Publications

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2019
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.

2016
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.

Newman, S, Xu XM, Gurney KR, Hsu YK, Li KF, Jiang X, Keeling R, Feng S, O'Keefe D, Patarasuk R, Wong KW, Rao P, Fischer ML, Yung YL.  2016.  Toward consistency between trends in bottom-up CO2 emissions and top-down atmospheric measurements in the Los Angeles megacity. Atmospheric Chemistry and Physics. 16:3843-3863. AbstractWebsite

Large urban emissions of greenhouse gases result in large atmospheric enhancements relative to background that are easily measured. Using CO2 mole fractions and delta C-14 and delta C-13 values of CO2 in the Los Angeles megacity observed in inland Pasadena (2006-2013) and coastal Palos Verdes peninsula (autumn 2009-2013), we have determined time series for CO2 contributions from fossil fuel combustion (C-ff) for both sites and broken those down into contributions from petroleum and/or gasoline and natural gas burning for Pasadena. We find a 10 % reduction in Pasadena C-ff during the Great Recession of 2008-2010, which is consistent with the bottom-up inventory determined by the California Air Resources Board. The isotopic variations and total atmospheric CO2 from our observations are used to infer seasonality of natural gas and petroleum combustion. The trend of CO2 contributions to the atmosphere from natural gas combustion is out of phase with the seasonal cycle of total natural gas combustion seasonal patterns in bottom-up inventories but is consistent with the seasonality of natural gas usage by the area's electricity generating power plants. For petroleum, the inferred seasonality of CO2 contributions from burning petroleum is delayed by several months relative to usage indicated by statewide gasoline taxes. Using the high-resolution Hestia-LA data product to compare C-ff from parts of the basin sampled by winds at different times of year, we find that variations in observed fossil fuel CO2 reflect seasonal variations in wind direction. The seasonality of the local CO2 excess from fossil fuel combustion along the coast, on Palos Verdes peninsula, is higher in autumn and winter than spring and summer, almost completely out of phase with that from Pasadena, also because of the annual variations of winds in the region. Variations in fossil fuel CO2 signals are consistent with sampling the bottom-up Hestia-LA fossil CO2 emissions product for sub-city source regions in the LA megacity domain when wind directions are considered.

Forkel, M, Carvalhais N, Rodenbeck C, Keeling R, Heimann M, Thonicke K, Zaehle S, Reichstein M.  2016.  Enhanced seasonal CO2 exchange caused by amplified plant productivity in northern ecosystems. Science. 351:696-699.   10.1126/science.aac4971   AbstractWebsite

Atmospheric monitoring of high northern latitudes (above 40 degrees N) has shown an enhanced seasonal cycle of carbon dioxide (CO2) since the 1960s, but the underlying mechanisms are not yet fully understood. The much stronger increase in high latitudes relative to low ones suggests that northern ecosystems are experiencing large changes in vegetation and carbon cycle dynamics. We found that the latitudinal gradient of the increasing CO2 amplitude is mainly driven by positive trends in photosynthetic carbon uptake caused by recent climate change and mediated by changing vegetation cover in northern ecosystems. Our results underscore the importance of climate-vegetation-carbon cycle feedbacks at high latitudes; moreover, they indicate that in recent decades, photosynthetic carbon uptake has reacted much more strongly to warming than have carbon release processes.

2013
Rafelski, LE, Paplawsky B, Keeling RF.  2013.  An Equilibrator System to Measure Dissolved Oxygen and Its Isotopes. Journal of Atmospheric and Oceanic Technology. 30:361-377.   10.1175/jtech-d-12-00074.1   AbstractWebsite

An equilibrator is presented that is designed to have a sufficient equilibration time even for insoluble gases, and to minimize artifacts associated with not equilibrating to the total gas tension. A gas tension device was used to balance the pressure inside the equilibrator with the total gas tension. The equilibrator has an e-folding time of 7.36 +/- 0.74 min for oxygen and oxygen isotopes, allowing changes on hourly time scales to be easily resolved. The equilibrator delivers "equilibrated" air at a flow rate of 3 mL min(-1) to an isotope ratio mass spectrometer. The high gas sampling flow rate would allow the equilibrator to be interfaced with many potential devices, but further development may be required for use at sea. This system was tested at the Scripps Institution of Oceanography pier, in La Jolla, California. A mathematical model validated with performance tests was used to assess the sensitivity of the equilibrated air composition to headspace pressure and makeup gas composition. Parameters in this model can be quantified to establish corrections under different operating conditions. For typical observed values, under the operating conditions presented here, the uncertainty in the measurement due to the equilibrator system is 2.2 per mil for delta(O-2/N-2), 1.5 per mil for delta(O-2/Ar), 0.059 per mil for delta O-18, and 0.0030 per mil for Delta O-17.

2012
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.

2011
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.

Manning, AC, Nisbet EG, Keeling RF, Liss PS.  2011.  Greenhouse gases in the Earth system: setting the agenda to 2030. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 369:1885-1890.   10.1098/rsta.2011.0076   AbstractWebsite

What do we need to know about greenhouse gases? Over the next 20 years, how should scientists study the role of greenhouse gases in the Earth system and the changes that are taking place? These questions were addressed at a Royal Society scientific Discussion Meeting in London on 22-23 February 2010, with over 300 participants.

Keeling, CD, Piper SC, Whorf TP, Keeling RF.  2011.  Evolution of natural and anthropogenic fluxes of atmospheric CO2 from 1957 to 2003. Tellus Series B-Chemical and Physical Meteorology. 63:1-22.   10.1111/j.1600-0889.2010.00507.x   AbstractWebsite

An analysis is carried out of the longest available records of atmospheric CO(2) and its 13C/12C ratio from the Scripps Institution of Oceanography network of fixed stations, augmented by data in the 1950s and 1960s from ships and ice floes. Using regression analysis, we separate the interhemispheric gradients of CO(2) and 13C/12C into: (1) a stationary (possibly natural) component that is constant with time, and (2) a time-evolving component that increases in proportion to fossil fuel emissions. Inverse calculations using an atmospheric transport model are used to interpret the components of the gradients in terms of land and ocean sinks. The stationary gradients in CO(2) and 13C/12C are both satisfactorily explained by ocean processes, including an ocean carbon loop that transports 0.5 PgC yr-1 southwards in the ocean balanced by an atmospheric return flow. A stationary northern land sink appears to be ruled out unless its effect on the gradient has been offset by a strong rectifier effect, which seems doubtful. A growing northern land sink is not ruled out, but has an uncertain magnitude (0.3-1.7 PgC yr-1 centred on year 2003) dependent on the rate at which CO(2) from fossil fuel burning is dispersed vertically and between hemispheres.

2009
Graven, HD, Stephens BB, Guilderson TP, Campos TL, Schimel DS, Campbell JE, Keeling RF.  2009.  Vertical profiles of biospheric and fossil fuel-derived CO2 and fossil fuel CO2: CO ratios from airborne measurements of Δ14C, CO2 and CO above Colorado, USA. Tellus Series B-Chemical and Physical Meteorology. 61:536-546.   10.1111/j.1600-0889.2009.00421.x   AbstractWebsite

Measurements of Delta C-14 in atmospheric CO2 are an effective method of separating CO2 additions from fossil fuel and biospheric sources or sinks of CO2. We illustrate this technique with vertical profiles of CO2 and Delta C-14 analysed in whole air flask samples collected above Colorado, USA in May and July 2004. Comparison of lower tropospheric composition to cleaner air at higher altitudes (>5 km) revealed considerable additions from respiration in the morning in both urban and rural locations. Afternoon concentrations were mainly governed by fossil fuel emissions and boundary layer depth, also showing net biospheric CO2 uptake in some cases. We estimate local industrial CO2: CO emission ratios using in situ measurements of CO concentration. Ratios are found to vary by 100% and average 57 mole CO2:1 mole CO, higher than expected from emissions inventories. Uncertainty in CO2 from different sources was +/- 1.1 to +/- 4.1 ppm for addition or uptake of -4.6 to 55.8 ppm, limited by Delta 14C measurement precision and uncertainty in background Delta C-14 and CO2 levels.

2008
1999
Langenfelds, RL, Francey RJ, Steele LP, Battle M, Keeling RF, Budd WF.  1999.  Partitioning of the global fossil CO2 sink using a 19-year trend in atmospheric O2. Geophysical Research Letters. 26:1897-1900.   10.1029/1999gl900446   AbstractWebsite

O-2/N-2 is measured in the Cape Grim Air Archive (CGAA), a suite of tanks filled with background air at Cape Grim, Tasmania (40.7 degrees S, 144.8 degrees E) between April 1978 and January 1997. Derived trends are compared with published O-2/N-2 records and assessed against limits on interannual variability of net terrestrial exchanges imposed by trends of delta(13)C in CO2. Two old samples from 1978 and 1987 and eight from 1996/97 survive critical selection criteria and give a mean 19-year trend in delta(O-2/N-2) of -16.7 +/- 0.5 per meg y(-1), implying net storage of +2.3 +/- 0.7 GtC (10(15) g carbon) yr(-1) of fossil fuel CO2 in the oceans and +0.2 +/- 0.9 GtC yr(-1) in the terrestrial biosphere. The uptake terms are consistent for both O-2/N-2 and delta(13)C tracers if the mean C-13 isotopic disequilibrium flux, combining terrestrial and oceanic contributions, is 93 +/- 15 GtC parts per thousand yr(-1).

1996
Keeling, RF, Piper SC, Heimann M.  1996.  Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration. Nature. 381:218-221.   10.1038/381218a0   AbstractWebsite

THE global budget for sources and sinks of anthropogenic CO2 has been found to be out of balance unless the oceanic sink is supplemented by an additional 'missing sink', plausibly associated with land biota(1,25). A similar budgeting problem has been found for the Northern Hemisphere alone(2,3), suggesting that northern land biota may be the sought-after sink, although this interpretation is not unique(2-5); to distinguish oceanic and land carbon uptake, the budgets rely variously, and controversially, on ocean models(2,6,7), (CO2)-C-13/(CO2)-C-12 data(2,4,5), sparse oceanic observations of p(CO2) (ref. 3) or C-13/C-12 ratios of dissolved inorganic carbon, (4,5,8) or single-latitude trends in atmospheric O-2 as detected from changes in O-2/N-2 ratio.(9,10). Here we present an extensive O-2/N-2 data set which shows simultaneous trends in O-2/N-2 in both northern and southern hemispheres and allows the O-2/N-2 gradient between the two hemispheres to be quantified. The data are consistent with a budget in which, for the 1991-94 period, the global oceans and the northern land biota each removed the equivalent of approximately 30% of fossil-fuel CO2 emissions, while the tropical land biota as a whole were not a strong source or sink.