Methane (CH4) is an important biogenic trace gas with a relatively long tropospheric lifetime of around a decade (Lelieveld et al., 1998). It is infrared-active, accounting for about 20% of enhanced greenhouse forcing (IPCC, 2007). Tropospheric methane has a variety of global sources, of which more than one half are anthropogenic. Natural sources are dominated by wetlands but marine emissions also contribute (IPCC, 2007). Reaction with tropospheric OH is the primary atmospheric CH4 sink (Crutzen, 1995). Short-term changes in the relative strengths of source and sink functions are reflected in a tropospheric CH4 growth rate that is annually variable. Since the beginning of 2007, the trend has been one of consistent growth (Rigby et al., 2008), but unfortunately the interactions that govern this variability are not well understood.
Methane is generally supersaturated in surface ocean waters. Current ‘best’ estimates are that <5% of tropospheric CH4 is of marine origin, however data coverage is rather poor and these estimates still tend to be dominated by open ocean regions where the degree of surface CH4 supersaturation is relatively mild. Contributions from estuaries and shallow coastal shelves are far larger per unit area (e.g. Bange et al., 1994; Upstill-Goddard et al., 2000), implying that such estimates require revision.
Improving our current estimates of marine CH4 emissions is therefore a key priority. A recent initiative under COST Action 735, called MEMENTO, aims to integrate existing oceanic and atmospheric measurements into a global database (see Bange et al., 2009). For further details of MEMENTO, please also see the outline document. Details concerning the spreadsheets necessary for submission are available along with instructions for submitting data.
Implementation Working Group 3 also examines the following long-lived, climatically active gases: