Role of terrestrial ecosystems in determining CO₂ stabilization and recovery behaviour

Abstract

Terrestrial ecosystems are sensitive to climate and can also influence it through both biophysical and biogeochemical feedbacks. Natural carbon uptake by ecosystems will control future evolution of CO₂ and climate, but the ecosystems themselves may be committed to long-term changes. Here we use a coupled climate-carbon cycle GCM with dynamic vegetation to investigate the policy-relevance of these feedbacks in several idealized scenarios. Our results show that the natural carbon cycle in the ocean and on land controls the recovery of atmospheric CO₂ following emissions reductions at three action points during the 2^1st century. Initial rates of recovery are similar but for different reasons. Ocean carbon uptake exceeds terrestrial uptake, with higher CO₂ levels leading to increased ocean uptake whereas on land greater climate change at higher CO₂ leads to decreased carbon storage. There are long-term committed changes to terrestrial ecosystems which vary in sign regionally and create a complex dynamic response of terrestrial carbon storage as it slowly approaches a new steady state. Neither stabilization nor CO₂ recovery allows ecosystems to recover back to their initial state and the ecosystems continue to respond for decades or even centuries following emissions reductions. These long-term committed changes, in addition to realized, transient changes, must be considered when defining dangerous climate change and identifying emission-pathways to avoid it.