Global cycles of greenhouse gases during the last 100 years implied by firn air data

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Air samples were collected from different depths of the firn layer at YM85, Antarctica, and NGRIP, Greenland, respectively, and analyzed for CO_2, and CH_4, concentrations and their isotopic ratios. The CO_2, and CH_4 concentrations at 70m depth at YM85 are 304ppm and 970ppb, respectively. Their effective age was estimated to be about 90 years by using a one-dimensional numerical model. The vertical distribution of CH_4 concentration in the firn layer can be reconstructed by a linear combination of the age distribution functions weighted by the atmospheric CH_4 concentrations in the past.Therefore, the most plausible past atmospheric record of CH_4 can be derived iteratively so that its observed profile in the firn layer was reproduced well. Same method was also applied to the reconstructions of carbon and hydrogen isotopic ratios. Temporal variations of atmospheric CH_4 budget was calculated from the atmospheric histories of concentration and its carbon and hydrogen isotopic ratios. This result shows that the CH_4 emission from biomass burning would be important during last 100 years.