocol, using boundary layer CO2 data from a subset of GLOBALVIEW surface sites. Global annual net BS + BB + OC CO2 fluxes over 2004–2006 for GEOS-4 (GEOS-5) meteorology are −4.4 ± 0.9 (−4.2 ± 0.9), −3.9 ± 0.9 (−4.5 ± 0.9), and −5.2 ± 0.9 (−4.9 ± 0.9) PgC yr−1, respectively. After taking into account anthropogenic fossil fuel and bio-fuel emissions, the global annual net CO2 emissions for 2004–2006 are estimated to be 4.0 ± 0.9 (4.2 ± 0.9), 4.8 ± 0.9 (4.2 ± 0.9), and 3.8 ± 0.9 (4.1 ± 0.9) PgC yr−1, respectively. The estimated 3-yr total net emission for GEOS-4 (GEOS-5) meteorology is equal to 12.5 (12.4) PgC, agreeing with other recent top-down estimates (12–13 PgC). The regional a posteriori fluxes are broadly consistent in the sign and magnitude of the TransCom-3 study for 1992–1996, but we find larger net sinks over northern and southern continents. We find large departures from our a priori over Europe during summer 2003, over temperate Eurasia during 2004, and over North America during 2005, reflecting an incomplete description of terrestrial carbon dynamics. We find GEOS-4 (GEOS-5) a posteriori CO2 concentrations reproduce the observed surface trend of 1.91–2.43 ppm yr−1 (parts per million per year), depending on latitude, within 0.15 ppm yr−1 (0.2 ppm yr−1) and the seasonal cycle within 0.2 ppm (0.2 ppm) at all latitudes. We find the a posteriori model reproduces the aircraft vertical profile measurements of CO2 over North America and Siberia generally within 1.5 ppm in the free and upper troposphere but can be biased by up to 4–5 ppm in the boundary layer at the start and end of the growing season. The model has a small negative bias in the free troposphere CO2 trend (1.95–2.19 ppm yr−1) compared to AIRS data which has a trend of 2.21–2.63 ppm yr−1 during 2004–2006, consistent with surface data. Model CO2 concentrations in the upper troposphere, evaluated using CONTRAIL (Comprehensive Observation Network for TRace gases by AIrLiner) aircraft measurements, reproduce the magnitude and phase of the seasonal cycle of CO2 in both hemispheres. We generally find that the GEOS meteorology reproduces much of the observed tropospheric CO2 variability, suggesting that these meteorological fields will help make significant progress in understanding carbon fluxes as more data become available.

Open ocean gas transfer velocity derived from long‐term direct measurements of the CO2 flux

Air‐sea open ocean CO2 flux measurements have been made using the Eddy Covariance (EC) technique onboard the weathership Polarfront in the North Atlantic between September 2006 and December 2009. Flux measurements were made using an autonomous system ‘AutoFlux’. CO2 mass density was measured with an open‐path infrared gas analyzer. Following quality control procedures, 3938 20‐minute flux measurements were made at mean wind speeds up to 19.6 m/s, significantly higher wind speeds than previously published results. The uncertainty in the determination of gas transfer velocities is large, but the mean relationship to wind speed allows a new parameterisation of the gas transfer velocity to be determined. A cubic dependence of gas transfer on wind speed is found, suggesting a significant influence of bubble‐mediated exchange on gas transfer.

Comparative performance of CO2 measuring methods: Marine aquaculture recirculation system application

Many methods are available for the measurement of dissolved carbon dioxide in an aqueous environment. Standard titration is the typical field method for measuring dissolved CO2 in aquaculture systems. However, titrimetric determination of dissolved CO2 in marine water aquaculture systems is unsuitable because of the high dissolved solids, silicates, and other dissolved minerals that interfere with the determination. Other methods used to measure dissolved carbon dioxide in an aquaculture water included use of a wetted CO2 probe analyzer, standard nomographic methods, and calculation by direct measurements of the water's pH, temperature, and alkalinity. The determination of dissolved CO2 in saltwater based on partial pressure measurements and non-dispersive infra-red (NDIR) techniques with a CO2 gas analyzer are widely employed for oceanic surveys of surface ocean CO2 flux and are similar to the techniques employed with the head space unit (HSU) in this study. Dissolved carbon dioxide (DC) determination with the HSU using a infra-red gas analyzer (IRGA) was compared with titrimetric, nomographic, calculated, and probe measurements of CO2 in freshwater and in saltwater with a salinity ranging from 5.0 to 30ppt, and a CO2 range from 8 to 50mg/L. Differences in CO2 measurements between duplicate HSUs (0.1–0.2mg/L) were not statistically significant different. The coefficient of variation for the HSU readings averaged 1.85% which was better than the CO2 probe (4.09%) and that for the titrimetric method (5.84%). In all low, medium and high salinity level trials HSU precision was good, averaging 3.39%. Differences existed between comparison testing of the CO2 probe and HSU measurements with the CO2 probe readings, on average, providing DC estimates that were higher than HSU estimates. Differences between HSU and titration based estimates of DC increased with salinity and reached a maximum at 32.2ppt. These differences were statistically significant (P<0.05) at all salinity levels greater than 0.3ppt. Results indicated reliable replicated results from the head space unit with varying salinity and dissolved carbon dioxide concentrations.

The global carbon cycle in the Canadian Earth system model (CanESM1): Preindustrial control simulation

The preindustrial carbon cycle is described for the Canadian Centre for Climate Modelling and Analysis Earth system model (CanESM1). The interhemispheric gradient of surface atmospheric CO2 concentration (xCO2) is reversed from the present day, with higher concentrations in the Southern Hemisphere, and southward interhemispheric transport by the ocean, estimated at 0.38 Pg C yr−1. The seasonal cycles of xCO2 and surface CO2 exchange are dominated by Northern Hemisphere terrestrial processes; the ocean contribution to CO2 flux is in phase with the larger terrestrial flux in the tropics and out of phase in the extratropics. Ocean processes dominate the relatively small Southern Hemisphere variability. Interannual variability of land carbon exchange is much larger than ocean exchange; both are comparable to results from previously published models with possibly larger variability in the terrestrial flux. Terrestrial net primary production (NPP) is determined largely by water availability at low latitudes, with temperature becoming more important at high latitudes. Temperature and moisture affect both NPP and heterotrophic respiration such that respiration effects tend to dampen the effect of fluctuations in NPP on CO2 exchange. Ocean CO2 flux variability is controlled by a variety of physical and biological processes with greater control by physical processes in the tropics and a larger biological contribution in the extratropics. Ocean CO2 flux is more strongly correlated with tropical sea surface temperature (SST) than terrestrial, but the variance associated with tropical SST is larger on land, in absolute terms, because of the much greater total variance of the land carbon flux. A novel hypothesis is advanced to explain how biological drawdown can cause recently upwelled water to be a net sink rather than source for atmospheric CO2. This process occurs over large areas of extratropical ocean and forms a natural sink for atmospheric CO2 that is potentially sensitive to both ocean acidification and anthropogenic perturbations of the aeolian iron flux.

Simulation and assimilation of global ocean pCO2 and air–sea CO2 fluxes using ship observations of surface ocean pCO2 in a simplified biogeochemical offline model

We used an offline tracer transport model, driven by reanalysis ocean currents and coupled to a simple biogeochemical model, to synthesize the surface ocean pCO2 and air–sea CO2 flux of the global ocean from 1996 to 2004, using a variational assimilation method. This oceanic CO2 flux analysis system was developed at the National Institute for Environmental Studies (NIES), Japan, as part of a project that provides prior fluxes for atmospheric inversions using CO2 measurements made from an on-board instrument attached to the Greenhouse gas Observing SATellite (GOSAT). Nearly 250 000 pCO2 observations from the database of Takahashi et al. (2007) have been assimilated into the model with a strong constraint provide by ship-track observations while maintaining a weak constraint of 20% on global averages of monthly mean pCO2 in regions where observations are limited. The synthesized global air–sea CO2 flux shows a net sink of 1.48 PgC yr-1. The Southern Ocean air–sea CO2 flux is a sink of 0.41 PgC yr-1. The interannual variability of synthesized CO2 flux from the El Niño region suggests a weaker source (by an amplitude of 0.4 PgC yr-1) during the El Niño events in 1997/1998 and 2003/2004. The assimilated air–sea CO2 flux shows remarkable correlations with the CO2 fluxes obtained from atmospheric inversions on interannual time-scales.

Can bottom-up ocean CO2 fluxes be reconciled with atmospheric 13C observations?

The rare stable carbon isotope, 13C, has been used previously to partition CO2 fluxes into land and ocean components. Net ocean and land fluxes impose distinctive and predictable fractionation patterns upon the stable isotope ratio, making it an excellent tool for distinguishing between them. Historically, isotope constrained inverse methods for calculating CO2 surface fluxes—the ‘double deconvolution’—have disagreed with bottom-up ocean flux estimates. In this study, we use the double deconvolution framework, but add, as a constraint, independent estimates of time histories of ocean fluxes to the atmospheric observations of CO2 and 13CO2. We calculate timeseries of net land flux, total disequilibrium flux and terrestrial disequilibrium flux from 1991 to 2008 that are consistent with bottom-up net ocean fluxes.We investigate possible drivers of interannual variability in terrestrial disequilibrium flux, including terrestrial discrimination, and test the sensitivity of our results to those mechanisms. We find that C3 plant discrimination and shifts in the global composition of C3 and C4 vegetation are likely drivers of interannual variability in terrestrial disequilibrium flux, while contributions from heterotrophic respiration and disturbance anomalies are also possible.

Estimating surface CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes from space-borne CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; dry air mole fraction observations using an ensemble Kalman Filter

Abstract. We have developed an ensemble Kalman Filter (EnKF) to estimate 8-day regional surface fluxes of CO2 from space-borne CO2 dry-air mole fraction observations (XCO2) and evaluate the approach using a series of synthetic experiments, in preparation for data from the NASA Orbiting Carbon Observatory (OCO). The 32-day duty cycle of OCO alternates every 16 days between nadir and glint measurements of backscattered solar radiation at short-wave infrared wavelengths. The EnKF uses an ensemble of states to represent the error covariances to estimate 8-day CO2 surface fluxes over 144 geographical regions. We use a 12×8-day lag window, recognising that XCO2 measurements include surface flux information from prior time windows. The observation operator that relates surface CO2 fluxes to atmospheric distributions of XCO2 includes: a) the GEOS-Chem transport model that relates surface fluxes to global 3-D distributions of CO2 concentrations, which are sampled at the time and location of OCO measurements that are cloud-free and have aerosol optical depths &lt;0.3; and b) scene-dependent averaging kernels that relate the CO2 profiles to XCO2, accounting for differences between nadir and glint measurements, and the associated scene-dependent observation errors. We show that OCO XCO2 measurements significantly reduce the uncertainties of surface CO2 flux estimates. Glint measurements are generally better at constraining ocean CO2 flux estimates. Nadir XCO2 measurements over the terrestrial tropics are sparse throughout the year because of either clouds or smoke. Glint measurements provide the most effective constraint for estimating tropical terrestrial CO2 fluxes by accurately sampling fresh continental outflow over neighbouring oceans. We also present results from sensitivity experiments that investigate how flux estimates change with 1) bias and unbiased errors, 2) alternative duty cycles, 3) measurement density and correlations, 4) the spatial resolution of estimated flux estimates, and 5) reducing the length of the lag window and the size of the ensemble. At the revision stage of this manuscript, the OCO instrument failed to reach its orbit after it was launched on 24 February 2009. The EnKF formulation presented here is also applicable to GOSAT measurements of CO2 and CH4.

A Bayesian calibration of a simple carbon cycle model: The role of observations in estimating and reducing uncertainty

The strengths of future carbon dioxide (CO2) sinks are highly uncertain. A sound methodology to characterize current and predictive uncertainties in carbon cycle models is crucial for the design of efficient carbon management strategies. We demonstrate such a methodology, Markov Chain Monte Carlo (MCMC), by performing a Bayesian calibration of a simple global‐scale carbon cycle model with historical carbon cycle observations to (1) estimate probability density functions (PDFs) of key carbon cycle parameters, (2) derive statistically sound probabilistic predictions of future CO2 sinks, and (3) assess the utility of hypothetical observation systems to reduce prediction uncertainties. We find that the PDFs of model parameter estimates are not normally distributed, and the residuals show statistically significant temporal autocorrelation. The assumption of normally distributed PDFs likely causes biased results, and the neglect of autocorrelation in the residual of the annual CO2 time series causes overconfidence in parameter estimates and predictions. Using interannually varying global temperature observations as forcing provides important information: terrestrial parameter PDFs are shifted and are more sharply constrained when compared to PDFs estimated when forcing the carbon cycle with a simple energy‐balance model. Although CO2 observations provide a strong constraint on the total carbon sink, adding independent observations of terrestrial and oceanic fluxes has the potential to reduce uncertainty in predictions of this total sink more rapidly. Assimilating hypothetical annual observations of terrestrial and oceanic CO2 fluxes with realistic uncertainties reduces predictive uncertainties about CO2 sinks in the year 2050 by as much as a factor of 2 compared to assimilating CO2 concentrations alone.

An empirical estimate of the Southern Ocean air‐sea CO2 flux

Despite improvements in our understanding of the Southern Ocean air‐sea flux of CO2, discrepancies still exist between a variety of differing ocean/atmosphere methodologies. Here we employ an independent method to estimate the Southern Ocean air‐sea flux of CO2 that exploits all available surface ocean measurements for dissolved inorganic carbon (DIC) and total alkalinity (ALK) beyond 1986. The DIC concentrations were normalized to the year 1995 using coinciding CFC measurements in order to account for the anthropogenic CO2 signal. We show that independent of season, surface‐normalized DIC and ALK can be empirically predicted to within ∼8 μmol/kg using standard hydrographic properties. The predictive equations were used in conjunction with World Ocean Atlas (2001) climatologies to give a first estimate of the annual cycle of DIC and ALK in the surface Southern Ocean. These seasonal distributions will be very useful in both validating biogeochemistry in general circulation models and for use in situ biological studies within the Southern Ocean. Using optimal CO2 dissociation constants, we then estimate an annual cycle of pCO2 and associated net air‐sea CO2 flux. Including the effects of sea ice, we estimate a Southern Ocean (&gt;50°S) CO2 sink of 0.4 ± 0.25 Pg C/yr. Our analysis also indicates a substantial CO2 sink of 1.1 ± 0.6 Pg C/yr within the sub‐Antarctic zone (40°S–50°S), associated with strong cooling and high winds. Our results imply the Southern Ocean CO2 flux south of 50°S to be very similar to those found by Takahashi et al. (2002), but on the higher end of a range of atmospheric/oceanic CO2 inversion methodologies. This paper estimates for the first time basic seasonal carbon cycle parameters within the circumpolar Southern Ocean, which have up to now been extremely difficult to measure and sparse. The application of such an empirical technique using more widely available hydrographic parameters in the Southern Ocean provides an important independent estimate to not only CO2 uptake, but also for other future biogeochemical studies. Refining and testing these empirical methods with new carbon measurements will be important to further reduce uncertainties and extend our understanding of Southern Ocean CO2 dynamics.

Natural Variability in a Stable, 1000-Yr Global Coupled Climate–Carbon Cycle Simulation

Abstract A new 3D global coupled carbon–climate model is presented in the framework of the Community Climate System Model (CSM-1.4). The biogeochemical module includes explicit land water–carbon coupling, dynamic carbon allocation to leaf, root, and wood, prognostic leaf phenology, multiple soil and detrital carbon pools, oceanic iron limitation, a full ocean iron cycle, and 3D atmospheric CO2 transport. A sequential spinup strategy is utilized to minimize the coupling shock and drifts in land and ocean carbon inventories. A stable, 1000-yr control simulation [global annual mean surface temperature ±0.10 K and atmospheric CO2 ± 1.2 ppm (1σ)] is presented with no flux adjustment in either physics or biogeochemistry. The control simulation compares reasonably well against observations for key annual mean and seasonal carbon cycle metrics; regional biases in coupled model physics, however, propagate clearly into biogeochemical error patterns. Simulated interannual-to-centennial variability in atmospheric CO2 is dominated by terrestrial carbon flux variability, ±0.69 Pg C yr−1 (1σ global net annual mean), which in turn reflects primarily regional changes in net primary production modulated by moisture stress. Power spectra of global CO2 fluxes are white on time scales beyond a few years, and thus most of the variance is concentrated at high frequencies (time scale &amp;lt;4 yr). Model variability in air–sea CO2 fluxes, ±0.10 Pg C yr−1 (1σ global annual mean), is generated by variability in sea surface temperature, wind speed, export production, and mixing/upwelling. At low frequencies (time scale &amp;gt;20 yr), global net ocean CO2 flux is strongly anticorrelated (0.7–0.95) with the net CO2 flux from land; the ocean tends to damp (20%–25%) slow variations in atmospheric CO2 generated by the terrestrial biosphere. The intrinsic, unforced natural variability in land and ocean carbon storage is the “noise” that complicates the detection and mechanistic attribution of contemporary anthropogenic carbon sinks.

Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models

Measurements of atmospheric O2/N2 ratios and CO2 concentrations can be combined into a tracer known as atmospheric potential oxygen (APO ≈ O2/N2 + CO2) that is conservative with respect to terrestrial biological activity. Consequently, APO reflects primarily ocean biogeochemistry and atmospheric circulation. Building on the work of Stephens et al. (1998), we present a set of APO observations for the years 1996–2003 with unprecedented spatial coverage. Combining data from the Princeton and Scripps air sampling programs, the data set includes new observations collected from ships in the low‐latitude Pacific. The data show a smaller interhemispheric APO gradient than was observed in past studies, and different structure within the hemispheres. These differences appear to be due primarily to real changes in the APO field over time. The data also show a significant maximum in APO near the equator. Following the approach of Gruber et al. (2001), we compare these observations with predictions of APO generated from ocean O2 and CO2 flux fields and forward models of atmospheric transport. Our model predictions differ from those of earlier modeling studies, reflecting primarily the choice of atmospheric transport model (TM3 in this study). The model predictions show generally good agreement with the observations, matching the size of the interhemispheric gradient, the approximate amplitude and extent of the equatorial maximum, and the amplitude and phasing of the seasonal APO cycle at most stations. Room for improvement remains. The agreement in the interhemispheric gradient appears to be coincidental; over the last decade, the true APO gradient has evolved to a value that is consistent with our time‐independent model. In addition, the equatorial maximum is somewhat more pronounced in the data than the model. This may be due to overly vigorous model transport, or insufficient spatial resolution in the air‐sea fluxes used in our modeling effort. Finally, the seasonal cycles predicted by the model of atmospheric transport show evidence of an excessive seasonal rectifier in the Aleutian Islands and smaller problems elsewhere.

Interannual and decadal changes in the sea‐air CO2 flux from atmospheric CO2 inverse modeling

The atmosphere‐land‐ocean fluxes of CO2 were derived for 64 partitioned areas of the globe (22 over the ocean and 42 over the land) using a time‐dependent inverse (TDI) model for the period of January 1988 to December 2001. The model calculation partially follow the TransCom‐3 protocol, and is constrained by atmospheric CO2 concentration data from 87 stations and fully time‐dependent atmospheric transport model simulations. The air‐to‐land and air‐to‐sea fluxes averaged over the 1990s are estimated at 1.15 ± 0.74 and 1.88 ± 0.53 Pg‐C yr−1, respectively. These estimates, however, remain uncertain owing to sampling biases arising from the sparse distribution of atmospheric CO2 data, are compared with other estimates by various methods. The sensitivity analysis indicates that the differences in fluxes and flux variability caused by the choices of initial conditions for the TDI model are smaller compared to those due to the selection of measurement networks. Our model results capture interannual variations in global and regional CO2 fluxes realistically. The estimated oceanic CO2 flux anomalies appear to be closely related to prominent climate modes such as El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific Decadal Oscillation (PDO). The results from the correlation analyses show that the oceanic CO2 flux in the tropics is strongly influenced by the ENSO dynamical cycle, and that in the sub‐polar regions by upwelling of sub‐surface waters in the winter and plankton blooms in the spring.