Effect of increasing CO₂ on the terrestrial carbon cycle

Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO₂ concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO₂ effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO₂ that do not show large tropical uptake. Our results, however, show significant tropical uptake and, combining tropical and extratropical fluxes, suggest that up to 60% of the present-day terrestrial sink is caused by increasing atmospheric CO₂. This conclusion is consistent with a validated subset of atmospheric analyses, but uncertainty remains. Improved model diagnostics and new space-based observations can reduce the uncertainty of tropical and temperate zone carbon flux estimates. This analysis supports a significant feedback to future atmospheric CO₂ concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO₂ concentrations. This feedback will have substantial tropical contributions, but the magnitude of future carbon uptake by tropical forests also depends on how they respond to climate change and requires their protection from deforestation.