chapter - Atmospheric and Oceanic Science

Background on other regional aspects: Land use change, aerosols and trace gases
10.1 Introduction
Relevant examples of the complexity of the interactions of the non-linear relations
of the Climate System have been detected in S. America in the context of the
effect of biomass burning aerosols and land use. Land use change is associated with
ecosystems changes and it is known that ecosystems have an impact on the concentration
of gases in the atmosphere both as a source and as a sink for many atmospheric
constituents including the greenhouse gases. Changes in land use and
ecosystems also impact the water flow (hydrology), the energy balance (reflection
and absorption of solar radiation), and air circulation (surface properties and aerodynamics).
Biomass burning is a major and sometimes unique source of several
gases that affect climate and air quality. It is a common and natural phenomenon
(Haberle and Maslin, 1999; Liu and Colinvaux, 1988, Martin et al. 1992) in many
savanna areas such as the Cerrado in central South America and also in some higher
latitude forests, as well as being a common land management practice. Reasons
for biomass burning include deforestation, shifting cultivation, grazing in savannas,
clearing agricultural residue, and fuel wood.
Changes in physical properties of surface can modify the fluxes of water
(hydrological cycle) and energy (solar radiation, radiative forcing, heat exchange)
that can have significant climate impacts at the local regional scale, affecting air circulation,
precipitation patterns and temperatures. The impacts are highly dependent
of the geographical location and season and quite often the mean global impact is
very small and therefore difficult to be detected is global averages. However, locally,
the effects associated with land use change of regional changes in the atmospheric
composition may be very important. The main physical processes impacting
regional climate change through changes in land use and atmospheric chemical
composition are:
a) Surface albedo (fraction of solar radiation reflected back into the atmosphere).
Surface albedo depends on the vegetation cover, and is lower in a forested landscape
than in open land or agriculture.
b) Evapotranspiration. The transport of water from the plants to the atmosphere
directly impacts the hydrological cycle, radiative forcing (indirectly via clouds),
and energy budgets. It is controlled by vegetation rooting depth, leaf area and
soil moisture.
c) Aerodynamic roughness of surfaces. The surface of the landscape affects the
circulation of air passing over it and thus also influences evapotranspiration and
energy fluxes.
d) Aerosols. Aerosols directly influence the radiative balance of the atmosphere
with positive and negative feedbacks. Some aerosols are highly reflective and
therefore decrease the solar energy available at the surface, having a cooling
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