Amazon deforestation could trigger drop in rainfall across South America

/ Rhett A. Butler

Deforestation could cause rainfall across the Amazon rainforest to drop precipitously, warns a new study published in the journal Nature. Using a computer model that accounts for forest cover and rainfall patterns, Dominick Spracklen of the University of Leeds and colleagues estimate that large-scale deforestation in the Amazon could reduce basin-wide rainfall 12 percent during the wet season and 21 percent in the dry season by 2050. Localized swings would be greater.

Study reveals the effects of deforestation on rainfall.

Deforestation in the Brazilian Amazon

Deforestation could cause rainfall across the Amazon rainforest to drop precipitously, warns a new study published in the journal Nature.

Using a computer model that accounts for forest cover and rainfall patterns, Dominick Spracklen of the University of Leeds and colleagues estimate that large-scale deforestation in the Amazon could reduce basin-wide rainfall 12 percent during the wet season and 21 percent in the dry season by 2050. Localized swings would be greater. Forest clearing in the Congo Basin would produce similar results.

The shift would primarily result from disrupting the forest’s water cycle. Trees absorb water from rain and then release moisture back into the air via the process of evapotranspiration. That moisture fuels further rainfall. When forests are cleared, evapotranspiration and more water runs off into rivers leaving less moisture for the formation of rain.

While the cycle has long been understood, the new study attempts to quantify the effects for a large forest area using newly available data from NASA.

Rainfall over the Amazon River in Colombia. Photo by Rhett A Butler.

“We looked at what had been happening to the air over previous days – where it came from and how much forest it had traveled over,” said Spracklen in a statement. “Our study implies that deforestation of the Amazon and Congo forests could have catastrophic consequences for the people living thousands of kilometers away in surrounding countries.”

Other research suggests that up to 70 percent of South America’s GDP is produced in areas that receive precipitation or surface water from the Amazon.

“The Amazon forest maintains rainfall over important agricultural regions of Southern Brazil, while preserving the forests of the Congo Basin increases rainfall in regions of Southern Africa where rainfed agriculture is important. Increased drought in these regions would have severe implications for their mostly subsistence farmers.”

However the results alone don’t provide a complete picture, at least for the Amazon. Other climate models indicate that warmer temperatures in the tropical Atlantic could reduce precipitation in the Southern Amazon, exacerbating the effects of drought in the region.

“Changes in regional climate could exacerbate drought-related tree mortality, which in turn would reduce carbon stocks, increase fire risk and lower biodiversity,” noted Luiz Aragao of the University of Exeter in a separate commentary also published in Nature. “Such changes might also directly threaten agriculture, which generates US$15 billion per year in Amazonia, and the hydropower industry, which supplies 65% of Brazil’s electricity. Society should therefore take urgent action now, to curb tropical deforestation and avert future environmental problems.”

There are positive signs that society — or at least Brazilian society — is responding to the deforestation threat. Forest clearing in the Brazilian Amazon has fallen by about 60 percent (7,000 sq km per year from 2009-2011) relative to the 1997-2002 baseline of 17,600 sq km used in the study. Should the trend hold, forest loss may be significantly lower than assumed by Spracklen and colleagues.

Nevertheless there remains cause for concern. Brazil’s Congress last week wrapped up language on a revision to the country’s Forest Code, which limits how much forest a landowner can clear. Environmentalists worry that the new Forest Code could usher in a new period of deforestation and reverse recent progress on saving the Amazon.

The effect of deforestation on rainfall.
Effect of deforestation on rainfall in the tropics. (a) Much of the rainfall over tropical forests
comes from water vapour that is carried by the atmosphere from elsewhere. But a large component
is ‘recycled’ rain — water that is pumped by trees from soil into the atmosphere through a process
called evapotranspiration. Water exits from forests either as run-off into streams and rivers, or as
evapotranspirated vapour that is carried away by the atmosphere. The atmospheric transport of water
vapor into the forest is balanced by the exit of water in the form of vapor and run-off. (b) Spracklen
and colleagues’ analysis suggests that deforestation reduces evapotranspiration and so inhibits water
recycling. This decreases the amount of moisture carried away by the atmosphere, reducing rainfall in
regions to which the moisture is transported. Decreasing evapotranspiration may also increase localized
run-off and raise river levels. Image and caption courtesy of NATURE


Luiz Aragão (2012). Environmental science: The rainforest’s water pump. Nature 05 September 2012 doi:10.1038/nature11485

D. V. Spracklen, S. R. Arnold & C. M. Taylor (2012). Observations of increased tropical rainfall preceded by air passage over forests. Nature 05 September 2012 doi:10.1038/nature11390

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