Three-dimensional representation of the latitudinal distributions of atmospheric carbon dioxide (top) and methane (bottom) in the marine boundary layer during the period 1990-1999 based on measurements from the NOAA CMDL cooperative air sampling network. Each surface represents data smoothed in time and latitude and highlights significant features of the atmospheric variation of each trace gas. Carbon Dioxide: The global abundance of CO2 in the atmosphere (measured as moles CO2 per 106 moles air, "ppm") continues to increase at an average rate of 1.5 ppm yr-1. Annual mean CO2 mixing ratios are 3-4 ppm higher in the northern hemisphere where anthropogenic emissions are greatest. Strong seasonality in the northern hemisphere is due to photosynthesis and respiration of the terrestrial biosphere. Seasonality in the southern hemisphere is much smaller and opposite in phase. Interannual variability in the seasonal cycle is due to variation in the balance between photosynthesis and respiration, and ocean uptake and release. Methane: The global abundance of atmospheric CH4 (measured as moles CH4 per 109 moles air, "ppb") also continues to increase. Unlike CO2, the rate of increase has been slowing. CH4 mixing ratios are ~150 ppb higher in the northern hemisphere where anthropogenic and natural sources of atmospheric methane are predominantly located. Seasonal patterns in the northern hemisphere are caused by the interaction of seasonally varying sources, sinks, and atmospheric transport. Seasonality in the southern hemisphere is opposite in phase and driven mostly by chemical destruction by OH radical. These smoothed surfaces are derived from thousands of CO2 and CH4 observations and provide powerful constraints on the global carbon cycle.
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