Deforestation reduces rainfall in tropics, says study

If deforestation continues, rainfall rates could drop, say scientists Copyright: Flickr/CIFOR

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[SANTIAGO] Large-scale deforestation in tropical rainforests can dramatically reduce rainfall rates both locally and thousands of kilometres away, according to a study published in Nature yesterday (5 September). This could have a potentially devastating impact on communities living in or close to the Amazon and Congo rainforests.

This drop occurs because deforestation reduces the natural recycling of moisture from soils, through vegetation, and into the atmosphere, from where it returns as rainfall.

When forests are replaced by pasture or crops, this water recycling process changes, "leading to reduced atmospheric humidity and potentially suppressing precipitation", according to researchers from the University of Leeds and the Centre for Ecology and Hydrology, in the United Kingdom.

To measure this activity, researchers combined satellite data on vegetation with data on the direction of prevailing winds. They combined this with satellite data on precipitation to determine whether the air’s increased exposure to vegetation had an effect on rainfall. The model was applied in the Amazon and Congo basins, the two largest river basins in the world.

The study showed that "for more than 60 per cent of the tropical land surface, air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation".

Based on their findings, the authors say the Amazon basin could experience an estimated 12 per cent reduction in wet-season rainfall and a 21 per cent decrease in dry season rainfall by 2050, if Amazon deforestation rates reach the level predicted.

It is even possible that these reductions could extend as far as the Río de la Plata river basin, thousands of kilometres south of the Amazon in southern Brazil, northern Argentina, Paraguay and Uruguay.  

"Our study implies that deforestation of the Amazon and Congo forests could have catastrophic consequences for the people living thousands of kilometres away in surrounding countries," said Dominick Spracklen, lead author of the study.

"For more than 20 years, climate models have been predicting that precipitation over tropical forest regions would decrease in [the] case of large-scale deforestation," said Marcos Heil Costa, head of the Research Group on Biosphere-Atmosphere Interaction at the Federal University of Viçosa, Brazil. "[But], the verification of these predictions has been limited to the small scale [until now]".

Luiz Aragão, a senior lecturer in earth systems science at the University of Exeter, said the scenario used by the authors "is likely to overestimate the extent and magnitude of rainfall reduction in the mid-twenty-first century".

This is because Brazil "is committed to limiting historical rates [of deforestation] by 80 per cent by the year 2020," thus invalidating forecasts based on current rates.

But Spracklen disagreed, saying that due to an unpredictable political landscape, "it is not yet clear that deforestation rates will remain low".

Link to full paper in Nature


Nature doi:10.1038/nature11390 (2012)