Carbon Dioxide (CO₂) and Methane (CH₄)

L. Bruhwiler and E. Dlugokencky

NOAA, Earth System Research Laboratory (ESRL),
Global Monitoring Division, Boulder, CO

November 10, 2011

Highlights

• Global increases in greenhouse gases from human sources continue.
• NOAA ESRL weekly air samples from 8 Arctic sites (north of 53°N) show that, as yet, there is no direct atmospheric evidence either that Arctic emissions of CH₄, or the net balance of C from CO₂, are changing.

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Carbon dioxide (CO₂) and methane (CH₄) are the two largest contributors to radiative forcing by long-lived greenhouse gases, accounting for about 80% of the total (2.29 out of 2.81 W m^-2 in 2010; see: http://www.esrl.noaa.gov/gmd/aggi/). In contrast to short-lived species like black carbon, whose Arctic emissions only affect Arctic climate, CO₂ and CH₄ emissions from anywhere on Earth will impact Arctic climate. Both these greenhouse gases have long atmospheric residence times; the residence time of CH₄ is about a decade due to photochemical loss (Forster et al., 2007), and for CO₂, whose loss from the atmosphere is controlled by many processes with different time scales, it is much longer (Tans, 2010). CO₂ released in the past and future decade will remain a global warming driver for most of the century.

The Arctic has great potential to influence climate through positive feedbacks. The topmost 3 m of ice-rich permafrost is estimated to hold an amount of carbon about equal to the carbon in known coal reserves, i.e., ~1000 PgC (where 1 Pg [petagram] = 10¹⁵ g) (Tarnocai et al., 2009). If that permafrost were to melt and become water-saturated soil, microbes could convert the carbon into CH₄. If the soils drain, the carbon will be respired as CO₂ into the atmosphere. Currently, however, peat formation at high northern latitudes takes up an estimated 0 to 0.8 PgC yr^-1 (McGuire et al., 2009). A major uncertainty is whether continued Arctic warming and permafrost thawing could cause these high latitude ecosystems to become a net source of CO₂. See the essay on Permafrost for more information about permafrost warming and thawing.

The NOAA ESRL measures atmospheric CO₂ and CH₄ in weekly air samples from 8 Arctic sites (north of 53°N, Table A1). Figure A7 shows the inter-polar difference (IPD, defined as the difference in zonally-averaged CH₄ and CO₂ annual mean abundances for polar zones covering 53° to 90° in each hemisphere) for CO₂ and CH₄. The IPD is a potential indicator of changes in Arctic emissions of CO₂ and CH₄ since there are no significant sources of either gas in southern polar latitudes. The IPD of CO₂ increases with time because increasing CO₂ emissions from fossil fuel combustion at northern mid-latitudes are transported to the Arctic. Independent evidence from model analyses (not shown) suggests that Arctic biogenic emissions of CO₂ are not increasing (see: http://www.esrl.noaa.gov/gmd/ccgg/carbontracker/). Trends in mid-latitude anthropogenic emissions of CH₄ are likely small; therefore, trends in IPD mainly reflect changes in Arctic emissions. No trend in IPD is seen for CH₄, which is also supported by model analyses. The economic collapse of the former Soviet Union from 1991 to 1992 shows the sensitivity of IPD to changing emissions of CH₄. During this period, high northern latitude CH₄ emissions (typically expressed in teragrams, Tg, where 1 Tg = 10¹² g) are estimated to have decreased by ~10 Tg CH₄ yr^-1, and IPD decreased by ~10 ppb (parts per billion) (Dlugokencky et al., 2003), but has not recovered. As yet, there is no direct atmospheric evidence that either Arctic emissions of CH₄, or the net balance of C from CO₂, are changing.

Table A1. NOAA ESRL measures CO2 and CH4 in air samples taken at these eight sites. All are classified as Arctic, i.e, north of 53°N.

Site	Latitude (°N)	Longitude (°)*
ALT: Alert, Nunavut, Canada	82.45	-62.51
BAL: Baltic Sea, Poland	55.35	17.22
BRW: Barrow, Alaska, USA	71.32	-156.61
CBA: Cold Bay, Alaska, USA	55.21	-162.72
ICE: Storhofdi, Vestmannaeyjar, Iceland	63.40	-20.29
STM: Ocean Station M, Norway	66.00	2.00
SUM: Summit, Greenland	72.58	-38.48
ZEP: Ny Ålesund, Svalbard, Norway	78.90	11.88

*Positive and negative values are east and west of the Greenwich meridian, respectively.

Fig. A7. Differences in annual mean abundances of CO2 (top) and CH4 (bottom) in parts per billion (ppb) for polar northern (53° to 90°N, PNH) and polar southern (53° to 90°S, PSH) regions determined from the NOAA ESRL global cooperative air sampling network. Data are available at http://www.esrl.noaa.gov/gmd/dv/iadv/.

References

Dlugokencky, E. J., S. Houweling, L. Bruhwiler, K. A. Masarie, P. M. Lang, J. B. Miller, and P. P. Tans (2003), Atmospheric methane levels off: Temporary pause or a new steady-state?, Geophys. Res. Lett., 30, 1992, doi:10.1029/2003GL018126.

Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

McGuire, D. A., L. G. Anderson, T. R. Christensen, S. Dallimore, L. Guo, D. J. Hayes, M. Heimann, T. D. Lorenson, R. W. Macdonald, and N. Roulet (2009), Sensitivity of the carbon cycle in the Arctic to climate change, Ecol. Monogr., 79(4), 523-555.

Tans, P. (2009), An Accounting of the Observed Increase in Oceanic and Atmospheric CO₂ and an Outlook for the Future, Oceanography, 26-35.

Tarnocai, C., J. G. Canadell, E. A. G. Schuur, P. Kuhry, G. Mazhitova, and S. Zimov (2009), Soil organic carbon pools in the northern circumpolar permafrost region, Global Biogeochem. Cycles, 23, GB2023, doi:10.1029/2008GB003327.