Carbon Dioxide (CO₂) and Methane (CH₄)

L. Bruhwiler and E. Dlugokencky

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

November 11, 2012

Highlights

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

Carbon dioxide (CO₂) and methane (CH₄) are the two largest contributors to radiative forcing by long-lived greenhouse gases, accounting for about 82% of the total (2.32 out of 2.84 W m^-2 in 2011; see: http://www.esrl.noaa.gov/gmd/aggi/). 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. CH₄ is a potent greenhouse gas; it causes about 25 times more warming over 100 years than emission of an equal mass of CO₂ (Forster et al. 2007).

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, ~1000 PgC (where 1 Petagram (Pg) = 10¹⁵ g) (Tarnocai et al., 2009). If Arctic permafrost thaws, then the carbon stored in Arctic soils will decay and be emitted to the atmosphere as some combination of CO₂ and CH₄. If Arctic soils remain water-saturated, a larger fraction of carbon will be emitted as CH₄ as a result of anaerobic microbial activity. On the other hand, if Arctic soils drain as permafrost thaws, a larger proportion of carbon will be emitted as CO₂. Currently, the Arctic is thought to be a small sink for atmospheric CO₂ (McGuire et al., 2009). Model studies that attempt to describe permafrost dynamics as the atmospheric warms in the future suggest that even with a more productive Arctic biosphere capable of taking up more carbon, the Arctic will become a net source of carbon sometime in the first half of the 21st Century (e.g. Schaefer et al., 2011).

Shallow Arctic sea sediments, especially offshore of Siberia, are thought to be rich in organic matter that may be emitted to the atmosphere as the seawater temperature increases. In addition, ice hydrates deep within the Arctic sea shelf sediments may destabilize due to warmer water temperatures and release methane to the atmosphere. Currently, the amount of CH₄ emitted to the atmosphere by these processes is thought to be about one third of that emitted from wetlands in the Arctic tundra (Shakova et al., 2010; McGuire et al., 2012); however, the sparseness of atmospheric observations makes this difficult to confirm.

It is important to monitor Arctic greenhouse gases as they have great potential to influence global climate through positive feedbacks. Consequently, NOAA ESRL currently measures atmospheric CO₂ and CH₄ weekly in air samples from 6 Arctic sites (north of 53°N, Table 1.1). This is down from 8 sites in 2011; sites in the Baltic Sea and Station M in the North Atlantic were discontinued due to budget cuts.

Table 1.1. 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. The BAL and STM sites were discontinued in 2011.

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.

Figures 1.13 and 1.14 show time series of CO₂ and CH₄ at polar northern latitudes (53 to 90°N) averaged over all NOAA network sites. Both species show large annual cycles related to summertime uptake by the land biosphere in the case of CO₂ and emissions from wetlands and other biogenic sources in the case of CH₄. Note that uptake of CO₂ and biogenic emissions of methane are largest in the warm months, so the seasonal cycles are approximately out of phase. Over many years, the behavior of CO₂ is dominated by a positive trend related to fossil fuel combustion that occurs mostly in the populated mid-latitudes. The recent upward trend in CH₄ is thought to be related mainly to growth of natural emissions in the tropics after a prolonged period of lower-than-average precipitation (Dlugokencky et al., 2009; Bousquet et al., 2011).

Zonal mean abundance of CO2 in parts per million

Fig. 1.13. Zonal mean abundance of CO₂ in parts per million (ppm) for the northern and polar region (53° to 90°N, PNH) determined from the NOAA ESRL global cooperative air sampling network. Data are available at http://www.esrl.noaa.gov/gmd/dv/iadv/.

Zonal mean abundance of CH4 in parts per billion

Fig. 1.14. Zonal mean abundance of CH₄ in parts per billion (ppb) for the polar northern (53° to 90°N, PNH) region determined from the NOAA ESRL global cooperative air sampling network. Data are available at http://www.esrl.noaa.gov/gmd/dv/iadv/.

Figure 1.15. shows the inter-polar difference of CH₄ (IPD, defined as the difference in zonally-averaged CH₄ annual mean abundances for polar zones covering 53° to 90° in each hemisphere). The IPD is a potential indicator of changes in Arctic CH₄ emissions because there are no significant sources in southern polar latitudes, and trends in mid-latitude sources are transported to high latitudes of both hemispheres; therefore, trends in IPD mainly reflect changes in Arctic emissions (e.g., Dlugokencky at al., 2003). No upward trend in IPD is seen for CH₄ since the 1990s, suggesting that Arctic emissions have not been increasing in recent years. 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 emissions are estimated to have decreased by ~10 Tg CH₄ yr^-1 (where 1 Teragram (Tg) = 10¹² g), and IPD decreased by ~10 ppb [parts per billion]) (Dlugokencky et al., 2003), but has not recovered. As yet, multi-decadal observations of atmospheric CH₄ do not suggest that Arctic emissions are not increasing rapidly. Trends in CO₂ emission or uptake are difficult to detect because the global budget is dominated by the global increase due to fossil fuels. However, given the large inter-annual variability, trends in emissions of both species that are too small to be detected by the atmospheric network over several decades cannot be ruled out.

Inter-polar difference in annual mean abundance of CH4

Fig. 1.15. Inter-polar difference (IPD) in annual mean abundance of CH₄ 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. The trend lines (broken lines) emphasize the change in Arctic emissions that occurred in the early 1990s, possibly related to changes in anthropogenic emissions in the former Soviet Union. Data are available at http://www.esrl.noaa.gov/gmd/dv/iadv/.

References

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Carbon Dioxide (CO2) and Methane (CH4)