Bibliography - M. N. Gloor

1. Crevoisier, C., C. Sweeney, M. N. Gloor, Jorge Sarmiento, and P. P. Tans, October 2010: Regional US carbon sinks from three-dimensional atmospheric CO₂ sampling. Proceedings of the National Academy of Sciences of the United States of America, 107(43), doi:10.1073/pnas.0900062107 18348-18353
   [ Abstract ]

   Studies diverge substantially on the actual magnitude of the North American carbon budget. This is due to the lack of appropriate data and also stems from the difficulty to properly model all the details of the flux distribution and transport inside the region of interest. To sidestep these difficulties, we use here a simple budgeting approach to estimate land-atmosphere fluxes across North America by balancing the inflow and outflow of CO₂ from the troposphere. We base our study on the unique sampling strategy of atmospheric CO₂ vertical profiles over North America from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory aircraft network, from which we infer the three-dimensional CO₂ distribution over the continent. We find a moderate sink of 0.5 ± 0.4 PgC y^-1 for the period 2004–2006 for the coterminous United States, in good agreement with the forest-inventory-based estimate of the first North American State of the Carbon Cycle Report, and averaged climate conditions. We find that the highest uptake occurs in the Midwest and in the Southeast. This partitioning agrees with independent estimates of crop uptake in the Midwest, which proves to be a significant part of the US atmospheric sink, and of secondary forest regrowth in the Southeast. Provided that vertical profile measurements are continued, our study offers an independent means to link regional carbon uptake to climate drivers.

2. Gloor, M. N., Jorge Sarmiento, and N. Gruber, 2010: What can be learned about carbon cycle climate feedbacks from the CO₂ airborne fraction? Atomsphere Chemistry and Physics Discussion, www.atmos-chem-phys.net/10/7739/2010/, (10), doi:10.5194/acp-10-7739-2010 7739–7751
   [ Abstract ]

   The ratio of CO₂ accumulating in the atmosphere to the CO₂ flux into the atmosphere due to human activity, the airborne fraction AF, is central to predict changes in earth’s surface temperature due to greenhouse gas induced warming. This ratio has remained remarkably constant in the past five decades, but recent studies have reported an apparent increasing trend and interpreted it as an indication for a decrease in the efficiency of the combined sinks by the ocean and terrestrial biosphere. We investigate here whether this interpretation is correct by analyzing the processes that control long-term trends and decadal-scale variations in the AF. To this end, we use simplified linear models for describing the time evolution of an atmospheric CO₂ perturbation. We find firstly that the spin-up time of the system for the AF to converge to a constant value is on the order of 200–300 years and differs depending on whether exponentially increasing fossil fuel emissions only or the sum of fossil fuel and land use emissions are used. We find secondly that the primary control on the decadal time-scale variations of the AF is variations in the relative growth rate of the total anthropogenic CO₂ emissions. Changes in sink efficiencies tend to leave a smaller imprint. Therefore, before interpreting trends in the AF as an indication of weakening carbon sink efficiency, it is necessary to account for trends and variations in AF stemming from anthropogenic emissions and other extrinsic forcing events, such as volcanic eruptions. Using atmospheric CO₂ data and emission estimates for the period 1959 through 2006, and our simple predictive models for the AF, we find that likely omissions in the reported emissions from land use change and extrinsic forcing events are sufficient to explain the observed long-term trend in AF. Therefore, claims for a decreasing long-term trend in the carbon sink efficiency over the last few decades are currently not supported by atmospheric CO₂ data and anthropogenic emissions estimates.

3. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. E. Mikaloff-Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2010: Trends and regional distributions of land and ocean carbon sinks. Biogeosciences, www.biogeosciences.net/7/2351/2010/, 7, doi:10.5194/bg-7-2351-2010 2351-2367
   [ Abstract ]

   We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 PgC yr⁻¹ between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) PgC yr⁻¹ to a new relatively constant mean of −1.15 PgC yr⁻¹ between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO₂, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

4. Gruber, N., M. N. Gloor, S. E. Mikaloff-Fletcher, S. C. Doney, S. Dutkiewicz, M. Follows, M. Gerber, A. R. Jacobson, F. Joos, K. Lindsay, , and , et al., 2009: Oceanic Sources, Sinks, and Transport of Atmospheric CO₂. Global Biogeochemical Cycles, 23(GB 1005), doi:10.1029/2008GB003349
   [ Abstract ]

   We synthesize estimates of the contemporary net air-sea CO₂ flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO₂ (pCO₂) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO₂ for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a^-1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in the high latitudes. Both estimates point toward a small (˜ -0.3 Pg C a^-1) contemporary CO₂ sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO₂ and a strong uptake of anthropogenic CO₂. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO₂-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO₂ is estimated to be -1.7 ± 0.4 Pg C a^-1 (inversion) and -1.4 ± 0.7 Pg C a^-1 (pCO₂-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ˜+0.5 Pg C a^-1, and an uptake flux of anthropogenic carbon of -2.2 ± 0.3 Pg C a^-1 (inversion) and -1.9 ± 0.7 Pg C a^-1 ((pCO₂-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between -0.2 and -0.3 Pg C a^-1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO₂ fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

5. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. M. Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2009: Trends and Regional Distributions of Land and Ocean Carbon Sink. Biogeosciences, http://www.biogeosciences-discuss.net/6/10583/2009/bgd-6-10583-2009.html, (6), 10583-10624
   [ Abstract ]

   We show here a new estimate of the variability and long-term trends in the net land carbon sink from 1960 onwards calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by 5 recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. The net land carbon sink appears to have increased by −0.88 (−0.77 to −1.04) PgCyr−1 after 1988/1989 from a relatively constant mean of −0.27 PgCyr−1 before then to −1.15 PgCyr−1 thereafter (the sign convention is negative out of the atmosphere). This result is significant at the 1% critical level. The increase in net land 10 uptake is partially compensated by a reduction in the expected oceanic uptake, which we estimate from model simulations as about 0.35 (0.26 to 0.49) PgCyr−1. This implies that the atmospheric growth rate must have decreased by about −0.53 (−0.51 to −0.55) PgCyr−1 (equivalent to −0.25 ppm yr−1) below what would have been projected if the ocean uptake had continued to grow at the rate expected from a constant 15 climate model and if the net land uptake had continued at its pre-1988/1989 level. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the northern hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

6. Gloor, M. N., N. Gruber, Jorge Sarmiento, C. L. Sabine, R. A. Feely, and C. Rödenbeck, 2008: A first estimate of present and pre-industrial CO₂ flux patterns based on ocean interior carbon measurements and models. Geophysical Research Letters, 30(1), doi:10.1029/2002GL015594
   [ Abstract ]

   The exchange of CO₂ across the air-sea interface is a main determinant of the distribution of atmospheric CO₂ from which major conclusions about the carbon cycle are drawn, yet our knowledge of atmosphere-ocean fluxes still has major gaps. A new analysis based on recent ocean dissolved inorganic carbon data and on models permits us to separately estimate the preindustrial and present air-sea CO₂ flux distributions without requiring knowledge of the gas exchange coefficient. We find a smaller carbon sink at mid to high latitudes of the southern hemisphere than previous data based estimates and a shift of ocean uptake to lower latitude regions compared to estimates and simulations. The total uptake of anthropogenic CO₂ for 1990 is 1.8 (±0.4) Pg C yr^-1. Our ocean based results support the interpretation of the latitudinal distribution of atmospheric CO₂ data as evidence for a large northern hemisphere land carbon sink.

7. Crevoisier, C., E. Shevliakova, M. N. Gloor, C. Wirth, and Stephen W. Pacala, 2007: Drivers of fires in the boreal forests: data constrained design of a prognostic model for burned area for use in dynamic global vegetation models. Journal of Geophysical Research, 112(D24112), doi:10.1029/2006JD008372
   [ Abstract ]

   Boreal regions are an important component of the global carbon cycle because they host large stocks of aboveground and belowground carbon. Since boreal forest evolution is closely related to fire regimes, shifts in climate are likely to induce changes in ecosystems, potentially leading to a large release of carbon and other trace gases to the atmosphere. Prediction of the effect of this potential climate feedback on the Earth system is therefore important and requires the modeling of fire as a climate driven process in dynamic global vegetation models (DGVMs). Here, we develop a new data-based prognostic model, for use in DGVMs, to estimate monthly burned area from four climate (precipitation, temperature, soil water content and relative humidity) and one humanrelated (road density) predictors for boreal forest. The burned area model is a function of current climatic conditions and is thus responsive to climate change. Model parameters are estimated using a Markov Chain Monte Carlo method applied to on ground observations from the Canadian Large Fire Database. The model is validated against independent observations from three boreal regions: Canada, Alaska and Siberia. Provided realistic climate predictors, the model is able to reproduce the seasonality, intensity and interannual variability of burned area, as well as the location of fire events. In particular, the model simulates well the timing of burning events, with two thirds of the events predicted for the correct month and almost all the rest being predicted 1 month before or after the observed event. The predicted annual burned area is in the range of various current estimates. The estimated annual relative error (standard deviation) is twelve percent in a grid cell, which makes the model suitable to study quantitatively the evolution of burned area with climate.

8. Gloor, M. N., E. Dlugokensky, C. Brenninkmeijer, L. W. Horowitz, D. F. Hurst, G. Dutton, C. Crevoisier, T. Machida, and P. P. Tans, 2007: Three-dimensional SF₆ data and tropospheric transport simulations: Signals, modeling accuracy, and implications for inverse modeling. Journal of Geophysical Research, 112(D15112), doi:10.1029/2006JD007973
   [ Abstract ]

   Surface emissions of SF₆ are closely tied to human activity and thus fairly well known. They therefore can and have been used to evaluate tropospheric transport predicted by models. A range of new atmospheric SF₆ data permit us to expand on earlier studies. The purpose of this first of two papers is to characterize known and new transport constraints provided by the data and to use them to quantify predictive skill of the MOZART-2 atmospheric chemistry and transport model. Main noteworthy observational constraints are (1) a well-known steep N-S gradient at the surface confined to an ≈40° wide latitude band in the tropics; (2) a fairly uniform N-S gradient in the upper troposphere; (3) an increase in the temporal variation in upper troposphere Northern Hemisphere records with increasing latitude; (4) a negative SF₆ gradient in Northern Hemisphere vertical profiles from the surface to 8 km height, but a positive gradient in the Southern Hemisphere; and (5) a clear reflection in surface records of large-scale seasonal atmosphere movements like the undulations of the Intertropical Convergence Zone (ITCZ). Comparison of observations with simulations reveal excellent modeling skills with regards to (1) large-scale annual mean latitudinal gradients at remote surface sites (relative bias of N-S hemisphere difference ≤ 5%) and aloft (≈10 km, relative bias ≤ 25%); (2) seasonality in signals at remote sites caused by large-scale movements of the atmosphere; (3) time variation in upper troposphere records; (4) ‘‘faithfulness’’ of advective transport on timescales up to ≈1 week; and (5) the general shapes and seasonal variation of vertical profiles. The model (1) underestimates the variation in the vertical of profiles, particularly those from locations close to high emissions regions, and (2) overestimates the difference in SF₆ between the planetary boundary layer (PBL) and free troposphere over North America, and thus likely Eurasia, during winter by approximately a factor of 2 (STD ≈ 100%). The comparisons permit estimating lower bounds on representation errors which are large for sites close to continental outflow regions. Given the magnitude of the signals and signal variance, SF₆ provides a strong constraint on interhemispheric transport, PBL ventilation, dispersion pathways of northern midlatitude surface emissions through the upper troposphere, and large-scale movements of the atmosphere.

9. Jacobson, A. R., S. E. Mikaloff-Fletcher, N. Gruber, Jorge Sarmiento, and M. N. Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: I. Methods and global-scale fluxes. Global Biogeochemical Cycles, 21(GB1019), doi:10.1029/2005GB002556
   [ Abstract ]

   We have constructed an inverse estimate of surface fluxes of carbon dioxide using both atmospheric and oceanic observational constraints. This global estimate is spatially resolved into 11 land regions and 11 ocean regions, and is calculated as a temporal mean for the period 1992–1996. The method interprets in situ observations of carbon dioxide concentration in the ocean and atmosphere with transport estimates from global circulation models. Uncertainty in the modeled circulation is explicitly considered in this inversion by using a suite of 16 atmospheric and 10 oceanic transport simulations. The inversion analysis, coupled with estimates of river carbon delivery, indicates that the open ocean had a net carbon uptake from the atmosphere during the period 1992–96 of 1.7 PgC yr^-1, consisting of an uptake of 2.1 PgC yr^-1 of anthropogenic carbon and a natural outgassing of about 0.5 PgC yr^-1 of carbon fixed on land and transported through rivers to the open ocean. The formal uncertainty on this oceanic uptake, despite a comprehensive effort to quantify sources of error due to modeling biases, uncertain riverine carbon load, and biogeochemical assumptions, is driven down to 0.2 PgC yr^-1 by the large number and relatively even spatial distribution of oceanic observations used. Other sources of error, for which quantifiable estimates are not currently available, such as unresolved transport and large region inversion bias, may increase this uncertainty.

10. Jacobson, A. R., S. E. Mikaloff-Fletcher, N. Gruber, Jorge Sarmiento, and M. N. Gloor, 2007: A joint atmosphere-ocean inversion for surface fluxes of carbon dioxide: II. Regional results. Global Biogeochemical Cycles, 21(GB1020), doi:10.1029/2006GB002703
    [ Abstract ]

    We report here the results from a coupled ocean-atmosphere inversion, in which atmospheric CO₂ gradients and transport simulations are combined with observations of ocean interior carbon concentrations and ocean transport simulations to provide a jointly constrained estimate of air-sea and air-land carbon fluxes. While atmospheric data have little impact on regional air-sea flux estimates, the inclusion of ocean data drives a substantial change in terrestrial flux estimates. Our results indicate that the tropical and southern land regions together are a large source of carbon, with a 77% probability that their aggregate source size exceeds 1 PgC yr^-1. This value is of similar magnitude to estimates of fluxes in the tropics due to land-use change alone, making the existence of a large tropical CO₂ fertilization sink unlikely. This terrestrial result is strongly driven by oceanic inversion results that differ from flux estimates based on ΔpCO₂ climatologies, including a relatively small Southern Ocean sink (south of 44°S) and a relatively large sink in the southern temperate latitudes (44°S–18°S). These conclusions are based on a formal error analysis of the results, which includes uncertainties due to observational error transport and other modeling errors, and biogeochemical assumptions. A suite of sensitivity tests shows that these results are generally robust, but they remain subject to potential sources of unquantified error stemming from the use of large inversion regions and transport biases common to the suite of available transport models.

11. Mikaloff-Fletcher, S. E., N. Gruber, A. R. Jacobson, M. N. Gloor, S. C. Doney, S. Dutkiewicz, M. Gerber, M. Follows, F. Joos, K. Lindsay, D. Menemenlis, and A. Mouchet, et al., 2007: Inverse estimates of the oceanic sources and sinks of natural CO₂ and the implied oceanic carbon transport. Global Biogeochemical Cycles, 21(GB1010), doi:10.1029/2006GB002751
    [ Abstract ]

    We use an inverse method to estimate the global-scale pattern of the air-sea flux of natural CO₂, i.e., the component of the CO₂ flux due to the natural carbon cycle that already existed in preindustrial times, on the basis of ocean interior observations of dissolved inorganic carbon (DIC) and other tracers, from which we estimate ΔC_gasex, i.e., the component of the observed (DIC that is due to the gas exchange of natural CO₂. We employ a suite of 10 different Ocean General Circulation Models (OGCMs) to quantify the error arising from uncertainties in the modeled transport required to link the interior ocean observations to the surface fluxes. The results from the contributing OGCMs are weighted using a model skill score based on a comparison of each model’s simulated natural radiocarbon with observations. We find a pattern of air-sea flux of natural CO₂ characterized by outgassing in the Southern Ocean between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres, and strong outgassing in the tropics. In the Northern Hemisphere and the tropics, the inverse estimates generally agree closely with the natural CO₂ flux results from forward simulations of coupled OGCM-biogeochemistry models undertaken as part of the second phase of the Ocean Carbon Model Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less air-sea exchange than the inversion south of 20°S, but more recent forward OGCM studies are in better agreement with the inverse estimates in the Southern Hemisphere. The strong source and sink pattern south of 20°S was not apparent in an earlier inversion study, because the choice of region boundaries led to a partial cancellation of the sources and sinks. We show that the inversely estimated flux pattern is clearly traceable to gradients in the observed ΔC_gasex, and that it is relatively insensitive to the choice of OGCM or potential biases in ΔC_gasex. Our inverse estimates imply a southward interhemispheric transport of 0.31 ± 0.02 Pg C yr^-1, most of which occurs in the Atlantic. This is considerably smaller than the 1 Pg C yr^-1 of Northern Hemisphere uptake that has been inferred from atmospheric CO₂ observations during the 1980s and 1990s, which supports the hypothesis of a Northern Hemisphere terrestrial sink.

12. Sweeney, C., M. N. Gloor, A. R. Jacobson, R. M. Key, G. McKinley, Jorge Sarmiento, and R. Wanninkhof, 2007: Constraining global air-sea gas exchange for CO₂ with recent bomb ¹⁴C measurements. Global Biogeochemical Cycles, 21(GB2015), doi:10.1029/2006GB002784
    [ Abstract ]

    The ¹⁴CO₂ released into the stratosphere during bomb testing in the early 1960s provides a global constraint on air-sea gas exchange of soluble atmospheric gases like CO₂. Using the most complete database of dissolved inorganic radiocarbon, DI¹⁴C, available to date and a suite of ocean general circulation models in an inverse mode we recalculate the ocean inventory of bomb-produced DI¹⁴C in the global ocean and confirm that there is a 25% decrease from previous estimates using older DI¹⁴C data sets. Additionally, we find a 33% lower globally averaged gas transfer velocity for CO₂ compared to previous estimates (Wanninkhof, 1992) using the NCEP/NCAR Reanalysis 1 1954–2000 where the global mean winds are 6.9 m s^-1. Unlike some earlier ocean radiocarbon studies, the implied gas transfer velocity finally closes the gap between small-scale deliberate tracer studies and global-scale estimates. Additionally, the total inventory of bomb-produced radiocarbon in the ocean is now in agreement with global budgets based on radiocarbon measurements made in the stratosphere and troposphere. Using the implied relationship between wind speed and gas transfer velocity ks = 0.27u²₁₀ 2 i(Sc/660)^-0.5 and standard partial pressure difference climatology of CO₂ we obtain an net air-sea flux estimate of 1.3 ± 0.5 PgCyr^-1 for 1995. After accounting for the carbon transferred from rivers to the deep ocean, our estimate of oceanic uptake (1.8 ± 0.5 PgCyr^-1) compares well with estimates based on ocean inventories, ocean transport inversions using ocean concentration data, and model simulations.

13. Crevoisier, C., M. N. Gloor, E. Gloaguen, L. W. Horowitz, Jorge Sarmiento, C. Sweeney, and P. P. Tans, 2006: A direct carbon budgeting approach to infer carbon sources and sinks. Design and synthetic application to complement the NACP observation network. Tellus, 58(5), doi:10.1111/j.1600-0889.2006.00214.x 366-375
    [ Abstract ]

    In order to exploit the upcoming regular measurements of vertical carbon dioxide (CO₂) profiles over North America implemented in the framework of the North American Carbon Program (NACP), we design a direct carbon budgeting approach to infer carbon sources and sinks over the continent using model simulations. Direct budgeting puts a control volume on top of North America, balances air mass in- and outflows into the volume and solves for the surface fluxes. The flows are derived from the observations through a geostatistical interpolation technique called Kriging combined with transport fields from weather analysis. The use of CO₂ vertical profiles simulated by the atmospheric transport model MOZART-2 at the planned 19 stations of the NACP network has given an estimation of the error of 0.39 GtC yr^-1 within the model world. Reducing this error may be achieved through a better estimation of mass fluxes associated with convective processes affecting North America. Complementary stations in the north-west and the north-east are also needed to resolve the variability of CO₂ in these regions. For instance, the addition of a single station near 52°N; 110°W is shown to decrease the estimation error to 0.34 GtC yr^-1.

14. Patra, P. K., K. R. Gurney, A. S. Denning, S. T. Maksyutov, T. Nakazawa, D. Baker, P. Bousquet, L. Bruhwiler, Y.-H. Chen, P. Ciais, S. Fan, I. Y. Fung, and M. N. Gloor, et al., 2006: Sensitivity of inverse estimation of annual mean CO₂ sources and sinks to ocean-only sites versus allsites observational networks. Geophysical Research Letters, 33(L05814), doi:10.1029/2005GL025403
    [ Abstract ]

    Inverse estimation of carbon dioxide (CO₂) sources and sinks uses atmospheric CO₂ observations, mostly made near the Earth’s surface. However, transport models used in such studies lack perfect representation of atmospheric dynamics and thus often fail to produce unbiased forward simulations. The error is generally larger for observations over the land than those over the remote/marine locations. The range of this error is estimated by using multiple transport models (16 are used here). We have estimated the remaining differences in fluxes due to the use of ocean-only versus all-sites (i.e., over ocean and land) observations of CO₂ in a time-independent inverse modeling framework. The fluxes estimated using the ocean-only networks are more robust compared to those obtained using all-sites networks. This makes the global, hemispheric, and regional flux determination less dependent on the selection of transport model and observation network.

15. Tiwari, Y. K., M. N. Gloor, R. J. Engelen, F. Chevallier, C. Rödenbeck, S. Korner, P. Peylin, B. H. Brasswell, and M. Heimann, 2006: Comparing CO₂ retrieved from Atmospheric Infrared Sounder with model predictions: implications for constraining surface fluxes and lower–to-upper troposphere transport. Journal of Geophysical Research, 111(D17106), doi:10.1029/2005JD006681
    [ Abstract ]

    Large-scale carbon sources and sinks can be estimated by combining atmospheric CO₂ concentration data with atmospheric transport inverse modeling. This approach has been limited by sparse spatiotemporal tropospheric sampling. CO₂ estimates from space using observations on recently launched satellites (Atmospheric Infrared Sounder (AIRS)), or platforms to be launched (Infrared Atmospheric Sounding Interferometer (IASI), Orbiting Carbon Observatory (OCO)) have the potential to fill some of these gaps. Here we assess the realism of initial AIRS-based mid-to-upper troposphere CO₂ estimates from European Centre for Medium-Range Weather Forecasts by comparing them with simulations of two transport models (TM3 and Laboratoire Meteorologie Dynamique Zoom (LMDZ)) forced with one data-based set of surface fluxes. The simulations agree closer with one another than with the retrievals. Nevertheless, there is good overall agreement between all estimates of seasonal cycles and north-south gradients within the latitudinal band extending from 30°S to 30°N, but not outside this region. At smaller spatial scales, there is a contrast in the satellite-based retrievals above continents versus above oceans that is absent in the model predictions. Hovmoeller diagrams indicate that in the models, high Northern Hemisphere winter CO₂ concentrations propagate toward the tropical upper troposphere via Northern Hemisphere high latitudes, while in retrievals, elevated winter CO₂ appears instantaneously throughout the Northern Hemisphere. This raises questions about lower-to-upper troposphere transport pathways. Prerequisites for use of retrievals to provide an improved constraint on surface fluxes are therefore further improvements in retrievals and better understanding/validation of lower-to-upper troposphere transport and its modeling. This calls for more independent upper troposphere transport tracer data like SF₆ and CO₂.

16. Tohjima, Y., H. Mukai, T. Machida, Y. Nojiri, and M. N. Gloor, 2005: First measurements of the latitudinal atmospheric O₂ and CO₂ distributions across the Western Pacific. Geophysical Research Letters, 32(L17805), doi:10.1029/2005GL023311
    [ Abstract ]

    We examine the latitudinal distribution of the tracer ‘‘atmospheric potential oxygen’’ APO = O₂ + 1.1 X CO₂ between 40°S and 50°N using new atmospheric CO₂ and O₂ measurements from flask samples collected onboard cargo ships between Japan and the United States, and Japan and Australia (or New Zealand) during the period from December 2001 to August 2004. Because APO is unaltered during photosynthesis and respiration by land vegetation, its atmospheric distribution is tightly linked to the air-sea gas exchange of O₂ and CO₂ and the underlying processes. We find that the seasonal amplitude of APO increases towards high latitudes in both hemispheres and its minimum is located approximately 10-degree north of the equator. The latitudinal distribution of annual mean APO shows a maximum in the tropics, confirming the recent coupled ocean-atmosphere model predictions for this region.

17. Gurney, K. R., R. M. Law, A. S. Denning, P. J. Rayner, D. Baker, P. Bousquet, L. Bruhwiler, Y.-H. Chen, P. Ciais, Y. Fan, I. Y. Fung, and M. N. Gloor, 2003: TransCom 3 CO₂ inversion intercomparison: 1. Annual mean control results and sensitivity to transport and prior flux information. Tellus, Series B, International Meteorological Institute in Stockholm, 55(2), doi:10.1034/j.1600-0889.2003.00049.x 555-579
    [ Abstract ]

    Spatial and temporal variations of atmospheric CO₂ concentrations contain information about surface sources and sinks, which can be quantitatively interpreted through tracer transport inversion. Previous CO₂ inversion calculations obtained differing results due to different data, methods and transport models used. To isolate the sources of uncertainty, we have conducted a set of annual mean inversion experiments in which 17 different transport models or model variants were used to calculate regional carbon sources and sinks from the same data with a standardized method. Simulated transport is a significant source of uncertainty in these calculations, particularly in the response to prescribed “background” fluxes due to fossil fuel combustion, a balanced terrestrial biosphere, and air–sea gas exchange. Individual model-estimated fluxes are often a direct reflection of their response to these background fluxes. Models that generate strong surface maxima near background exchange locations tend to require larger uptake near those locations. Models with weak surface maxima tend to have less uptake in those same regions but may infer small sources downwind. In some cases, individual model flux estimates cannot be analyzed through simple relationships to background flux responses but are likely due to local transport differences or particular responses at individual CO₂ observing locations. The response to the background biosphere exchange generates the greatest variation in the estimated fluxes, particularly over land in the Northern Hemisphere. More observational data in the tropical regions may help in both lowering the uncertain tropical land flux uncertainties and constraining the northern land estimates because of compensation between these two broad regions in the inversion. More optimistically, examination of the model-mean retrieved fluxes indicates a general insensitivity to the prior fluxes and the prior flux uncertainties. Less uptake in the Southern Ocean than implied by oceanographic observations, and an evenly distributed northern land sink, remain in spite of changes in this aspect of the inversion setup.

18. Gurney, K. R., R. M. Law, A. S. Denning, P. J. Rayner, D. Baker, P. Bousquet, L. Bruhwiler, Y.-H. Chen, P. Ciais, S. Fan, I. Y. Fung, and M. N. Gloor, et al., 2002: Towards robust regional estimates of CO₂ sources and sinks using atmospheric transport models. Nature, 415, doi:10.1038/415626a 626-630
    [ Abstract ]

    Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO₂ concentrations via inverse modeling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface atmosphere CO₂ fluxes from an intercomparison of atmospheric CO₂ inversion models (the TransCom 3 project), which includes 16transportmodelsandmodelvariants. We find an uptake of CO₂ in the southern extra tropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO₂. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO₂ observation network within the tropics. We estimate annual average fluxes for the 1992-96 period using each transport model and a common inversion set-up (see Methods). Methodological choices for this 'control' inversion have been selected on the basis of knowledge gained from a wide range of sensitivity tests (to be reported elsewhere). Performing the inversion with multiple transport models gives mean estimated fluxes that are relatively insensitive to reasonable variations in the set-up-and estimated uncertainties that represent a more complete estimate of the true uncertainty. The maximum number of Southern 0cean fluxes had been noted a decade ago. 0ur sensitivity tests find that the near-uniformity of observed concentration in the Southern Hemisphere and the small uncertainty associated with those measurements make this result robust to the choice of observing network, prior flux estimates, global ocean constraint, and transport (see Fig 2 in Supplementary Information). The discrepancy also cannot be explained by a systematic bias in transport models, as the north-south transport has been investigated in a recent intercomparison where successful simulations of the observed meridional gradient in SF₆ suggested reasonable veracity in gross interhemispheric transport. 0ne possible reconciliation between the pCO₂ database and the inverse result presented here is suggested by recent ocean measurements taken during January and August 2000 in the Indian Ocean.

19. Gloor, M. N., N. Gruber, T.M.C. Hughes, and Jorge Sarmiento, 2001: Estimating Net Air-Sea Fluxes from Ocean Bulk Data: Methodology and Application to the Heat Cycle. Global Biogeochemical Cycles, 15(4), doi:10.1029/2000GB001301 767-782
    [ Abstract ]

    A novel method to estimate annual mean heat, water, and gas exchange fluxes between the ocean and the atmosphere is proposed that is complementary to the traditional approach based on air-sea gradients and bulk exchange parameterization. The new approach exploits the information on surface exchange fluxes contained in the distribution of temperature, salinity, and dissolved gases in the ocean interior. We use an Ocean General Circulation Model to determine how the distribution in the ocean interior is linked to surface fluxes. We then determine with least squares the surface fluxes that are most compatible with the observations. To establish and test the method, we apply it to ocean temperature data to estimate heat fluxes across the air-sea interface for which a number of climatological estimates exists. We also test the sensitivity of the inversion results to data coverage,differences in ocean transport, variations in the surface flux pattern and a range of spatial resolutions. We find, on the basis of the World Ocean Circulation Experiment( WOCE) data network augmented with selected high-quality pre-WOCE data, t hat we are able to constrain heat exchange fluxes for 10-15 regions of the ocean, whereby these fluxes nearly balance globally without enforcing a conservation constraint. Our results agree well with heat flux estimates on the basis of bulk exchange parameterization, which generally require constraints to ensure a global net heat flux of zero. We also find that the heat transports implied by our inversely estimated fluxes are in good agreement with a large range of heat transport estimates based on hydrographic data. Increasing the number of regions beyond the 10-15 regions considered here is severely limited because of modeling errors. The inverse method is fairly robust to the modeling of the spatial patterns of the surface fluxes; however, it is quite sensitive to the modeling of ocean transport. The most striking difference between our estimates and the heat flux climatologies is a large heat loss of 0.64 PW to the atmosphere from the Southern Ocean and a large heat gain by the subpolar South Atlantic of 0.56 PW. These results are consistent with the large gain of carbon dioxide called for by Takahashi et al. [1999] in his recent analysis of the air-sea flux of carbon dioxide but inconsistent with the large loss of oxygen and carbon dioxide such as those of Stephens et al. [1998].

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