The turnover time of atmospheric carbon is very short. It is less than 5 years. Therefore, the balance of atmospheric carbon strongly depends on the gas exchange between the atmosphere and ocean as well as on the gas exchange between the atmosphere and terrestrial ecosystems.
Atmospheric CO2 content. The atmospheric content of carbon dioxide gradually increased from 680 PgC in 1960-69 to 760 PgC in 1990-99. The rate of increase was 3.3 ± 0.1 PgC yr^1 during 1980-89 and 3.2 ± 0.1 PgC yr^1 during 1990-99. Interannual variations were significantly wider. The lowest rate of 1.9PgCyr~ was observed in 1992, and the highest rate of 6.0 PgC yr~ was observed in 1998. Since fossil fuel emission does not show short-term variability of this magnitude, the interannual variations are normally attributed to the climate-induced variations in the land-atmosphere flux, or the ocean-atmosphere flux, or both.
Air/sea exchange. The gross carbon exchange between the atmosphere and ocean is estimated at 90 PgC yr~ . Since atmospheric CO2 concentration is increasing, there is net uptake of carbon by the ocean, driven by the atmosphere-ocean difference in partial pressure of CO2. The magnitude of the uptake slightly varies on decadal scale: 1.9 ± 0.6 PgC yr-1 for 1980-89, 1.7 ± 0.5 PgC yr-1 for 1990-99. This variability is attributed to El Niño events. The natural efflux of CO2 from equatorial Pacific is reduced during El Niño mainly due to the reduced upwel-ling of CO2-rich waters. The decrease in the efflux varies between 0.2 and 1.0 PgC yr-1.
Air/land exchange. The gross carbon exchange between the atmosphere and land is estimated at 60PgCyr-1. Despite the net release of CO2 associated with land use, the balance between emissions from fossil fuel combustion, net ocean uptake, and accumulation in the atmosphere is positive suggesting net uptake of carbon by the land. The net terrestrial uptake widely varies on decadal scale: 0.2 ± 0.7 PgC yr-1 for 1980-89 and 1.4 ± 0.7 PgC yr-1 for 1990-99.
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