Methane and nitrous oxide emissions from the ocean: A reassessment using basin‐wide observations in the Atlantic

Abstract

We measured the concentrations of nitrous oxide (N 2 O) and methane (CH 4 ) in the marine boundary layer and surface waters of the Atlantic Ocean from ∼50°N to ∼50°S during the Atlantic Meridional Transect expedition (AMT‐7) in 1998. The cruise track transects a variety of meteorological and oceanographic regimes. Unusually high mixing ratios of atmospheric CH 4 were observed in the extratropical Northern Hemisphere, coinciding with globally high levels of CH 4 associated with the El Niño event of 1998. Atmospheric N 2 O remained nearly invariable during the expedition, with only a small hemispheric difference (0.82 ppb). Throughout the cruise, these gases were saturated or supersaturated in the water. The coastal region was observed to be a significant source of CH 4 , while upwelling regions acted as strong N 2 O emission sources. We estimated the global oceanic emission of CH 4 to be 0.6–1.2 Tg a −1 , comparable to previous estimates from basin‐wide observations. However, our estimate turns out to be ∼10 times lower than the value in the 1990 to 2007 Intergovernmental Panel on Climate Change (IPCC) reports, which essentially all relied on the estimate by Ehhalt (1974). A bias toward high CH 4 saturation anomalies is probably responsible for the overestimation of the marine CH 4 source in the IPCC reports. The CH 4 saturation anomaly in the ocean appears to have remained constant over an interval of 20 years in spite of the increase of atmospheric CH 4 , suggesting that the increase of the surface water temperature driven by global warming may be a major factor. Meanwhile, the N 2 O emission from the ocean, estimated in the present study to be 0.9–1.7 Tg N a −1 , is ∼3 times lower than the value in the recent IPCC report [ Denman et al. , 2007], implying either weak upwelling activity or low amounts of dissolved N 2 O in upwelling subsurface waters, or both, in the Atlantic.