Publications

Export 111 results:
Sort by: [ Author  (Desc)] Title Type Year
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
Y
Yver, CE, Graven HD, Lucas DD, Cameron-Smith PJ, Keeling RF, Weiss RF.  2013.  Evaluating transport in the WRF model along the California coast. Atmospheric Chemistry and Physics. 13:1837-1852.   10.5194/acp-13-1837-2013   AbstractWebsite

This paper presents a step in the development of a top-down method to complement the bottom-up inventories of halocarbon emissions in California using high frequency observations, forward simulations and inverse methods. The Scripps Institution of Oceanography high-frequency atmospheric halocarbons measurement sites are located along the California coast and therefore the evaluation of transport in the chosen Weather Research Forecast (WRF) model at these sites is crucial for inverse modeling. The performance of the transport model has been investigated by comparing the wind direction and speed and temperature at four locations using aircraft weather reports as well at all METAR weather stations in our domain for hourly variations. Different planetary boundary layer (PBL) schemes, horizontal resolutions (achieved through nesting) and two meteorological datasets have been tested. Finally, simulated concentration of an inert tracer has been briefly investigated. All the PBL schemes present similar results that generally agree with observations, except in summer when the model sea breeze is too strong. At the coarse 12 km resolution, using ERA-interim (ECMWF Re-Analysis) as initial and boundary conditions leads to improvements compared to using the North American Model (NAM) dataset. Adding higher resolution nests also improves the match with the observations. However, no further improvement is observed from increasing the nest resolution from 4 km to 0.8 km. Once optimized, the model is able to reproduce tracer measurements during typical winter California large-scale events (Santa Ana). Furthermore, with the WRF/CHEM chemistry module and the European Database for Global Atmospheric Research (EDGAR) version 4.1 emissions for HFC-134a, we find that using a simple emission scaling factor is not sufficient to infer emissions, which highlights the need for more complex inversions.

Yadav, V, Duren R, Mueller K, Verhulst KR, Nehrkorn T, Kim J, Weiss RF, Keeling R, Sander S, Fischer ML, Newman S, Falk M, Kuwayama T, Hopkins F, Rafiq T, Whetstone J, Miller C.  2019.  Spatio-temporally resolved methane fluxes from the Los Angeles megacity. Journal of Geophysical Research-Atmospheres. 124:5131-5148.   10.1029/2018jd030062   AbstractWebsite

We combine sustained observations from a network of atmospheric monitoring stations with inverse modeling to uniquely obtain spatiotemporal (3-km, 4-day) estimates of methane emissions from the Los Angeles megacity and the broader South Coast Air Basin for 2015-2016. Our inversions use customized and validated high-fidelity meteorological output from Weather Research Forecasting and Stochastic Time-Inverted Lagrangian model for South Coast Air Basin and innovatively employ a model resolution matrix-based metric to disentangle the spatiotemporal information content of observations as manifested through estimated fluxes. We partially track and constrain fluxes from the Aliso Canyon natural gas leak and detect closure of the Puente Hills landfill, with no prior information. Our annually aggregated fluxes and their uncertainty excluding the Aliso Canyon leak period lie within the uncertainty bounds of the fluxes reported by the previous studies. Spatially, major sources of CH4 emissions in the basin were correlated with CH4-emitting infrastructure. Temporally, our findings show large seasonal variations in CH4 fluxes with significantly higher fluxes in winter in comparison to summer months, which is consistent with natural gas demand and anticorrelated with air temperature. Overall, this is the first study that utilizes inversions to detect both enhancement (Aliso Canyon leak) and reduction (Puente Hills) in CH4 fluxes due to the unintended events and policy decisions and thereby demonstrates the utility of inverse modeling for identifying variations in fluxes at fine spatiotemporal resolution.

W
Welp, LR, Keeling RF, Meijer HAJ, Bollenbacher AF, Piper SC, Yoshimura K, Francey RJ, Allison CE, Wahlen M.  2011.  Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Nino. Nature. 477:579-582.   10.1038/nature10421   AbstractWebsite

The stable isotope ratios of atmospheric CO2 (O-18/O-16 and C-13/C-12) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way(1). Interpreting the O-18/O-16 variability has proved difficult, however, because oxygen isotopes in CO2 are influenced by both the carbon cycle and the water cycle(2). Previous attention focused on the decreasing O-18/O-16 ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration(3); a global increase in C-4 crops at the expense of C-3 forests(4); and environmental conditions, such as atmospheric turbulence(5) and solar radiation(6), that affect CO2 exchange between leaves and the atmosphere. Here we present 30 years' worth of data on O-18/O-16 in CO2 from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Nino/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Nino increases the O-18/O-16 ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO2 by biosphere-atmosphere gas exchange. We show how the decay time of the El Nino anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Nino events, implying a shorter cycling time of CO2 with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year(7), may be too low, and that a best guess of 150-175 petagrams of carbon per year better reflects the observed rapid cycling of CO2. Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.

Welp, LR, Patra PK, Rodenbeck C, Nemani R, Bi J, Piper SC, Keeling RF.  2016.  Increasing summer net CO2 uptake in high northern ecosystems inferred from atmospheric inversions and comparisons to remote-sensing NDVI. Atmospheric Chemistry and Physics. 16:9047-9066.   10.5194/acp-16-9047-2016   AbstractWebsite

Warmer temperatures and elevated atmospheric CO2 concentrations over the last several decades have been credited with increasing vegetation activity and photosynthetic uptake of CO2 from the atmosphere in the high northern latitude ecosystems: the boreal forest and arctic tundra. At the same time, soils in the region have been warming, permafrost is melting, fire frequency and severity are increasing, and some regions of the boreal forest are showing signs of stress due to drought or insect disturbance. The recent trends in net carbon balance of these ecosystems, across heterogeneous disturbance patterns, and the future implications of these changes are unclear. Here, we examine CO2 fluxes from northern boreal and tundra regions from 1985 to 2012, estimated from two atmospheric inversions (RIGC and Jena). Both used measured atmospheric CO2 concentrations and wind fields from interannually variable climate reanalysis. In the arctic zone, the latitude region above 60 degrees N excluding Europe (10 degrees W-63 degrees E), neither inversion finds a significant long-term trend in annual CO2 balance. The boreal zone, the latitude region from approximately 50-60 degrees N, again excluding Europe, showed a trend of 8-11 Tg C yr(-2) over the common period of validity from 1986 to 2006, resulting in an annual CO2 sink in 2006 that was 170-230 Tg C yr(-1) larger than in 1986. This trend appears to continue through 2012 in the Jena inversion as well. In both latitudinal zones, the seasonal amplitude of monthly CO2 fluxes increased due to increased uptake in summer, and in the arctic zone also due to increased fall CO2 release. These findings suggest that the boreal zone has been maintaining and likely increasing CO2 sink strength over this period, despite browning trends in some regions and changes in fire frequency and land use. Meanwhile, the arctic zone shows that increased summer CO2 uptake, consistent with strong greening trends, is offset by increased fall CO2 release, resulting in a net neutral trend in annual fluxes. The inversion fluxes from the arctic and boreal zones covering the permafrost regions showed no indication of a large-scale positive climate-carbon feedback caused by warming temperatures on high northern latitude terrestrial CO2 fluxes from 1985 to 2012.

Welp, LR, Keeling RF, Weiss RF, Paplawsky W, Heckman S.  2013.  Design and performance of a Nafion dryer for continuous operation at CO2 and CH4 air monitoring sites. Atmos. Meas. Tech.. 6:1217-1226.: Copernicus Publications   10.5194/amt-6-1217-2013   AbstractWebsite

In preparation for routine deployment in a network of greenhouse gas monitoring stations, we have designed and tested a simple method for drying ambient air to near or below 0.2% (2000 ppm) mole fraction H2O using a Nafion dryer. The inlet system was designed for use with cavity ring-down spectrometer (CRDS) analyzers such as the Picarro model G2301 that measure H2O in addition to their principal analytes, in this case CO2 and CH4. These analyzers report dry-gas mixing ratios without drying the sample by measuring H2O mixing ratio at the same frequency as the main analytes, and then correcting for the dilution and peak broadening effects of H2O on the mixing ratios of the other analytes measured in moist air. However, it is difficult to accurately validate the water vapor correction in the field. By substantially lowering the amount of H2O in the sample, uncertainties in the applied water vapor corrections can be reduced by an order of magnitude or more, thus eliminating the need to determine instrument-specific water vapor correction coefficients and to verify the stability over time. Our Nafion drying inlet system takes advantage of the extra capacity of the analyzer pump to redirect 30% of the dry gas exiting the Nafion to the outer shell side of the dryer and has no consumables. We tested the Nafion dryer against a cryotrap (−97 °C) method for removing H2O and found that in wet-air tests, the Nafion reduces the CO2 dry-gas mixing ratios of the sample gas by as much as 0.1 ± 0.01 ppm due to leakage across the membrane. The effect on CH4 was smaller and varied within ± 0.2 ppb, with an approximate uncertainty of 0.1 ppb. The Nafion-induced CO2 bias is partially offset by sending the dry reference gases through the Nafion dryer as well. The residual bias due to the impact of moisture differences between sample and reference gas on the permeation through the Nafion was approximately −0.05 ppm for CO2 and varied within ± 0.2 ppb for CH4. The uncertainty of this partial drying method is within the WMO compatibility guidelines for the Northern Hemisphere, 0.1 ppm for CO2 and 2 ppb for CH4, and is comparable to experimentally determining water vapor corrections for each instrument but less subject to concerns of possible drift in these corrections.

Wagner, TJW, Dell RW, Eisenman I, Keeling RF, Padman L, Severinghaus JP.  2018.  Wave inhibition by sea ice enables trans-Atlantic ice rafting of debris during Heinrich events. Earth and Planetary Science Letters. 495:157-163.   10.1016/j.epsl.2018.05.006   AbstractWebsite

The last glacial period was punctuated by episodes of massive iceberg calving from the Laurentide Ice Sheet, called Heinrich events, which are identified by layers of ice-rafted debris (IRD) in ocean sediment cores from the North Atlantic. The thickness of these IRD layers declines more gradually with distance from the iceberg sources than would be expected based on present-day iceberg drift and decay. Here we model icebergs as passive Lagrangian particles driven by ocean currents, winds, and sea surface temperatures. The icebergs are released in a comprehensive climate model simulation of the last glacial maximum (LGM), as well as a simulation of the modern climate. The two simulated climates result in qualitatively similar distributions of iceberg meltwater and hence debris, with the colder temperatures of the LGM having only a relatively small effect on meltwater spread. In both scenarios, meltwater flux falls off rapidly with zonal distance from the source, in contrast with the more uniform spread of IRD in sediment cores. To address this discrepancy, we propose a physical mechanism that could have prolonged the lifetime of icebergs during Heinrich events. The mechanism involves a surface layer of cold and fresh meltwater formed from, and retained around, large densely packed armadas of icebergs. This leads to wintertime sea ice formation even in relatively low latitudes. The sea ice in turn shields the icebergs from wave erosion, which is the main source of iceberg ablation. We find that sea ice could plausibly have formed around the icebergs during four months each winter. Allowing for four months of sea ice in the model results in a simulated IRD distribution which approximately agrees with the distribution of IRD in sediment cores. (C) 2018 Elsevier B.V. All rights reserved.

V
Volk, T, Keeling R.  1993.  Summary of workshop on interannual variations in the carbon cycle. The Global carbon cycle. ( Heimann M, Ed.).:579-581., Berlin; New York: Springer-Verlag Abstract
n/a
Verhulst, KR, Karion A, Kim J, Salameh PK, Keeling RF, Newman S, Miller J, Sloop C, Pongetti T, Rao P, Wong C, Hopkins FM, Yadav V, Weiss RF, Duren RM, Miller CE.  2017.  Carbon dioxide and methane measurements from the Los Angeles Megacity Carbon Project - Part 1: calibration, urban enhancements, and uncertainty estimates. Atmospheric Chemistry and Physics. 17:8313-8341.   10.5194/acp-17-8313-2017   AbstractWebsite

We report continuous surface observations of carbon dioxide (CO2) and methane (CH4) from the Los Angeles (LA) Megacity Carbon Project during 2015. We devised a calibration strategy, methods for selection of background air masses, calculation of urban enhancements, and a detailed algorithm for estimating uncertainties in urban-scale CO2 and CH4 measurements. These methods are essential for understanding carbon fluxes from the LA megacity and other complex urban environments globally. We estimate background mole fractions entering LA using observations from four "extra-urban" sites including two "marine" sites located south of LA in La Jolla (LJO) and offshore on San Clemente Island (SCI), one "continental" site located in Victorville (VIC), in the high desert northeast of LA, and one "continental/mid-troposphere" site located on Mount Wilson (MWO) in the San Gabriel Mountains. We find that a local marine background can be established to within similar to 1 ppm CO2 and similar to 10 ppb CH4 using these local measurement sites. Overall, atmospheric carbon dioxide and methane levels are highly variable across Los Angeles. "Urban" and "suburban" sites show moderate to large CO2 and CH4 enhancements relative to a marine background estimate. The USC (University of Southern California) site near downtown LA exhibits median hourly enhancements of similar to 20 ppm CO2 and similar to 150 ppb CH4 during 2015 as well as similar to 15 ppm CO2 and similar to 80 ppb CH4 during mid-afternoon hours (12:00-16:00 LT, local time), which is the typical period of focus for flux inversions. The estimated measurement uncertainty is typically better than 0.1 ppm CO2 and 1 ppb CH4 based on the repeated standard gas measurements from the LA sites during the last 2 years, similar to Andrews et al. (2014). The largest component of the measurement uncertainty is due to the single-point calibration method; however, the uncertainty in the background mole fraction is much larger than the measurement uncertainty. The background uncertainty for the marine background estimate is similar to 10 and similar to 15% of the median mid-afternoon enhancement near downtown LA for CO2 and CH4, respectively. Overall, analytical and background uncertainties are small relative to the local CO2 and CH4 enhancements; however, our results suggest that reducing the uncertainty to less than 5% of the median mid-afternoon enhancement will require detailed assessment of the impact of meteorology on background conditions.

T
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.

Tanhua, T, Keeling RF.  2012.  Changes in column inventories of carbon and oxygen in the Atlantic Ocean. Biogeosciences. 9:4819-4833.   10.5194/bg-9-4819-2012   AbstractWebsite

Increasing concentrations of dissolved inorganic carbon (DIC) in the interior ocean are expected as a direct consequence of increasing concentrations of CO2 in the atmosphere. This extra DIC is often referred to as anthropogenic carbon (C-ant), and its inventory, or increase rate, in the interior ocean has previously been estimated by a multitude of observational approaches. Each of these methods is associated with hard to test assumptions since C-ant cannot be directly observed. Results from a simpler concept with fewer assumptions applied to the Atlantic Ocean are reported on here using two large data collections of carbon relevant bottle data. The change in column inventory on decadal time scales, i.e. the storage rate, of DIC, respiration compensated DIC and oxygen is calculated for the Atlantic Ocean. We report storage rates and the confidence intervals of the mean trend at the 95% level (CI), reflecting the mean trend but not considering potential biasing effects of the spatial and temporal sampling. For the whole Atlantic Ocean the mean trends for DIC and oxygen are non-zero at the 95% confidence level: DIC: 0.86 (CI: 0.72-1.00) and oxygen: -0.24 (CI: -0.41-(-0.07)) mol m(-2) yr(-1). For oxygen, the whole Atlantic trend is dominated by the subpolar North Atlantic, whereas for other regions the O-2 trends are not significant. The storage rates are similar to changes found by other studies, although with large uncertainty. For the subpolar North Atlantic the storage rates show significant temporal and regional variation of all variables. This seems to be due to variations in the prevalence of subsurface water masses with different DIC and oxygen concentrations leading to sometimes different signs of storage rates for DIC compared to published C-ant estimates. This study suggest that accurate assessment of the uptake of CO2 by the oceans will require accounting not only for processes that influence C-ant but also additional processes that modify CO2 storage.

S
Sundquist, ET, Keeling RF.  2009.  The Mauna Loa carbon dioxide record: lessons for long-term earth observations. Carbon sequestration and its role in the global carbon cycle. ( McPherson BJ, Sundquist ET, Eds.).:27-35., Washington, DC: American Geophysical Union Abstract

"For carbon sequestration, the issues of monitoring, assessment and verification of carbon content and storage efficacy are perhaps the most uncertain yet most critical issues facing the broader context of climate change in relation to carbon sequestration. As a result, this book presents current perspectives and research that combine four major areas: verification and assessment of global carbon sources and sinks; potential capacity and temporal/spatial scales of terrestrial, oceanic, and geologic carbon storage; assessing risks and benefits associated with terrestrial, oceanic, and geologic carbon storage; and predicting, monitoring, and verifying effectiveness of terrestrial, oceanic and geologic carbon storage. This volume is based on a Chapman conference and will appeal to the rapidly growing group of scientists and engineers in examining methods for deliberate carbon sequestration through storage in plants, soils, the oceans, and geological repositories."--Publisher's description.

Stephens, BB, Wofsy SC, Keeling RF, Tans PP.  2000.  The CO2 budget and rectification airborne study. Inverse methods in global biogeochemical cycles. ( Kasibhatla P, Ed.).:311-324., Washington, DC: American Geophysical Union Abstract

The CD-ROM contains the code and data files for the Exercises outlined in the paper by Rayner, et at., (p. 81-106).

Stephens, BB, Keeling RF, Heimann M, Six KD, Murnane R, Caldeira K.  1998.  Testing global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration. Global Biogeochemical Cycles. 12:213-230.   10.1029/97gb03500   AbstractWebsite

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O-2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO approximate to O-2 + CO2), which is conservative with respect to terrestrial photosynthesis and respiration. We then compare observations of APO to the simulations of an atmospheric transport model which uses ocean-model air-sea fluxes and fossil fuel combustion estimates as lower boundary conditions. We present observations of the annual-average concentrations of CO2, O-2, and APO at 10 stations in a north-south transect. The observations of APO show a significant interhemispheric gradient decreasing towards the north. We use air-sea CO2, O-2, and N-2 fluxes from the Princeton ocean biogeochemistry model, the Hamburg model of the ocean carbon cycle, and the Lawrence Livermore ocean biogeochemistry model to drive the TM2 atmospheric transport model. The latitudinal variations in annual-average APO predicted by the combined models are distinctly different from the observations. All three models significantly underestimate the interhemispheric difference in APO, suggesting that they underestimate the net southward transport of the sum of O-2 and CO2 in the oceans. Uncertainties in the model-observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the physical and biological components of the ocean models. Potential deficiencies in the physical components of the ocean models, which have previously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the APO observations. These deficiencies include the inadequate parameterization of subgrid-scale isopycnal eddy mixing, a lack of subgrid-scale vertical convection, too much Antarctic sea-ice formation, and an overestimation of vertical diffusivities in the main thermocline.

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.

Stephens, BB, Keeling RF.  2000.  The influence of Antarctic sea ice on glacial-interglacial CO2 variations. Nature. 404:171-174.   10.1038/35004556   AbstractWebsite

Ice-core measurements indicate that atmospheric CO(2) concentrations during glacial periods were consistently about 80 parts per million lower than during interglacial periods(1). Previous explanations for this observation(2-9) have typically had difficulty accounting for either the estimated glacial O(2) concentrations in the deep sea, (13)C/(12)C ratios in Antarctic surface waters, or the depth of calcite saturation; also lacking is an explanation for the strong link between atmospheric CO(2) and Antarctic air temperature(1). There is growing evidence that the amount of deep water upwelling at low latitudes is significantly overestimated in most ocean general circulation models(10,11) and simpler box models previously used to investigate this problem. Here we use a box model with deep-water upwelling confined to south of 55 degrees S to investigate the glacial-interglacial linkages between Antarctic air temperature and atmospheric CO(2) variations. We suggest that low glacial atmospheric CO(2) levels might result from reduced deep-water ventilation associated with either year-round Antarctic sea-ice coverage, or wintertime coverage combined with ice-induced stratification during the summer. The model presented here reproduces 67 parts per million of the observed glacial-interglacial CO(2) difference, as a result of reduced air-sea gas exchange in the Antarctic region, and is generally consistent with the additional observational constraints.

Stephens, BB, Keeling RF, Paplawsky WJ.  2003.  Shipboard measurements of atmospheric oxygen using a vacuum-ultraviolet absorption technique. Tellus Series B-Chemical and Physical Meteorology. 55:857-878.   10.1046/j.1435-6935.2003.00075.x   AbstractWebsite

We have developed an instrument for making continuous, field-based, part-per-million (ppm) level measurements of atmospheric oxygen concentration, and have implemented it on research cruises in the equatorial Pacific and Southern Oceans. The instrument detects changes in oxygen by the absorption of vacuum ultraviolet (VUV) radiation as it passes through a flowing gas stream, and has a precision comparable to existing laboratory techniques. Here we describe the VUV instrument and present atmospheric O-2 and CO2 data collected from the NOAA ship Ka' imimoana in the equatorial Pacific during April and May of 1998, and from the NSF ship Lawrence M. Gould in the Southern Ocean during October 1998. These data represent the first field-based measurements of atmospheric O-2, and significant additions to the O-2 datasets in these regions. Our boreal-springtime equatorial measurements reveal significant short-term variations in atmospheric O-2, resulting from variations in atmospheric mixing relative to the strong interhemispheric gradient. Our austral-springtime Southern Ocean observations confirm the low O-2 concentrations seen in flask samples from this region, allow the separate identification of oceanic and industrial influences on CO2, and provide evidence of a Southern Ocean source for CO2 at this time of year. These shipboard VUV observations do not provide any evidence to support coupled ocean-atmosphere model predictions of a large decreasing atmospheric O-2 gradient between equatorial and high-southern latitudes.

Staudigel, H, Albarede F, Blichert-Toft J, Edmond J, McDonough B, Jacobsen SB, Keeling R, Langmuir CH, Nielsen RL, Plank T, Rudnick R, Shaw HF, Shirey S, Veizer J, White W.  1998.  Geochemical Earth Reference Model (GERM): description of the initiative. Chemical Geology. 145:153-159.   10.1016/S0009-2541(97)00141-1   Abstract

The Geochemical Earth Reference Model (GERM) initiative is a grass root effort with the goals of establishing a community consensus on a chemical characterization of the Earth, its major reservoirs, and the flu?;es between them. The GERM initiative will provide a review of available scientific constraints for: (1) the composition of all major chemical reservoirs of the present-day Earth, from core to atmosphere; (2) present-day fluxes between reservoirs; (3) the Earth's chemical and isotopic evolution since accretion; and (4) the chemical and isotopic evolution of seawater as a record of global tectonics and climate, Even though most of the constraints for the GERM will be drawn from chemical data sets, some data will have to come from other disciplines, such as geophysics, nuclear physics, and cosmochemistry. GERM also includes a diverse chemical and physical data base and computer codes that are useful for our understanding of how the Earth works as a dynamic chemical and physical system. The GERM initiative is developed in an open community discussion on the World Wide Web (http://www-ep.es.llnl.gov/germ/germ-home.html) that is moderated by editors with responsibilities for different reservoirs, fluxes, data bases, and other scientific or technical aspects. These editors have agreed to lay out an initial, strawman GERM for their respective sections and to moderate community discussions leading to a first, preliminary consensus. The development of the GERM began with an initial workshop in Lyon, France in March, 1996. Since then, the GERM has continued to be developed on the Internet, punctuated by workshops and special sessions at professional meetings. A second GERM workshop will be held in La Jolla, CA USA on March 10-13, 1998. (C) 1998 Elsevier Science B.V. All nights reserved.

Severinghaus, JP, Keeling RF, Miller BR, Weiss RF, Deck B, Broecker WS.  1997.  Feasibility of using sand dunes as archives of old air. Journal of Geophysical Research-Atmospheres. 102:16783-16792.   10.1029/97jd00525   AbstractWebsite

Large unaltered samples of the atmosphere covering the past century would complement the history of atmospheric gases obtained from bubbles in ice cores, enabling measurement of geochemically important species such as O-2, (CH4)-C-14, and (CO)-C-14. Sand dunes are a porous media with interstitial air in diffusive contact with the atmosphere, somewhat analogous to the unconsolidated layer of firn atop glaciers. Recent studies have demonstrated the value of firn as an archive of old air [Battle et al., 1996; Bender et al., 1994a]. Unlike firn, sand dunes are incompressible and so remain permeable to greater depths and may extend the firn record into the past century. To evaluate the feasibility of using sand dunes as archives of old air, we drilled 60 m deep test holes in the Algodones Dunes, Imperial Valley, California. The main objective was to see if the air in a sand dune is as old as predicted by a diffusion model, or if the dune is rapidly flushed by advective pumping during windstorms and barometric pressure changes. We dated the air with chlorofluorocarbons and krypton-85, anthropogenic tracers whose atmospheric concentrations are known and have been increasing rapidly in the past half century. These tracer data match the pure diffusion model well, showing that advection in this dune is negligible compared to diffusion as a transport mechanism and that the mean age of the air at 61 m depth is similar to 10 years. Dunes therefore do contain old air. However, dunes appear to suffer from two serious drawbacks as archives. Microbial metabolism is evident in elevated CO2 and N2O and depressed CH4 and O-2 concentrations in this dune, corrupting the signals of interest in this and probably most dunes. Second, isotopic analyses of N-2 and O-2 from the dune show that fractionation of the gases occurs due to diffusion of water vapor, complicating the interpretation of the O-2 signal beyond the point of viability for an air archive. Sand dunes may be useful for relatively inert gases with large atmospheric concentration changes such as chlorofluorocarbons.

Severinghaus, JP, Bender ML, Keeling RF, Broecker WS.  1996.  Fractionation of soil gases by diffusion of water vapor, gravitational settling, and thermal diffusion. Geochimica Et Cosmochimica Acta. 60:1005-1018.   10.1016/0016-7037(96)00011-7   AbstractWebsite

Air sampled from the moist unsaturated zone in a sand dune exhibits depletion in the heavy isotopes of N-2 and O-2. We propose that the depletion is caused by a diffusive flux of water vapor out of the dune, which sweeps out the other gases, forcing them to diffuse back into the dune. The heavy isotopes of N-2 and O-2 diffuse back more slowly, resulting in a steady-state depletion of the heavy isotopes in the dune interior. We predict the effect's magnitude with molecular diffusion theory and reproduce it in a laboratory simulation, finding good agreement between field, theory, and lab. The magnitude of the effect is governed by the ratio of the binary diffusivities against water vapor of a pair of gases, and increases similar to linearly with the difference between the water vapor mole fraction of the site and the advectively mixed reservoir with which it is in diffusive contact (in most cases the atmosphere). The steady-state effect is given by delta(i) = [i/j/i(0)/j(0) - 1] 10(3) parts per thousand congruent to [(1 - x(H2O)/1 - x(H2O0))((Dj-H2O/Di-H2O)-1) -1] 10(3) parts per thousand, where delta(i) is the fractional deviation in permil of the gas i/gas j ratio from the advectively mixed reservoir, x(H2O) and x(H2O0) are respectively the mole fractions of water vapor at the site and in the advectively mixed reservoir, and D-i-H2O is the binary diffusion coefficient of gas i with water vapor. The effect is independent of scale at steady state, but approaches steady state with the time constant of diffusion set by the length scale. Exploiting the mechanism, we make an experimental estimate of the relative diffusivities of O-2 and N-2 against water vapor, finding that O-2 diffuses 3.6 +/- 0.3% faster than N-2 despite its greater mass. We also confirm in the study dune the presence of two additional known processes: gravitational fractionation, heretofore seen only in the unconsolidated firn of polar ice sheets, and thermal diffusion, well described in laboratory studies but not seen previously in nature. We predict that soil gases in general will exhibit the three effects described here, the water vapor flux fractionation effect, gravitational fractionation, and thermal diffusion. However, our analysis neglects Knudsen diffusion and thus may be inapplicable to fine-grained soils.

R
Rodgers, KB, Aumont O, Fletcher SEM, Plancherel Y, Bopp L, Montegut CD, Iudicone D, Keeling RF, Madec G, Wanninkhof R.  2014.  Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring. Biogeosciences. 11:4077-4098.   10.5194/bg-11-4077-2014   AbstractWebsite

Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model, where the objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000-2006, with a relative reduction with wind stirring on the order of 0.9 Pg C yr(-1) over the region south of 45 degrees S. This impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other ocean biogeochemical tracers. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong reduction on the order of 25-30% in the concentrations of NO3 exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry through its impact over the Southern Ocean.

Rodenbeck, C, Zaehle S, Keeling R, Heimann M.  2018.  History of El Nino impacts on the global carbon cycle 1957-2017: a quantification from atmospheric CO2 data. Philosophical Transactions of the Royal Society B-Biological Sciences. 373   10.1098/rstb.2017.0303   AbstractWebsite

Interannual variations in the large-scale net ecosystem exchange (NEE) of CO2 between the terrestrial biosphere and the atmosphere were estimated for 1957-2017 from sustained measurements of atmospheric CO2 mixing ratios. As the observations are sparse in the early decades, available records were combined into a 'quasi-homogeneous' dataset based on similarity in their signals, to minimize spurious variations from beginning or ending data records. During El Nino events, CO2 is anomalously released from the tropical band, and a few months later also in the northern extratropical band. This behaviour can approximately be represented by a linear relationship of the NEE anomalies and local air temperature anomalies, with sensitivity coefficients depending on geographical location and season. The apparent climate sensitivity of global total NEE against variations in pan-tropically averaged annual air temperature slowly changed over time during the 1957-2017 period, first increasing (though less strongly than in previous studies) but then decreasing again. However, only part of this change can be attributed to actual changes in local physiological or ecosystem processes, the rest probably arising from shifts in the geographical area of dominating temperature variations. This article is part of a discussion meeting issue 'The impact of the 2015/2016 El Nino on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.

Rodenbeck, C, Bakker DCE, Metzl N, Olsen A, Sabine C, Cassar N, Reum F, Keeling RF, Heimann M.  2014.  Interannual sea-air CO2 flux variability from an observation-driven ocean mixed-layer scheme. Biogeosciences. 11:4599-4613.   10.5194/bg-11-4599-2014   AbstractWebsite

Interannual anomalies in the sea-air carbon dioxide (CO2) exchange have been estimated from surface-ocean CO2 partial pressure measurements. Available data are sufficient to constrain these anomalies in large parts of the tropical and North Pacific and in the North Atlantic, in some areas covering the period from the mid 1980s to 2011. Global interannual variability is estimated as about 0.31 Pg Cyr(-1) (temporal standard deviation 1993-2008). The tropical Pacific accounts for a large fraction of this global variability, closely tied to El Ni o-Southern Oscillation (ENSO). Anomalies occur more than 6 months later in the east than in the west. The estimated amplitude and ENSO response are roughly consistent with independent information from atmospheric oxygen data. This both supports the variability estimated from surface-ocean carbon data and demonstrates the potential of the atmospheric oxygen signal to constrain ocean biogeochemical processes. The ocean variability estimated from surface-ocean carbon data can be used to improve land CO2 flux estimates from atmospheric inversions.

Rodenbeck, C, Keeling RF, Bakker DCE, Metz N, Olsen A, Sabine C, Heimann M.  2013.  Global surface-ocean p(CO2) and sea-air CO2 flux variability from an observation-driven ocean mixed-layer scheme. Ocean Science. 9:193-216.   10.5194/os-9-193-2013   AbstractWebsite

A temporally and spatially resolved estimate of the global surface-ocean CO2 partial pressure field and the sea air CO2 flux is presented, obtained by fitting a simple data-driven diagnostic model of ocean mixed-layer biogeochemistry to surface-ocean CO2 partial pressure data from the SOCAT v1.5 database. Results include seasonal, interannual, and short-term (daily) variations. In most regions, estimated seasonality is well constrained from the data, and compares well to the widely used monthly climatology by Takahashi et al. (2009). Comparison to independent data tentatively supports the slightly higher seasonal variations in our estimates in some areas. We also fitted the diagnostic model to atmospheric CO2 data. The results of this are less robust, but in those areas where atmospheric signals are not strongly influenced by land flux variability, their seasonality is nevertheless consistent with the results based on surface-ocean data. From a comparison with an independent seasonal climatology of surface-ocean nutrient concentration, the diagnostic model is shown to capture relevant surface-ocean biogeochemical processes reasonably well. Estimated interannual variations will be presented and discussed in a companion paper.

Rodenbeck, C, Zaehle S, Keeling R, Heimann M.  2018.  How does the terrestrial carbon exchange respond to inter-annual climatic variations? A quantification based on atmospheric CO2 data Biogeosciences. 15:2481-2498.   10.5194/bg-15-2481-2018   AbstractWebsite

The response of the terrestrial net ecosystem exchange (NEE) of CO2 to climate variations and trends may crucially determine the future climate trajectory. Here we directly quantify this response on inter-annual timescales by building a linear regression of inter-annual NEE anomalies against observed air temperature anomalies into an atmospheric inverse calculation based on long-term atmospheric CO2 observations. This allows us to estimate the sensitivity of NEE to inter-annual variations in temperature (seen as a climate proxy) resolved in space and with season. As this sensitivity comprises both direct temperature effects and the effects of other climate variables co-varying with temperature, we interpret it as "inter-annual climate sensitivity". We find distinct seasonal patterns of this sensitivity in the northern extratropics that are consistent with the expected seasonal responses of photosynthesis, respiration, and fire. Within uncertainties, these sensitivity patterns are consistent with independent inferences from eddy covariance data. On large spatial scales, northern extratropical and tropical interannual NEE variations inferred from the NEE-T regression are very similar to the estimates of an atmospheric inversion with explicit inter-annual degrees of freedom. The results of this study offer a way to benchmark ecosystem process models in more detail than existing effective global climate sensitivities. The results can also be used to gap-fill or extrapolate observational records or to separate inter-annual variations from longer-term trends.

Rodenbeck, C, Le Quere C, Heimann M, Keeling RF.  2008.  Interannual variability in oceanic biogeochemical processes inferred by inversion of atmospheric O2/N2 and CO2 data. Tellus Series B-Chemical and Physical Meteorology. 60:685-705.   10.1111/j.1600-0889.2008.00375.x   AbstractWebsite

Atmospheric measurements of O(2)/N(2) and CO(2) at up to nine sites have been used to infer the interannual variations in oceanic O(2) exchange with an inverse method. The method distinguishes the regional contributions of three latitudinal bands, partly the individual contributions of the North Pacific and the North Atlantic also. The interannual variations of the inferred O(2) fluxes in the tropical band correlate significantly with the El Nino/Southern Oscillation. Tropical O(2) variations appear to be dominated by the ventilation of the O(2) minimum zone from variations in Pacific equatorial upwelling. The interannual variations of the northern and southern extratropical bands are of similar amplitude, though the attribution to mechanisms is less clear. The interannual variations estimated by the inverse method are larger than those estimated by the current generation of global ocean biogeochemistry models, especially in the North Atlantic, suggesting that the representation of biological processes plays a role. The comparison further suggests that O(2) variability is a more stringent test to validate models than CO(2) variability, because the processes driving O(2) variability combine in the same direction and amplify the underlying climatic signal.