Vegetation dynamics and plant CO2 responses as positive feedbacks in a greenhouse world

An atmosphere-ocean-vegetation coupled model is used to quantify the biogeophysical feedback that emerges as vegetation adjusts dynamically to a quadrupling of atmospheric CO₂. This feedback amplifies global warming by 13%. About half of it is due to climatically induced expansion of boreal forest into tundra, reinforced by reductions in snow and sea ice cover. The other half represents a global climatic effect of increased vegetative cover (an indirect consequence of plant physiological responses to CO₂) in the semi-arid subtropics. Enhanced absorption of shortwave radiation in these regions produces a net surface warming, which the atmosphere communicates poleward. The greatest vegetation-induced wanning is co-located with large, vulnerable carbon stores in the north. These lose carbon, so that in the long term, the biospheric response to CO₂ and climate change becomes dominated by positive feedbacks that overwhelm the effect of CO₂ fertilization on terrestrial carbon stocks.