Geography Reference
In-Depth Information
The agricultural activities also have significant impacts on the carbon cycle. For
example, the conversion of the natural vegetation to the cultivated land, loss of
plant biomass and increased decomposition of organic matters in the soil, all make
the agricultural activities become one of the major sources of CO 2 emission. By
contrast, the utilization of the high yield variety and fertilizers, irrigation and no-
till agriculture all contribute to the reduction of carbon loss and increase the
absorption of carbon in the agricultural regions (Friedlingstein et al. 2006 ). For
example, the no-till agriculture in the USA has increased the organic content in the
soil, reaching 1.4 Gt carbon in the past 30 years. However, the increase of the
organic matters in the soil can only last for 50-100 years, after that a new equi-
librium of carbon cycle will be reached (Smith et al. 1997 ).
A lot of BVOC are generated by the plants and then released into the atmo-
sphere. These compounds have significant influence on the physical and chemical
characteristics of the atmosphere due to their great amount. Although the BVOC
have no direct influence on the radiation balance of the atmosphere, they affect the
longevity of the methane in the atmosphere and play an important role in the
formulation of the ozone and secondary organic aerosols (SOAs). The SOAs can
directly influence the climate since they can scatter or absorb the solar radiation
and consequently decrease or increase the temperature (Spracklen et al. 2008 ). The
overall influence of the SOAs on the climate system has not been accurately
quantified so far, they may mainly play a role in decreasing the temperature
(Chapin et al. 2005 ). The SOAs also have significant indirect impacts on the
climate, i.e., they may act as the cloud condensation nuclei (CCN) in the formation
of the clouds. In fact, the BVOCs released by the boreal forest has made the local
CCN increased by 100 % (Spracklen et al. 2008 ), which in turn influences the
number of water drops in the cloud and makes the albedo of the atmosphere
increased by 3-8 %.
There will be an albedo difference ranging from -1.8 to -6.7 W/m 2 in the
regions between 60-90N since the albedo increases due to the indirect influence
of BVOC. This means that owing to the feedback among BVOC released by the
trees, SOA, CNN and land surface albedo, the boreal forest will make the local
climate colder. The influence of BVOC on CCN is considered to be the most
important in the boreal forest since the regional air pollution is slight (Spracklen
et al. 2008 ). In addition, the net emission of carbon released due to the defores-
tation depends on the land use type converted from the forest or the temporal scale
of the regeneration of the forest and the feedback mechanism mentioned above.
According to the simulation which only takes into account the biogeochemical
influence, the complete deforestation will make the global temperature increase by
0. 09 or 0.19 C if the forests between 50-60N and 0-10S were completely
felled (Chapin et al. 2005 ). This is because of the great amount of biomass in the
tropic zone and depends on the total biomass of different forest types.
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