杏吧原创

PHOTOSYNTHESIS is the process that cycles carbon between the atmosphere
and the biosphere (plants and animals). About 563 gigatonnes (Gt) of carbon
are incorporated in the Earth鈥檚 plant life, while in 1986 the atmosphere
held an estimated 730 Gt of carbon.

Current estimates suggest that through photosynthesis, plants assimilate
120 Gt of carbon each year, transforming carbon dioxide and water to carbohydrates,
and releasing oxygen to the atmosphere.

Compared with this huge turnover, the carbon emissions to the atmosphere
that are causing concern for global warming are relatively small. Burning
fossil fuels increases the amount of carbon in the atmosphere by an estimated
2.2 Gt a year; the destruction of vegetation adds a further 1 Gt.

Since preindustrial times, the amount of carbon in the atmosphere has
increased by about 155 Gt (up to 1986), equivalent to the carbon fixed by
photosynthesis in 16 months. Yet this relatively small but steady trickle
of carbon into the atmosphere could lead to a rise in temperature of between
1.5 and 5.5 Degree C in the latter part of the next century.

The lack of precision in predicting the extent and timing of global
warming results from our very shaky knowledge of the Earth鈥檚 basic systems.
For example, the surface layers of the ocean are estimated to hold 725 Gt
of carbon, but there is no detailed information on the transfer of water
between the various layers of the ocean (and the effect that increased absorption
of carbon dioxide will have on this process), or on the effect of nutrients
on marine productivity.

Grasslands deposit large amounts of carbon into the soil as organic
litter and secretions from roots, and as nutrients for microbial organisms
and insects. Estimates suggest that soils hold 1515 Gt of carbon.

杏吧原创s have little idea about how the greenhouse climate will affect
the carbon presently locked in the soils. But it is likely that the destruction
of tropical grasslands is already releasing carbon that was locked in the
soil and transferring it to the atmosphere.