Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Niño

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Abstract

The stable isotope ratios of atmospheric CO2 (18O/16O and 13C/12C) 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 way1. Interpreting the 18O/16O variability has proved difficult, however, because oxygen isotopes in CO2 are influenced by both the carbon cycle and the water cycle2. Previous attention focused on the decreasing 18O/16O 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 respiration3; a global increase in C4 crops at the expense of C3 forests4; and environmental conditions, such as atmospheric turbulence5 and solar radiation6, that affect CO2 exchange between leaves and the atmosphere. Here we present 30 years’ worth of data on 18O/16O in CO2 from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Niño/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Niño increases the 18O/16O 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 Niño anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Niño 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 year7, 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.

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Figure 1: Measurements of δ 18 O-CO 2 from the SIO flask network and CSIRO.
Figure 2: Correlations between precipitation δ 18 O from IsoGSM and ENSO.
Figure 3: Correlations between flask δ 18 O-CO 2 records and precipitation δ 18 O and relative humidity from IsoGSM.
Figure 4: ENSO index and two-box model results.

References

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Acknowledgements

The authors thank S. Walker for programming assistance and all those involved with flask collection and analysis. This work was supported by the US National Science Foundation (NSF) under grant ATM06-32770, by the US Department of Energy (DOE) under grant DE-SC0005099 and by the US National Aeronautics and Space Administration (NASA) under grant NNX11AF36G. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF, DOE or NASA.

Author information

L.R.W. analysed the data. R.F.K. supervised the project. L.R.W. and R.F.K. wrote the paper. H.A.J.M., A.F.B., R.J.F., C.E.A. and M.W. provided data. K.Y. provided the IsoGSM output. All authors discussed the results and commented on the manuscript.

Correspondence to Lisa R. Welp or Ralph F. Keeling.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Text 1-6, which includes a Supplementary Discussion and Supplementary Methods, Supplementary References, Supplementary Figure 1 with a legend and Supplementary Tables 1-3. (PDF 1630 kb)

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