Figure 4.2: Different ecosystems,
their components, and human activities. The carbon stocks associated with the
different ecosystems are stored in aboveground and belowground biomass, detrital
material (dead organic matter), and soils. Carbon is withdrawn from the atmosphere
through photosynthesis (vertical down arrow), and returned by oxidation processes
that include plant respiration, decomposition, and combustion (vertical up arrow).
Carbon is also transferred within ecosystems and to other locations (horizontal
arrows). Both natural processes and human activities affect carbon flows. Mitigation
activities directed at one ecosystem component generally have additional effects
influencing carbon accumulation in, or loss from, other components. Estimates
of ecosystem and atmospheric C stocks are adapted from Bolin et al. (2000). Values
for C stocks in some ecosystems are still very uncertain. Not shown are estimates
of C stocks in tundra (127GtC), deserts and semi-deserts (199GtC), and oceans
(approx. 39,000GtC) (numbers are taken from Special Report on LULUCF, Fig 1-1,
page 30; IPCC, 2000a).