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
- 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.
|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|
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).
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.
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