Bibliography - N. Gruber

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. Deutsch, C., Jorge Sarmiento, Daniel Sigman, N. Gruber, and J. P. Dunne, 2007: Spatial coupling of nitrogen inputs and losses in the ocean. Nature, 445, doi:10.1038/nature05392
   [ Abstract ]

   Nitrogen fixation is crucial for maintaining biological productivity in the oceans, because it replaces the biologically available nitrogen that is lost through denitrification. But, owing to its temporal and spatial variability, the global distribution of marine nitrogen fixation is difficult to determine from direct shipboard measurements. This uncertainty limits our understanding of the factors that influence nitrogen fixation, which may include iron, nitrogen-to-phosphorus ratios, and physical conditions such as temperature. Here we determine nitrogen fixation rates in the world’s oceans through their impact on nitrate and phosphate concentrations in surface waters, using an ocean circulation model. Our results indicate that nitrogen fixation rates are highest in the Pacific Ocean, where water column denitrification rates are high but the rate of atmospheric iron deposition is low. We conclude that oceanic nitrogen fixation is closely tied to the generation of nitrogen-deficient waters in denitrification zones, supporting the view that nitrogen fixation stabilizes the oceanic inventory of fixed nitrogen over time.

7. 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.

8. 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.

9. 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.

10. Najjar, R. G., X. Jin, F. Louanchi, O. Aumont, K. Caldeira, S. C. Doney, J. C. Dutay, M. Follows, N. Gruber, F. Joos, K. Lindsay, and E. Maier-Reimer, et al., 2007: Impact of circulation on export production, dissolved organic matter, and dissolved oxygen in the ocean: Results from Phase II of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Global Biogeochemical Cycles, 21(GB3007), doi:10.1029/2006GB002857
    [ Abstract ]

    Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr^-1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

11. Battle, M., S. E. Mikaloff-Fletcher, Michael Bender, R. F. Keeling, A. C. Manning, N. Gruber, P. P. Tans, M. B. Hendricks, D. T. Ho, C. Simonds, R. Mika, and B. Paplawsky, 2006: Atmospheric potential oxygen: New observations and their implications for some atmospheric and oceanic models. Global Biogeochemical Cycles, 20(GB1010), doi:10.1029/2005GB002534
    [ Abstract ]

    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.

12. Jin, X., N. Gruber, J. P. Dunne, Jorge Sarmiento, and R. A. Armstrong, 2006: Diagnosing the contribution of phytoplankton functional groups to the production and export of particulate organic carbon, CaCO₃, and opal from global nutrient and alkalinity distributions. Global Biogeochemical Cycles, 20(GB2015), doi:10.1029/2005GB002532
    [ Abstract ]

    We diagnose the contribution of four main phytoplankton functional groups to the production and export of particulate organic carbon (POC), CaCO₃, and opal by combining in a restoring approach global oceanic observations of nitrate, silicic acid, and alkalinity with a simple size-dependent ecological/biogeochemical model. In order to determine the robustness of our results, we employ three different variants of the ocean general circulation model (OGCM) required to transport and mix the nutrients and alkalinity into the upper ocean. In our standard model, the global export of CaCO₃ is diagnosed as 1.1 PgC yr^-1 (range of sensitivity cases 0.8 to 1.2 PgC yr^-1) and that of opal as 180 Tmol Si yr^-1 (range 160 to 180 Tmol Si yr^-11). CaCO₃ export is found to have three maxima at approximately 40°S, the equator, and around 40°N. In contrast, the opal export is dominated by the Southern Ocean with a single maximum at around 60°S. The molar export ratio of inorganic to organic carbon is diagnosed in our standard model to be about 0.09 (range 0.07 to 0.10) and found to be remarkably uniform spatially. The molar export ratio of opal to organic nitrogen varies substantially from values around 2 to 3 in the Southern Ocean south of 45°S to values below 0.5 throughout most of the rest of the ocean, except for the North Pacific. Irrespective of which OGCM is used, large phytoplankton dominate the export of POC, with diatoms alone accounting for 40% of this export, while the contribution of coccolithophorids is only about 10%. Small phytoplankton dominate net primary production (NPP) with a fraction of ≈70%. Diatoms and coccolithophorids account for about 15% and less than 2% of NPP, respectively. These diagnosed contributions of the main phytoplankton functional groups to NPP are also robust across all OGCMs investigated. Correlation and regression analyses reveal that the variations in the relative contributions of diatoms and coccolithophorids to NPP can be predicted reasonably well on the basis of a few key parameters.

13. Mikaloff-Fletcher, S. E., N. Gruber, A. R. Jacobson, S. C. Doney, S. Dutkiewicz, M. Gerber, M. Follows, F. Joos, K. Lindsay, D. Menemenlis, A. Mouchet, S. A. Müller, and Jorge Sarmiento, 2006: Inverse estimates of anthropogenic CO₂ uptake, transport, and storage by the ocean. Global Biogeochemical Cycles, 20(GB2002), doi:10.1029/2005GB002530
    [ Abstract ]

    Regional air-sea fluxes of anthropogenic CO₂ are estimated using a Green’s function inversion method that combines data-based estimates of anthropogenic CO₂ in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data-based anthropogenic CO₂ estimates. On the basis of the prescribed anthropogenic CO₂ storage, we find a global uptake of 2.2 ± 0.25 Pg C yr^-1, scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC-based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO₂ uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross-equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large-scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO₂ inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.

14. Sarmiento, Jorge, and N. Gruber, 2006: Ocean Biogeochemical Dynamics. Ocean Biogeochemical Dynamics, Princeton University Press, ISBN:13:978-0-691-01,
    [ Abstract ]

    Ocean Biogeochemical Dynamics provides a broad theoretical framework upon which graduate students and upper-level undergraduates can formulate an understanding of the processes that control the mean concentration and distribution of biologically utilized elements and compounds in the ocean. Though it is written as a textbook, it will also be of interest to more advanced scientists as a wide-ranging synthesis of our present understanding of ocean biogeochemical processes.
    The first two chapters of the book provide an introductory overview of biogeochemical and physical oceanography. The next four chapters concentrate on processes at the airsea interface, the production of organic matter in the upper ocean, the remineralization of organic matter in the water column, and the processing of organic matter in the sediments. The focus of these chapters is on analyzing the cycles of organic carbon, oxygen, and nutrients.
    The next three chapters round out the authors' coverage of ocean biogeochemical cycles with discussions of silica, dissolved inorganic carbon and alkalinity, and CaCO₃. The final chapter discusses applications of ocean biogeochemistry to our understanding of the role of the ocean carbon cycle in interannual to decadal variability, paleoclimatology, and the anthropogenic carbon budget. The problem sets included at the end of each chapter encourage students to ask critical questions in this exciting new field. While much of the approach is mathematical, the math is at a level that should be accessible to students with a year or two of college level mathematics and/or physics.

15. Orr, J. C., V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, and K. Lindsay, et al., 2005: Anthropogenic ocean acidification over the 21st century and its impact on marine calcifying organisms. Nature, 437, doi:10.1038/nature04095 681-686
    [ Abstract ]

    Today’s surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a ‘business-as-usual’ scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

16. Doney, S. C., K. Lindsay, K. Caldeira, J.-M. Campin, H. Drange, J. C. Dutay, M. Follows, Y. Gao, A. Gnanadesikan, N. Gruber, A. Ishida, and F. Joos, et al., 2004: Evaluating global ocean carbon models: The importance of realistic physics. Global Biogeochemical Cycles, 18(GB3017), doi:10.1029/2003GB002150
    [ Abstract ]

    A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO₂ flux, chlorofluorocarbon and anthropogenic CO₂ uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO₂. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

17. Matsumoto, K., Jorge Sarmiento, R. M. Key, O. Aumont, J. L. Bullister, K. Caldeira, J.-M. Campin, S. C. Doney, H. Drange, J. C. Dutay, Y. Gao, A. Gnanadesikan, and N. Gruber, et al., 2004: Evaluation of ocean carbon cycle models with data-based metrics. Geophysical Research Letters, 31(L07303), doi:10.1029/2003GL018970
    [ Abstract ]

    New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new databased metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

18. Sarmiento, Jorge, N. Gruber, M. A. Brezeinski, and J. P. Dunne, 2004: High latitude controls of thermodine nutrients and low latitude biological productivity. Nature, 427, doi:10.1038/nature02127 56-60
    [ Abstract ]

    The ocean’s biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean1 and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere 2,3 and North Atlantic Ocean3, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides4, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.

19. Brezeinski, M. A., C. J. Pride, V. M. Franck, Daniel Sigman, Jorge Sarmiento, K. Matsumoto, and N. Gruber, 2002: A switch from Si(OH)₄ to NO₃ depletion in the glacial Southern Ocean. Geophysical Research Letters, 29(12), doi:10.1029/2001GL014349 1564
    [ Abstract ]

    Phytoplankton in the Antarctic deplete silicic acid (Si(OH)₄) to a far greater extent than they do nitrate (NO₃). This pattern can be reversed by the addition of iron which dramatically lowers diatom Si(OH)₄: NO₃ uptake ratios. Higher iron supply during glacial times would thus drive the Antarctic towards NO₃ depletion with excess Si(OH)₄ remaining in surface waters. New δ³⁰SI and δ ¹⁵N records from Antarctic sediments confirm diminished Si(OH)₄ use and enhanced NO₃ depletion during the last three glaciations. The present low-Si(OH)₄ water is transported northward to at least the subtropics. We postulate that the glacial high-Si(OH)₄ water similarly may have been transported to the subtropics and beyond. This input of Si(OH)₄ may have caused diatoms to displace coccolithophores at low latitudes, weakening the carbonate pump and increasing the depth of organic matter remineralization. These effects may have lowered glacial atmospheric pCO₂ by as much as 60 ppm.

20. Dutay, J. C., J. L. Bullister, S. C. Doney, J. C. Orr, R. G. Najjar, K. Caldeira, J.-M. Campin, H. Drange, M. Follows, Y. Gao, and N. Gruber, et al., 2002: Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models. Ocean Modelling, 4(2), doi:10.1016/S1463-5003(01)00013-0 89-120
    [ Abstract ]

    We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO₂

21. Gnanadesikan, A., R. D. Slater, N. Gruber, and Jorge Sarmiento, 2002: Oceanic vertical exchange and new production: A comparison between models and observations. Deep Sea Research II, 49(1-3), doi:10.1016/S0967-0645(01)00107-2 363-401
    [ Abstract ]

    This paper explores the relationship between large-scale vertical exchange and the cycling of biologically active nutrients within the ocean. It considers how the parameterization of vertical and lateral mixing effects estimates of newproducti on (defined as the net uptake of phosphate). A baseline case is run with low vertical mixing in the pycnocline and a relatively lowlate ral diffusion coefficient. The magnitude of the diapycnal diffusion coefficient is then increased within the pycnocline, within the pycnocline of the Southern Ocean, and in the top 50 m; while the lateral diffusion coefficient is increased throughout the ocean. It is shown that it is possible to change lateral and vertical diffusion coefficients so as to preserve the structure of the pycnocline while changing the pathways of vertical exchange and hence the cycling of nutrients. Comparisons between the different models reveal that new production is very sensitive to the level of vertical mixing within the pycnocline, but only weakly sensitive to the level of lateral and upper ocean diffusion. The results are compared with two estimates of new production based on ocean color and the annual cycle of nutrients. On a global scale, the observational estimates are most consistent with the circulation produced with a low diffusion coefficient within the pycnocline, resulting in a new production of around 10 GtC yr¹: On a regional level, however, large differences appear between observational and model based estimates. In the tropics, the models yield systematically higher levels of new production than the observational estimates. Evidence from the Eastern Equatorial Pacific suggests that this is due to both biases in the data used to generate the observational estimates and problems with the models. In the North Atlantic, the observational estimates vary more than the models, due in part to the methodology by which the nutrient-based climatology is constructed. In the North Pacific, the modeled values of new production are all much lower than the observational estimates, probably as a result of the failure to form intermediate water with the right properties. The results demonstrate the potential usefulness of new production for evaluating circulation models.

22. Gruber, N., and Jorge Sarmiento, 2002: Large –scale biogeochemical-physical interactions in elemental cycles. The Sea, John Wiley & Sons, Inc., 12, 337-399
    [ Abstract ]

    Introduction Why is the distribution of most chemicals not uniform within the ocean? In the absence of sources and sinks the effect of ocean circulation and mixing would be to smooth the distribution of chemicals. An important reason for non-uniformity is the increase in concentration that occurs in regions of evaporation, and the reduction in concentration that occurs in regions of rainfall or river input at the ocean atmosphere interface. Evaporation, precipitation, and river input lead to concentration ranges usually well below 10% as evidenced in the oceanic variability of salinity. However, as Figure1 shows, many chemicals have concentration ranges much greater than this. The concentration of inorganic nitrogen, for example, is more than five times smaller near the surface compared to the deep ocean. Dissolved inorganic carbon shows a surface depletion that is also substantially larger than the 10% variation that could be induced by freshwater fluxes. The primary mechanism that drives such large concentration gradients is the use of dissolved inorganic chemicals by living organisms for formation of organic matter and inorganic solids. Photosynthetic organisms use the energy from light in the upper part of the ocean to produce both particulate and dissolved organic matter and a wide range of inorganic solids as well. These "biogenic" materials are often transported long distances before being converted back to dissolved inorganic chemicals by processes such as "remineralization" and dissolution. The largest such transports are vertical, giving rise to greatly reduced concentrations in the surface ocean, and enhanced concentrations in the abyss. A few chemicals, the most important being oxygen, are affected by these processes in the opposite direction. They are released during photosynthesis and consumed during remineralization. The wide range of biological and associated processes that affect such chemicals are referred to as "biogeochemical" processes. The view that therefore emerges with regard to the large scale distribution of chemicals in the ocean is one where biogeochemical processes are constantly creating spatial and temporal gradients in the chemicals, whereas ocean circulation and mixing are in general attempting to homogenize these gradients. The resulting distribution of the chemicals in the ocean can thus be understood as the result of a complex interplay between biogeochemical cycles and ocean physics. These large scale biogeochemical/physical interactions are the focus of this chapter. In particular, we are interested in determining the relative roles of the various ``gradient makers'' in creating the large-scale distribution of chemicals in the ocean. We will focus our discussion on the two most important biogeochemical elements, carbon and nitrogen. We will include the cycling of phosphorus and oxygen since they allow us to gain much additional information about carbon and nitrogen.

23. Sarmiento, Jorge, and N. Gruber, 2002: Sinks for anthropogenic carbon. Physics Today, http://www.gfdl.noaa.gov/bibliography/related_files/jls0202.pdf, 55, 30-36
    [ Abstract ]

    The ocean’s biological pump strips nutrients out of the surface waters and exports them into the thermocline and deep waters. If there were no return path of nutrients from deep waters, the biological pump would eventually deplete the surface waters and thermocline of nutrients; surface biological productivity would plummet. Here we make use of the combined distributions of silicic acid and nitrate to trace the main nutrient return path from deep waters by upwelling in the Southern Ocean1 and subsequent entrainment into subantarctic mode water. We show that the subantarctic mode water, which spreads throughout the entire Southern Hemisphere 2,3 and North Atlantic Ocean3, is the main source of nutrients for the thermocline. We also find that an additional return path exists in the northwest corner of the Pacific Ocean, where enhanced vertical mixing, perhaps driven by tides4, brings abyssal nutrients to the surface and supplies them to the thermocline of the North Pacific. Our analysis has important implications for our understanding of large-scale controls on the nature and magnitude of low-latitude biological productivity and its sensitivity to climate change.

24. Deutsch, C., N. Gruber, R. M. Key, and Jorge Sarmiento, 2001: Denitrification and N₂ fixation in the Pacific Ocean. Global Biogeochemical Cycles, http://www.agu.org/journals/gb/v015/i002/2000GB001291/2000GB001291.pdf, 15(2), 483-506
    [ Abstract ]

    We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water column denitrification rate of 48 ± 5 Tg N yr^-1 is calculated for the Eastern Tropical Pacific using N * [Gruber and Sarmiento,1997] and water mass age tracers. On the basis of rates in the literature, we estimate sedimentary denitrification to remove n additional 15 ± 3 Tg N yr^-1. We then calculate the total nitrogen divergence due to the large scale circulation through the basin, composed of flows through a zonal transect at 32°S, and through the Indonesian and Bering straits. Adding atmospheric deposition and riverine fluxes results in a net divergence of nitrogen from the basin of -4 ± 12 Tg N yr^-1. Pacific nitrogen fixation can be extracted as a residual component of the total budget, assuming steady state. We find that nitrogen fixation would have to contribute 59 ± 14 Tg N yr^-1 in order to balance the Pacific nitrogen budget. This result is consistent with the tentative global extrapolations of Gruber and Sarmiento[ 1997], based on nitrogen fixation rates estimated for the North Atlantic. Our estimated mean areal fixation rate is within the range of direct and geochemical rate estimates from a single location near Hawaii [Karl et al., 1997]. Pacific nitrogen fixation occurs primarily in the western part of the subtropical gyres where elevated N* signals are found. These regions are also supplied with significant amounts of iron via atmospheric dust deposition, lending qualitative support to the hypothesis that nitrogen fixation is regulated in part by iron suppy.

25. 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].

26. Orr, J. C., E. Maier-Reimer, U. Mikolajewicz, P. Monfray, Jorge Sarmiento, J. R. Toggweiler, N. K. Taylor, J. Palmer, N. Gruber, C. L. Sabine, C. Le Quere, R. M. Key, and J. Boutin, 2001: Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. Global Biogeochemical Cycles, http://www.agu.org/pubs/crossref/2001/2000GB001273.shtml, 15(1), 43-60
    [ Abstract ]

    We have compared simulations of anthropogenic CO₂ in the four three dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85 ± 0.35 Pg Cy r^-1 for the 1980-1989 average. Regionally, t he Southern Ocean dominates the present-day air-sea flux of anthropogenic CO₂ in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO₂ suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data-based estimates of anthropogenic CO₂ and bomb ¹⁴C. Column inventories of bomb ¹⁴C have become more similar to those for anthropogenic CO₂ with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and Word Ocean Circulation Experiment ( 1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO₂ would improve if systematic errors associated with the data-based estimates could be provided regionally.

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