The Amazon rainforest plays a very important role in the production of water vapor across the region. Around half of the area's rainfall comes from moisture evaporating from the forest (before it penetrates into the soil) and plant transpiration. The fewer trees there are, the less water is returned to the atmosphere.
Seventeen per cent of the rainforest has been felled in the last 30 years, and this figure continues to rise. In May 2005 the Brazilian government released figures showing deforestation in the Amazon reaching 10,088 square miles (26,129 square kilometers) for the year ending August 2004. This was the second worst year ever for Amazon deforestation, as huge swathes of rainforest were cleared for new cattle ranches and soy farms.
Restricted rainfall means drier forests, and drier forests are more susceptible to fire. This can lead to a feedback loop, as forest fires also inhibit rainfall production. The heavy smoke produced as the forest burns reduces the size of atmospheric water droplets and delays cloud formation.
While habitats like tropical savannah (open grassland) are adapted to fire, with many plant species able to thrive despite regular burning, the Amazon forest and its species are not. In the transition zones, between forest and savanna, when fires disturb both, the savannas recover more quickly and begin to replace the former forested areas.
As a result of this process Brazil has rocketed from low down on the list of CO2 producing nations to being one of the top four greenhouse gas emitters. About 75 per cent of Brazilian emissions come from deforestation, mostly from the Amazon forests. Globally, deforestation is responsible for about 25 per cent of greenhouse gases emissions around the world.
While scientists broadly agree that the Amazon's climate is getting hotter as a consequence of global warming, there is less concensus about the cause of the present drought. Brazilian research suggests that warmer temperatures in North Atlantic Ocean, the same factor which fuels storms and hurricanes over the North Atlantic, could be causing the reduction in cloud formation and rainfall over the Amazon.
There is another factor that could be contributing to this decline: a decline in the number of cold weather fronts which usually reach the Amazon during the autumn and winter. Rising temperatures on land have the effect of deflecting many of them out to sea. These two major factors would explain the very low level of the rivers across a vast area, stretching from Peru on the west coast of South America to the eastern Brazilian Amazon.
Due to the current lack of detailed information on the complex long-term cycles which drive ocean and atmospheric activity, the link between warmer sea temperatures and global warming cannot yet be conclusively proved.
But there are strong indications that we are facing a vicious circle in which Amazon deforestation decreases rainfall in the area and contributes to global warming, which in turn dries out the rainforest and causes it to die back. Without any attempts to break this cycle, the future for the Amazon is likely to be a bleak one - of more frequent and extreme droughts.