Publications

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2011
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.

2004
Keeling, RF, Blaine T, Paplawsky B, Katz L, Atwood C, Brockwell T.  2004.  Measurement of changes in atmospheric Ar/N2 ratio using a rapid-switching, single-capillary mass spectrometer system. Tellus Series B-Chemical and Physical Meteorology. 56:322-338.   10.1111/j.1600-0889.2004.00117.x   AbstractWebsite

The atmospheric Ar/N-2 ratio is expected to undergo very slight variations due to exchanges of Ar and N-2 across the air-sea interface, driven by ocean solubility changes. Observations of these variations may provide useful constraints on large-scale fluxes of heat across the air-sea interface. A mass spectrometer system is described that incorporates a magnet with a wide exit face, allowing a large mass spread, and incorporates an inlet with rapid (5 s) switching of sources gases through a single capillary, thus achieving high precision in the comparison of sample and reference gases. The system allows simultaneous measurement of Ar/N-2, O-2/N, and CO2/N-2 ratios. The system achieves a short-term precision in Ar/N-2 of 10 per meg for a 10 s integration, which can be averaged to achieve an internal precision of a few per meg in the comparison of reference gases. Results for Ar/N-2 are reported from flasks samples collected from nine stations in a north-to-south global network over about a 1 yr period. The imprecision on an individual flask, as estimated from replicate agreement, is 11 per meg. This imprecision is dominated by real variability between samples at the time of analysis. Seasonal cycles are marginally resolved at the extra-tropical stations with amplitudes of 5 to 15 per meg. Annual-mean values are constant between stations to within 5 per meg. The results are compared with a numerical simulation of the cycles and gradients in Ar/N-2 based on the TM2 tracer transport model in combination with air-sea Ar and N-2 fluxes derived from climatological air-sea heat fluxes. The possibility is suggested that Ar/N-2 ratios may be detectably enriched near the ground by gravimetric or thermal fractionation under conditions of strong surface inversions.

2001
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.

2000
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.

1997
Najjar, RG, Keeling RF.  1997.  Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean. Journal of Marine Research. 55:117-151.   10.1357/0022240973224481   AbstractWebsite

A global climatology of the dissolved oxygen anomaly (the excess over saturation) is created with monthly resolution in the upper 500 m of the ocean. The climatology is based on dissolved oxygen, temperature and salinity data archived at the National Oceanographic Data Center. Examination of this climatology reveals statistically significant annual cycles throughout the upper 500 m of the World Ocean, though seasonal variations are most coherent in the North Atlantic, where data density is greatest. Vertical trends in the phase and amplitude of the annual cycle are noted. The cycle in surface waters is characterized by a summer maximum and a winter minimum, consistent with warming and high rates of photosynthesis during the summer, and cooling and entrainment of oxygen-depleted water during the winter. In low and middle latitudes, the amplitude increases with depth and the maximum occurs later in the year, a trend consistent with the seasonal accumulation of oxygen associated with the shallow oxygen maximum. At a depth that varies between about 30 and 130 m, the phase of the annual cycle undergoes an abrupt shift. We call this depth the oxygen nodal depth. Below the nodal depth, the annual cycle is characterized by an early-spring maximum and a late-fall minimum, consistent with a cycle dominated by respiration during the spring and summer and replenishment of oxygen from the atmosphere by ventilation during the fall and winter. Below the nodal depth, the amplitude of the annual cycle generally decreases with depth, indicative of decreasing respiration and ventilation rates, or less seasonality in both processes. We postulate that the nodal depth in middle and high latitudes corresponds closely to the summertime compensation depth, where photosynthesis and net community respiration are equal. With this interpretation of the nodal depth and a simple model of the penetration of light in the water column, a compensation light intensity of 1 W m(-2) (4 mu E m(-2) s(-1)) is deduced, at the low end of independent estimates. Horizontal trends in the phase and amplitude of the annual cycle are also noted. We find that the nodal depth decreases toward the poles in both hemispheres and is generally greater in the Southern Hemisphere, patterns found to be consistent with light-based estimates of the compensation depth. The amplitude of the annual cycle in the oxygen anomaly increases monotonically with latitude, and higher latitudes lag lower latitudes. In the North Atlantic and North Pacific, the amplitude of the annual cycle tends to increase from east to west at all depths and latitudes, as expected considering that physical forcing has greater seasonal variability in the west. The tropics and the North Indian Ocean have features that distinguish them from other regions. Below about 75 m, these waters have pronounced annual cycles of the oxygen anomaly that areshown to be caused mainly by wind-driven adiabatic displacements of the thermocline. A semiannual cycle of the oxygen anomaly is found in the surface waters of the North Indian Ocean, consistent with the known semiannual cycle of surface heat flux in this region.