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Tropical dams emit considerably more greenhouse gas emissions than their temperate counterparts yet are being treated as a solution to climate change, warns a report published in Nature Climate Change.

The problem, argue Philip Fearnside and Salvador Pueyo, is the result of errors in calculations by energy companies. The authors single out ELETROBRÁS, a Brazilian energy giant that is in the midst of a dam-building spree in the Amazon.

“Various mathematical errors have resulted in Brazil’s electrical authorities estimating the magnitude of emissions from reservoir surfaces at a level of only one-fourth what it should be,” write Fearnside and Pueyo, who note that ELETROBRÁS’s estimates should be 345 percent higher.

“The myth can no longer be sustained that tropical dams produce clean energy,” said Fearnside.

Dams in the tropics have two principle greenhouse gas emissions sources: carbon released from soil carbon stocks and dying vegetation when the reservoir is flooded and methane formed where organic matter decays under low oxygen conditions at the bottom of the reservoir. Methane release is facilitated by a dam’s turbines, which usually draw from the bottom of the reservoir.

“The reservoir’s function in transforming renewable carbon [from algae and plankton] into methane gives it the role of a methane factory, continuously removing carbon from the atmosphere as carbon dioxide and returning it as methane, with a much greater impact on global warming.”

Fearnside and Pueyo highlight the urgency of the issue by noting that Brazil plans to add 30 dams in the legal Amazon by 2020, including the controversial Belo Monte dam which will flood tens of thousands of hectares and displace more than 20,000 people, including indigenous communities. Meanwhile ELETROBRÁS aims to build more than a dozen dams in Peru and other Amazonian countries.

New dams aren’t just limited to the Amazon. Countries in the Mekong region are building dozens of dams, while Sarawak in Malaysian Borneo aims to build a series of rainforest dams on lands used by traditional forest people.

It is possible to reduce the climate impact of a tropical dam by minimizing the size of its reservoir and capturing methane emissions. Yet neither of these fixes address social conflict that often arises from forced displacement in dam catchments.

CITATION: Philip M. Fearnside and Salvador Pueyo. Greenhouse gas emissions from tropical dams. NATURE CLIMATE CHANGE | VOL 2 | JUNE 2012

Mongabay, 27 May 2012