It is temporary. The increase in biomass due to CO2 saturates when the area locally runs out of other constraints like water or nitrogen. The IPCC predicts by

It is temporary. The increase in biomass due to CO2 saturates when the area locally runs out of other constraints like water or nitrogen. The IPCC predicts by 2100 the land is going to convert into a net source rather than sink of carbon -- so the positive feedback loops will exceed the negative feedback stabilizers. The increase in biomass also comes at a decrease in nutrient density as well, so its not like this is magic sauce for croplands.