Trends in land-atmosphere carbon fluxes and a new analysis of nonanthropogenic carbon sinks and sources

The evolution of the Earth\u27s terrestrial carbon sinks from 1980 to 2005 was investigated by using the TransCom 3, Level 2 (T3L2) inversion results comprised of 13 atmospheric transport models which calculated the carbon flux between Earth\u27s land surfaces and the atmosphere. Four measures of yearly biotic activity were defined: growing-season net flux (GSNF), dormant-season net flux (DSNF), amplitude (the magnitudes of GSNF and DSNF added together) and net carbon flux (NCF) and plotted for the T3L2 biomes. It was determined that the worldwide terrestrial carbon sink is intensifying at the rate of 0.029 ± 0.016 PgC y-1 y-1, or 0.76 ± 0.42 PgC y-1 over the period studied; this increase is dominated by a decrease in DSNF. The North American carbon sink is weakening at the rate of 0.024 ± 0.0095 PgC y-1 y-1. Stephens et al., (2007), published by Science, reported that consensus estimates of the extratropical Northern Hemisphere land (ENHL) sink were about 1 PgC y-1 too high and that tropical land surfaces are not a 1 PgC y-1 source, but near-neutral. In response, the T3L2 inversion was applied to CO2 data from 2003 to 2006 both with and without airborne data, with Globalview-CO2 uncertainties and approximate Stephens et al., (2007) uncertainties, and with nine of the 160 stations\u27 data excluded from analysis. For the inversion conditions that best matched S07, an ENHL of -2.9 ± 0.7 PgC y-1 and a tropical land source of 1.2 ± 0.6 PgC y-1 were detected. The inversion is robust in that excluding or including airborne data does not lead to significant changes in carbon flux, though it does remove variability in the relationship between CO2 profiles and carbon sinks. Reducing CO2 concentration uncertainties to Stephens et al., (2007) levels reduces the spread of model fluxes in the extratropical Northern Hemisphere, but increases their spread in the tropics. The effect of excluding 9 of the 160 stations is not statistically significant, even at 80% confidence