Box 13.1. Amazonia: a 'hotspot' of the Earth system
The Amazon Basin contains the largest contiguous extent of tropical forest on Earth, almost 5.8 million km2 (see Figure 13.3). It harbours perhaps 20% of the planet's plant and animal species. There is abundance of water resources and the Amazon River accounts for 18% of the freshwater input to the global oceans. Over the past 30 years almost 600,000 km2 have been deforested in Brazil alone (INPE-MMA, 2005a) due to the rapid development of Amazonia, making the region one of the 'hotspots' of global environmental change on the planet. Field studies carried out over the last 20 years clearly show local changes in water, energy, carbon and nutrient cycling, and in atmospheric composition, caused by deforestation, logging, forest fragmentation and biomass burning. The continuation of current trends shows that over 30% of the forest may be gone by 2050 (Alencar et al., 2004; Soares-Filho et al., 2006). In the last decade, research by the Large Scale Biosphere-Atmosphere (LBA) Experiment in Amazonia is uncovering novel features of the complex interaction between vegetated land surfaces and the atmosphere on many spatial and temporal scales. The LBA Experiment is producing new knowledge on the physical, chemical and biological functioning of Amazonia, its role for our planet, and the impacts on that functioning due to changes in climate and land use (http://lba.cptec.inpe.br/lba/site/).
There is observational evidence of sub-regional changes in surface energy budget, boundary layer cloudiness and regional changes in the lower troposphere radiative transfer due to biomass-burning aerosol loadings. The discovery of large numbers of cloud condensation nuclei (CCN) due to biomass burning has led to speculation about their possible direct and indirect roles in cloud formation and rainfall, possibly reducing dry-season rainfall (e.g., Andreae et al., 2004). During the rainy season, in contrast, there are very low amounts of CCN of biogenic origin and the Amazonian clouds show the characteristics of oceanic clouds. Carbon cycle studies of the LBA Experiment indicate that the Amazonian undisturbed forest may be a sink of carbon for about 100 to 400 Mt C/yr, roughly balancing CO2 emissions due to deforestation, biomass burning, and forest fragmentation of about 300 Mt C/yr (e.g., Ometto et al., 2005). On the other hand, the effect of deforestation and forest fragmentation is increasing the susceptibility of the forest to fires (Nepstad et al., 2004).
Observational evidence of changes in the hydrological cycle due to land-use change is inconclusive at present, although observations have shown reductions in streamflow and no change in rainfall for a large sub-basin, the Tocantins river basin (Costa et al., 2003). Modelling studies of large-scale deforestation indicate a probably drier and warmer post-deforestation climate (e.g., Nobre et al., 1991, among others). Reductions in regional rainfall might lead to atmospheric teleconnections affecting the climate of remote regions (Werth and Avissar, 2002). In sum, deforestation may lead to regional climate changes that would lead in turn to a 'savannisation' of Amazonia (Oyama and Nobre, 2003; Hutyra et al., 2005). That factor might be greatly amplified by global warming. The synergistic combination of both regional and global changes may severely affect the functioning of Amazonian ecosystems, resulting in large biome changes with catastrophic species disappearance (Nobre et al., 2005).
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