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Stephens, BB, Long MC, Keeling RF, Kort EA, Sweeney C, Apel EC, Atlas EL, Beaton S, Bent JD, Blake NJ, Bresch JF, Casey J, Daube BC, Diao MH, Diaz E, Dierssen H, Donets V, Gao BC, Gierach M, Green R, Haag J, Hayman M, Hills AJ, Hoecker-Martinez MS, Honomichl SB, Hornbrook RS, Jensen JB, Li RR, McCubbin I, McKain K, Morgan EJ, Nolte S, Powers JG, Rainwater B, Randolph K, Reeves M, Schauffler SM, Smith K, Smith M, Stith J, Stossmeister G, Toohey DW, Watt AS.  2018.  The O-2/N-2 Ratio and CO2 Airborne Southern Ocean Study. Bulletin of the American Meteorological Society. 99:381-402.   10.1175/bams-d-16-0206.1   AbstractWebsite

The Southern Ocean plays a critical role in the global climate system by mediating atmosphere-ocean partitioning of heat and carbon dioxide. However, Earth system models are demonstrably deficient in the Southern Ocean, leading to large uncertainties in future air-sea CO2 flux projections under climate warming and incomplete interpretations of natural variability on interannual to geologic time scales. Here, we describe a recent aircraft observational campaign, the O-2/N-2 Ratio and CO2 Airborne Southern Ocean (ORCAS) study, which collected measurements over the Southern Ocean during January and February 2016. The primary research objective of the ORCAS campaign was to improve observational constraints on the seasonal exchange of atmospheric carbon dioxide and oxygen with the Southern Ocean. The campaign also included measurements of anthropogenic and marine biogenic reactive gases; high-resolution, hyperspectral ocean color imaging of the ocean surface; and microphysical data relevant for understanding and modeling cloud processes. In each of these components of the ORCAS project, the campaign has significantly expanded the amount of observational data available for this remote region. Ongoing research based on these observations will contribute to advancing our understanding of this climatically important system across a range of topics including carbon cycling, atmospheric chemistry and transport, and cloud physics. This article presents an overview of the scientific and methodological aspects of the ORCAS project and highlights early findings.

Bender, ML, Battle M, Keeling RF.  1998.  The O2 balance of the atmosphere: A tool for studying the fate of fossil-fuel CO2. Annual Review of Energy and the Environment. 23:207-223.   10.1146/annurev.energy.23.1.207   AbstractWebsite

Carbon dioxide is a radiatively active gas whose atmospheric concentration increase is likely to affect Earth's climate. CO2 is added to the atmosphere by biomass burning and the combustion of fossil fuels. Some added CO2 remains in the atmosphere. However, substantial amounts are taken up by the oceans and land biosphere, attenuating the atmospheric increase. Atmospheric O-2 measurements provide one constraint for partitioning uptake rates between the ocean and the land biosphere. Here we review studies of atmospheric O-2 concentration variations and discuss their implications for CO2 uptake by the ocean and the land biosphere. We compare estimates of anthropogenic carbon fluxes from O-2 studies with estimates from other approaches and examine the contribution of natural ocean carbon fluxes to atmospheric O-2 variations.

Graven, HD, Guilderson TP, Keeling RF.  2012.  Observations of radiocarbon in CO2 at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles. Journal of Geophysical Research-Atmospheres. 117   10.1029/2011jd016535   AbstractWebsite

High precision measurements of Delta C-14 were conducted for monthly samples of CO2 from seven global stations over 2- to 16-year periods ending in 2007. Mean Delta C-14 over 2005-07 in the Northern Hemisphere was 5 parts per thousand lower than Delta C-14 in the Southern Hemisphere, similar to recent observations from I. Levin. This is a significant shift from 1988-89 when Delta C-14 in the Northern Hemisphere was slightly higher than the South. The influence of fossil fuel CO2 emission and transport was simulated for each of the observation sites by the TM3 atmospheric transport model and compared to other models that participated in the Transcom 3 Experiment. The simulated interhemispheric gradient caused by fossil fuel CO2 emissions was nearly the same in both 1988-89 and 2005-07, due to compensating effects from rising emissions and decreasing sensitivity of Delta C-14 to fossil fuel CO2. The observed 5 parts per thousand shift must therefore have been caused by non-fossil influences, most likely due to changes in the air-sea C-14 flux in the Southern Ocean. Seasonal cycles with higher Delta C-14 in summer or fall were evident at most stations, with largest amplitudes observed at Point Barrow (71 degrees N) and La Jolla (32 degrees N). Fossil fuel emissions do not account for the seasonal cycles of Delta C-14 in either hemisphere, indicating strong contributions from non-fossil influences, most likely from stratosphere-troposphere exchange.

Graven, HD, Guilderson TP, Keeling RF.  2012.  Observations of radiocarbon in CO2 at La Jolla, California, USA 1992-2007: Analysis of the long-term trend. Journal of Geophysical Research-Atmospheres. 117   10.1029/2011jd016533   AbstractWebsite

High precision measurements of Delta C-14 were performed on CO2 sampled at La Jolla, California, USA over 1992-2007. A decreasing trend in Delta C-14 was observed, which averaged -5.5 parts per thousand yr(-1) yet showed significant interannual variability. Contributions to the trend in global tropospheric Delta C-14 by exchanges with the ocean, terrestrial biosphere and stratosphere, by natural and anthropogenic C-14 production and by C-14-free fossil fuel CO2 emissions were estimated using simple models. Dilution by fossil fuel emissions made the strongest contribution to the Delta C-14 trend while oceanic C-14 uptake showed the most significant change between 1992 and 2007, weakening by 70%. Relatively steady positive influences from the stratosphere, terrestrial biosphere and C-14 production moderated the decreasing trend. The most prominent excursion from the average trend occurred when Delta C-14 decreased rapidly in 2000. The rapid decline in Delta C-14 was concurrent with a rapid decline in atmospheric O-2, suggesting a possible cause may be the anomalous ventilation of deep C-14-poor water in the North Pacific Ocean. We additionally find the presence of a 28-month period of oscillation in the Delta C-14 record at La Jolla.

Keeling, RF, Kortzinger A, Gruber N.  2010.  Ocean deoxygenation in a warming world. Annual Review of Marine Science. 2:199-229., Palo Alto: Annual Reviews   10.1146/annurev.marine.010908.163855   Abstract

Ocean warming and increased stratification of the upper ocean caused by global climate change will likely lead to declines in dissolved O(2) in the ocean interior (ocean deoxygenation) with implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitat. Ocean models predict declines of 1 to 7% in the global ocean O(2) inventory over the next century, with declines continuing for a thousand years or more into the future. An important consequence may be an expansion in the area and volume of so-called oxygen minimum zones, where O(2) levels are too low to support many macrofauna and profound changes in biogeochemical cycling occur. Significant deoxy enation has occurred over the past 50 years in the North Pacific and tropical oceans, suggesting larger changes are looming. The potential for larger O(2) declines in the future suggests the need for all improved observing system for tracking ocean O(2) changes.

Hamme, RC, Keeling RF.  2008.  Ocean ventilation as a driver of interannual variability in atmospheric potential oxygen. Tellus Series B-Chemical and Physical Meteorology. 60:706-717.   10.1111/j.1600-0889.2008.00376.x   AbstractWebsite

We present observations of interannual variability on 2-5 yr timescales in atmospheric potential oxygen (APO approximate to O(2) + CO(2)) from the Scripps Institution of Oceanography global flask sampling network. Interannual variations in the tracer APO are expected to arise from air-sea fluxes alone, because APO is insensitive to exchanges with the terrestrial biosphere. These interannual variations are shown to be regionally coherent and robust to analytical artefacts. We focus on explaining a feature dominant in records from the Northern Hemisphere stations, marked by increasing APO in the late 1990s, followed by an abrupt drawdown in 2000-2001. The timing of the drawdown matches a renewal of deep convection in the North Atlantic, followed the next year by a severe winter in the western North Pacific that may have allowed ventilation of denser isopycnals than usual. We find a weak correlation between changes in the interhemispheric APO difference and El Nino indices, and the observations show no strong features of the 1997-98 El Nino. Comparisons with estimates of variations in ocean productivity and ocean heat content demonstrate that these processes are secondary influences at these timescales. We conclude that the evidence points to variability in ocean ventilation as the main driver of interannual variability in APO.

Tans, PP, Berry JA, Keeling RF.  1993.  Oceanic 13C/12C observations: A new window on ocean CO2 uptake. Global Biogeochemical Cycles. 7:353-368.   10.1029/93gb00053   AbstractWebsite

Equations are developed describing the rate of change of carbon isotopic ratios in the atmosphere and oceans in terms of deltaC-13 quantities. The equations enable one to perform calculations directly with delta and epsilon quantities commonly reported in the literature. The main cause of the change occurring today is the combustion of fossil fuel carbon with lower deltaC-13 values. The course of this isotopic anomaly in atmosphere and oceans can provide new constraints on the carbon budgets of these reservoirs. Recently published deltaC-13 isotopic data of total inorganic carbon in the oceans [Quay et al., 1992] appear to lead to incompatible results with respect to the uptake of fossil fuel CO2 by the oceans if two different approaches Lo the data are taken. Consideration of the air-sea isotopic disequilibrium leads to an uptake estimate of only a few tenths of a gigaton C (Gt, for 10(15) g) per year, whereas the apparent change in the ocean deltaC-13 inventory leads to an estimate of more than 2 Gt C yr-1. Both results are very uncertain with presently available data. The isotopic ratio has the advantage that the signal-to-noise ratio for the measurement of the uptake of the isotopic signal by the oceans is better than for the uptake of total carbon. The drawback is that isotopic exchange with carbon reservoirs that are difficult to characterize introduces uncertainty into the isotopic budget. The accuracy requirements for the measurements are high, demanding careful standardization at all stages.

Keeling, RF.  2002.  On the freshwater forcing of the thermohaline circulation in the limit of low diapycnal mixing. Journal of Geophysical Research-Oceans. 107   10.1029/2000jc000685   AbstractWebsite

[1] A conjecture is offered on the stability characteristics of the thermohaline circulation in the limit of very low diapycnal mixing. In this limit the action of the winds on the Antarctic Circumpolar Current (ACC) can sustain a deep overturning pattern known as the "reconfigured conveyor,'' consisting of upwelling around Antarctica and sinking in the North Atlantic, as shown by the work of Toggweiler and others. It is conjectured that in this limit, northern sinking should be stabilized in an "on'' state because of the penetration of freshwater into the ocean interior via isopycnal layers that outcrop to the surface within and south of the ACC. This conjecture is supported by qualitative arguments and by a hydraulic model for the reconfigured conveyor. The hydraulic model takes into account the freshwater budgets of the Atlantic basin, Antarctic surface waters, and the remaining oceans. It also takes into account, in simple terms, wind-driven Antarctic upwelling, eddy transports and mixing within the ACC, changes in pycnocline depth, the role of temperature forcing, and advective feedbacks on salinity. The hydraulic model suggests that multiple "on/off'' states of the reconfigured conveyor are possible but only if the deep waters that form in the Northern Hemisphere are fresher than the intermediate waters that form in the vicinity of the ACC in the Southern Hemisphere, a condition that is not satisfied in the modern ocean.

Garcia, HE, Keeling RF.  2001.  On the global oxygen anomaly and air-sea flux. Journal of Geophysical Research-Oceans. 106:31155-31166.   10.1029/1999jc000200   AbstractWebsite

We present a new climatology of monthly air-sea oxygen fluxes throughout the ice-free surface global ocean. The climatology is based on weighted linear least squares regressions using heat flux monthly anomalies for spatial and temporal interpolation of historical O-2 data. The seasonal oceanic variations show that the tropical belt (20degreesS-20degreesN) is characterized by relatively small air-sea fluxes when compared to the middle to high latitudes (40degrees-70degrees). The largest and lowest seasonal fluxes occur during summer and winter in both hemispheres. By means of an atmospheric transport model we show that our climatology is in better agreement with the observed amplitude and phasing of the variations in atmospheric O-2/N-2 ratios because of seasonal air-sea exchanges at baseline stations in the Pacific Ocean than with previous air-sea O-2 climatologies. Our study indicates that the component of the air-sea O-2 flux that correlates with heat flux dominates the large-scale air-sea O-2 exchange on seasonal timescales. The contribution of each major oceanic basin to the atmospheric observations is described. The seasonal net thermal (SNOT) and biological (SNOB) outgassing components of the flux are examined in relation to latitudinal bands, basin-wide, and hemispheric contributions. The Southern Hemisphere's SNOB (similar to0.26 Pmol) and SNOT (similar to0.29 Pmol) values are larger than the Northern Hemisphere's SNOB (similar to0.15 Pmol) and SNOT (similar to0.16 Pmol) values (1 Pmol = 10(15) mol). We estimate a global extratropical carbon new production during the outgassing season of 3.7 Pg C (1 Pg = 10(15) g), lower than previous estimates with air-sea O-2 climatologies.

Keeling, RF, Visbeck M.  2011.  On the linkage between Antarctic surface water stratification and global deep-water temperature. Journal of Climate. 24:3545-3557.   10.1175/2011jcli3642.1   AbstractWebsite

The suggestion is advanced that the remarkably low static stability of Antarctic surface waters may arise from a feedback loop involving global deep-water temperatures. If deep-water temperatures are too warm, this promotes Antarctic convection, thereby strengthening the inflow of Antarctic Bottom Water into the ocean interior and cooling the deep ocean. If deep waters are too cold, this promotes Antarctic stratification allowing the deep ocean to warm because of the input of North Atlantic Deep Water. A steady-state deep-water temperature is achieved such that the Antarctic surface can barely undergo convection. A two-box model is used to illustrate this feedback loop in its simplest expression and to develop basic concepts, such as the bounds on the operation of this loop. The model illustrates the possible dominating influence of Antarctic upwelling rate and Antarctic freshwater balance on global deep-water temperatures.

Keeling, RF, Manning AC, Paplawsky WJ, Cox AC.  2007.  On the long-term stability of reference gases for atmospheric O2/N2 and CO2 measurements. Tellus Series B-Chemical and Physical Meteorology. 59:3-14.   10.1111/j.1600-0889.2006.00228.x   AbstractWebsite

Measurements of changes in the atmospheric O-2/N-2 ratio have typically relied on compressed air derived from high-pressure tanks as the reference material against which atmospheric changes are assessed. The validity of this procedure is examined here in the context of the history of 18 O-2/N-2 reference tanks compared over a 12-yr time-frame. By considering differences in tank sizes, material types, and by performing additional tests, the long-term stability of the delivered gas is evaluated with respect to surface reactions, leakage, regulator effects, and thermal diffusion and gravimetric fractionation. Results are also reported for the stability of CO2 in these tanks. The results emphasize the importance of orienting tanks horizontally within a thermally insulated enclosure to reduce thermal and gravimetric fractionation of both O-2/N-2 and CO2 concentrations, and they emphasize the importance of avoiding elastomeric O-rings at the head-valve base. With the procedures documented here, the long-term drift in O-2/N-2 appears to be zero to within approximately +/- 0.4 per meg yr(-1), which projects to an uncertainty of +/- 0.16 Pg C yr(-1) (1 sigma) in O-2-based global carbon budgets.

Manizza, M, Keeling RF, Nevison CD.  2012.  On the processes controlling the seasonal cycles of the air-sea fluxes of O2 and N2O: A modelling study. Tellus Series B-Chemical and Physical Meteorology. 64   10.3402/tellusb.v64i0.18429   AbstractWebsite

The seasonal dynamics of the air-sea gas flux of oxygen (O-2) are controlled by multiple processes occurring simultaneously. Previous studies showed how to separate the thermal component from the total O-2 flux to quantify the residual oxygen flux due to biological processes. However, this biological signal includes the effect of both net euphotic zone production (NEZP) and subsurface water ventilation. To help understand and separate these two components, we use a large-scale ocean general circulation model (OGCM), globally configured, and coupled to a biogeochemical model. The combined model implements not only the oceanic cycle of O-2 but also the cycles of nitrous oxide (N2O), argon (Ar) and nitrogen (N-2). For this study, we apply a technique to distinguish the fluxes of O-2 driven separately by thermal forcing, NEZP, and address the role of ocean ventilation by carrying separate O-2 components in the model driven by solubility, NEZP and ventilation. Model results show that the ventilation component can be neglected in summer compared to the production and thermal components polewards but not equatorward of 30 degrees in each hemisphere. This also implies that neglecting the role of ventilation in the subtropical areas would lead to overestimation of the component of O-2 flux due to NEZP by 20-30%. Model results also show that the ventilation components of air-sea O-2 and N2O fluxes are strongly anti-correlated in a ratio that reflects the subsurface tracer/tracer relationships (similar to 0.1 mmol N2O/mol O-2) as derived from observations. The results support the use of simple scaling relationships linking together the thermally driven fluxes of Ar, N-2 and O-2. Furthermore, our study also shows that for latitudes polewards of 30 degrees of both hemispheres, the Garcia and Keeling (2001) climatology, when compared to our model results, has a phasing error with the fluxes being too early by similar to 2-3 weeks.

Keeling, RF.  1993.  On the role of large bubbles in air-sea gas exchange and supersaturation in the ocean. Journal of Marine Research. 51:237-271.   10.1357/0022240933223800   AbstractWebsite

A parameterization of bubble-induced gas exchange is presented in which the bubble contribution to gas exchange is expressed in terms of separate transfer velocities for ingassing (K(b)in) and outgassing (K(b)out). The difference between the ingassing and outgassing velocities (K(b)in - K(b)out) is further separated into two components, the first caused by the injection of small bubbles into the water, the second caused by gas exchange across the surface of hydrostatically compressed larger bubbles. It is argued that both K(b)out and the exchange contribution to the difference K(b)in - K(b)out should be largely independent of the dissolved concentrations of the major gases N2 and O2. A simple model is presented which allows K(b)out and the exchange contribution to the difference K(b)in - K(b)out to be estimated. The model incorporates data from laboratory simulation experiments on the bubble production spectrum. The results indicate that bubbles larger than 0.05 cm in radius, which have often been assumed to play a negligible role, contribute significantly to bubble-induced gas exchange and supersaturation in the ocean. The model is used to explore the sensitivity of bubble-induced gas exchange to the overall air entrainment rate, size and depth distributions of the bubbles, and to the gas exchange rates across the surface of individual bubbles. The model suggests that bubbles may make an important contribution to overall gas exchange at windspeeds above 10 m sec-1. In this regime gas transfer velocities should depend, not just on diffusivity, but also on the solubility of the gases. It is suggested that K(b)(out) should scale roughly as alpha-0.3D0.35 where a is the solubility and D is the diffusivity. The model results, in combination with measurements on inert gas supersaturations, suggest that the global-mean supersaturation of CO2 induced by bubbles is not larger than 0.3% and most probably is around 0.08%. A major uncertainty results from a lack of information on production rates and distributions of large bubbles. Several possible experiments are proposed for improving estimates of bubble-induced gas exchange and supersaturation.

Lueker, TJ, Keeling RF, Dubey MK.  2001.  The oxygen to carbon dioxide ratios observed in emissions from a wildfire in Northern California. Geophysical Research Letters. 28:2413-2416.   10.1029/2000gl011860   AbstractWebsite

At Trinidad, California we observed elevated CO2 concentrations and concomitant lowered O-2 levels coincident with forest fires 70 kin distant (from 10/8/99 to 10/21/99). The precision of our O-2 data, 1 mu mol O-2 /mol dry air, revealed the reduction of atmospheric oxygen resulting from the combustion of biomass, and the stoichiometric ratios (-O-2/CO2) of the wildfire emissions. Estimates of daily -O-2/CO2 ratios were obtained by regression of CO2 against corresponding O-2 data (R-2, 0.86 to 0.96). Daily -O-2/CO2 ratios changed from 1.15 to 1.41 on a particularly smoky day that coincided with elevated levels of CH4 and increased CH4/CO2 ratios. The change to a higher ratio during smoky conditions illustrates the association between changing emissions and -O-2/CO2 ratios, possibly due to changing wildfire dynamics.