huh?Back in early November, I noted information suggesting hydropower may not be as carbon neutral as we thought. The word “may” was deliberately chosen.

At the time, I received an angry email from a leading hydropower supporter, which included links to some studies suggesting the data for high emissions was cherry-picked, or didn’t take net emissions into account adequately.

I soon found other studies dealing with these objections and suggesting significant net emissions from hydropower. At this point, I was reminded of the debate between warming deniers and climate scientists, except I was not sure which side was which.

The December 2006 issue of Nature contains a fascinating article on the subject: “The green image of hydropower may have been seriously overstated, warn Scientists.” The key paragraphs:

But that is where the agreement ends. On one side of the debate is Philip Fearnside, a conservation biologist at the National Institute for Research in the Amazon in Manaus. His work, based mainly on theoretical calculations, looks at water leaving dams. Many dams release water from several metres below the surface, so the flow goes through an abrupt pressure change. Fearnside calculates that this causes methane release, much as carbon dioxide fizzes out when carbonated drinks are opened. His latest results suggest that a typical tropical hydropower plant will, during the first ten years of its life, emit four times as much carbon as a comparable fossil-fuel station.

Lining up against him in a decade-long dispute are Luiz Pinguelli Rosa and his colleagues at the Federal University of Rio de Janeiro, who accuse Fearnside of exaggerating reservoir emissions. They complain in particular that Fearnside has extrapolated from measurements taken on the Petit Saut dam in French Guiana; the data were taken in the years immediately after the reservoir was created, when the store of organic matter would have been greatest.

With few data sets available on tropical dams, the debate has increased in acrimony without approaching a conclusion. Environmental groups question the impartiality of Rosa’s work, which is funded in part by the hydropower industry. Rosa strongly denies any bias, and in turn accuses Fearnside of seeking to show that “something is wrong with dams”.

One phrase is repeated throughout the article:

“… With few data sets available on tropical dams …”

“… Cullenward stresses that more data are needed …”

“… But matters are unlikely to change without more data …”

Overall, Nature seems to lean slightly toward Fearnside’s hypothesis — but also seems to regard the hypothesis as far from a settled question. In short, as I said in my first post: “Clearly this is an area where the IPCC needs to fund more research, quickly.”