Potential CO₂-Enhanced Carbon Storage by the Terrestrial Biosphere

Abstract

Geochemical models that deduce latitudinal source/sink relationships of atmospheric CO₂ suggest that, in tropical regions, there is almost zero net exchange of CO₂ between the atmosphere and the terrestrial biosphere. The implication is that CO₂-enhanced carbon storage (CO₂-ECS) by tropical biomes is negating the output of CO₂ from deforestation. We describe here a 10-biome model for CO₂-ECS, in which carbon accumulation in living vegetation is coupled to the Rothamsted soil carbon model. A biotic growth factor (β) was used to describe the relationship between literature estimates of net primary production (NPP) and atmospheric CO₂ concentration. Using β = 0.3 as a reference state, CO₂-ECS by the global biosphere in 1990 was 1.1 Gt. When more appropriate values of β were used (derived from a theoretical response of vegetation to increasing temperature and CO₂), CO₂-ECS was 1.3 Gt, of which tropical biomes accounted for 0.7 Gt. There are many uncertainties in this (and other) models; total CO₂-ECS is particularly sensitive to changes in NPP. Unless published surveys have underestimated tropical NPP by a factor of about 2, then it is unlikely that CO₂-ECS could have negated the 1.5-3.0 Gt of carbon that are estimated to have been emitted by tropical deforestation in 1990.