Observations of increasing carbon dioxide concentration in Earth’s thermosphere

Journal name:
Nature Geoscience
Volume:
5,
Pages:
868–871
Year published:
DOI:
doi:10.1038/ngeo1626
Received
Accepted
Published online

Carbon dioxide occurs naturally throughout Earth’satmosphere. In the thermosphere, CO2 is the primary radiative cooling agent and fundamentally affects the energy balance and temperature of this high-altitude atmospheric layer1, 2. Anthropogenic CO2 increases are expected to propagate upward throughout the entire atmosphere, which should result in a cooler, more contracted thermosphere3, 4, 5. This contraction, in turn, will reduce atmospheric drag on satellites and may have adverse consequences for the orbital debris environment that is already unstable6, 7. However, observed thermospheric mass density trends derived from satellite orbits are generally stronger than model predictions8, 9, indicating that our quantitative understanding of these changes is incomplete. So far, CO2 trends have been measured only up to 35km altitude10, 11, 12. Here, we present direct evidence that CO2 concentrations in the upper atmosphere—probably the primary driver of long-term thermospheric trends—are increasing. We analyse eight years of CO2 and carbon monoxide mixing ratios derived from satellite-based solar occultation spectra. After correcting for seasonal–latitudinal and solar influences, we obtain an estimated global increase in COx (CO2 and CO, combined) concentrations of 23.5±6.3ppm per decade at an altitude of 101km, about 10ppm per decade faster than predicted by an upper atmospheric model. We suggest that this discrepancy may explain why the thermospheric density decrease is stronger than expected.

At a glance

Figures

  1. Residual VMRs of CO, CO2 and COx[thinsp]=[thinsp]CO+CO2, after removal of seasonal-latitudinal effects.
    Figure 1: Residual VMRs of CO, CO2 and COx=CO+CO2, after removal of seasonal–latitudinal effects.

    Results are shown at pressure level Z=−6 (altitude ~ 101km) for ACE-FTS (blue circles) and the NCAR global mean model (green crosses). The solid pink line is the linear trend from a least-squares fit of the ACE-FTS COx residuals. Error bars denote the estimated 1σ uncertainty of the mean.

  2. Height dependence of 2004-2012 COx linear trends.
    Figure 2: Height dependence of 2004–2012 COx linear trends.

    a, Absolute trends. b, Relative trends, obtained by dividing the absolute trends by the profiles shown in Fig. 4. Results are shown for ACE-FTS (pink), the NCAR global mean model (solid green) and the global mean model with a 15% per decade eddy diffusion trend added (dotted green). The shaded areas encompass the estimated 95% confidence interval of the ACE-FTS trends. The dashed lines denote the portion of the ACE-FTS trend profiles that is influenced by the prescribed stratospheric CO2 trends.

  3. Temporal variation of carbon at pressure level
Z[thinsp]=[thinsp]-6(altitude [sim] 101[thinsp]km) from the NCAR global mean model simulation.
    Figure 3: Temporal variation of carbon at pressure level Z=−6(altitude ~ 101km) from the NCAR global mean model simulation.

    Shown are VMRs of CO (red), CO2 (blue) and COx=CO+CO2 (green). The data are plotted according to the colour-coordinated y axes. The bottom panel shows the 10.7cm solar radio flux (F10.7), a proxy for solar ultraviolet irradiance.

  4. Average carbon profiles.
    Figure 4: Average carbon profiles.

    2004–2012 average CO (red), CO2 (blue) and COx (green) VMR profiles, from ACE (solid lines) and the NCAR global mean model (dashed lines).

References

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Author information

Affiliations

  1. Space Science Division, Naval Research Laboratory, 4555 Overlook Avenue Southwest, Washington DC 20375, USA

    • J. T. Emmert,
    • M. H. Stevens &
    • D. P. Drob
  2. Department of Chemistry and Biochemistry, Old Dominion University, 5115 Hampton Boulevard, Norfolk, Virginia 23529, USA

    • P. F. Bernath
  3. Department of Chemistry, University of Waterloo, 200 University Avenue, West Waterloo, Ontario N2L 3G1, Canada

    • P. F. Bernath &
    • C. D. Boone
  4. Department of Chemistry, University of York, Heslington, York YO10 5DD, UK

    • P. F. Bernath

Contributions

J.T.E. conceived the study, analysed the data and model output, and prepared the manuscript. M.H.S. and D.P.D. conducted the model simulations. M.H.S. calculated carbon emissions from space vehicle launches. P.F.B. and C.D.B. provided guidance on the use of the ACE-FTS retrievals. C.D.B. developed the ACE-FTS retrieval algorithms. All authors discussed the results, interpretations and implications, and contributed to the manuscript.

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The authors declare no competing financial interests.

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