Synergy of rising nitrogen depositions and atmospheric CO₂ on land carbon uptake moderately offsets global warming

Increased carbon uptake of land in response to elevated atmospheric CO 2 concentration and nitrogen deposition could slow down the rate of CO 2 increase and facilitate climate change mitigation. Using a coupled model of climate, ocean, and land biogeochemistry, we show that atmospheric nitrogen deposition and atmospheric CO 2 have a strong synergistic effect on the carbon uptake of land. Our best estimate of the global land carbon uptake in the 1990s is 1.34 PgC/yr. The synergistic effect could explain 47% of this carbon uptake, which is higher than either the effect of increasing nitrogen deposition (29%) or CO 2 fertilization (24%). By 2030, rising carbon uptake on land has a potential to reduce atmospheric CO 2 concentration by about 41 ppm out of which 16 ppm reduction would come from the synergetic response of land to the CO 2 and nitrogen fertilization effects. The strength of the synergy depends largely on the cooccurrence of high nitrogen deposition regions with nonagricultural ecosystems. Our study suggests that reforestation and sensible ecosystem management in industrialized regions may have larger potential for climate change mitigation than anticipated.