A reevaluation of the open ocean source of methane to the atmosphere

Seawater and atmospheric methane (CH4) mixing ratios were measured on five cruises throughout the Pacific Ocean from 1987 to 1994 to assess the magnitude of the ocean-atmosphere flux. The results showed consistent regional and seasonal variations with surface seawater concentrations ranging from 1.6 to 3.6 nM and saturation ratios ranging from 0.95 to 1.17. The equatorial Pacific Ocean was supersaturated with respect to atmospheric CH4 partial pressures, while areas outside the tropics often were undersaturated during fall and winter. Although atmospheric CH4 mixing ratios over the North Pacific during April increased 3.4% from 1988 to 1993, the saturation ratios remained constant. Based on the concentration fields, the data were divided into two seasons and 10 latitude zones from 75 degrees S to 75 degrees N. Using monthly Comprehensive Ocean-Atmosphere Data Set (COADS) wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from -0.1 to 0.4 mu mol m(-2) d(-1). The combined seasonal and zonal fluxes result in a total global ocean-to-atmosphere flux of 25 Gmol yr(-1) (0.4 Tg CH4 yr(-1)), which is an order of magnitude less than previous estimates [Intergovernmental Panel on Climate Change (IPCC), 1994]. The estimated uncertainty in this number is approximately a factor of 2.