7.3.3 Terrestrial Carbon Cycle Processes and Feedbacks to Climate

The net exchange of carbon between the terrestrial biosphere and the atmosphere is the difference between carbon uptake by photosynthesis and release by plant respiration, soil respiration and disturbance processes (fire, windthrow, insect attack and herbivory in unmanaged systems, together with deforestation, afforestation, land management and harvest in managed systems). Over at least the last 30 years, the net result of all these processes has been uptake of atmospheric CO₂ by terrestrial ecosystems (Table 7.1, ‘land-atmosphere flux’ row). It is critical to understand the reasons for this uptake and its likely future course. Will uptake by the terrestrial biosphere grow or diminish with time, or even reverse so that the terrestrial biosphere becomes a net source of CO₂ to the atmosphere? To answer this question it is necessary to understand the underlying processes and their dependence on the key drivers of climate, atmospheric composition and human land management.

Drivers that affect the carbon cycle in terrestrial ecosystems can be classified as (1) direct climate effects (changes in precipitation, temperature and radiation regime); (2) atmospheric composition effects (CO₂ fertilization, nutrient deposition, damage by pollution); and (3) land use change effects (deforestation, afforestation, agricultural practices, and their legacies over time). This section first summarises current knowledge of the processes by which each of these drivers influence the terrestrial carbon balance, and then examines knowledge of the integrative consequences of all these processes in the key case of tropical forests.