Rotten business

2019-03-07 05:12:01

By Fred Pearce THANKS to rotting vegetation in the hydroelectric reservoir that powers Europe’s space programme, French Guiana has become one of the world’s biggest emitters of greenhouse gases per head of population. The Petit-Saut reservoir has been supplying electricity to the launch site for Ariane rockets since 1995. Despite warnings from engineers that rotting trees would generate large amounts of methane, French authorities opted simply to drown 365 kilometres of rainforest to create the reservoir rather than first clearing the vegetation (New Scientist, 19 October 1991, p 9). Robert Delmas and colleagues at the Laboratory of Aerology Observation in Toulouse, France, have measured the amount of methane and carbon dioxide bubbling out of the reservoir for the past three years. Then they worked out the likely total output for the reservoir until 2015. Most of the submerged carbon will eventually surface as CO2, but around 5 per cent will become methane, a much more potent greenhouse gas. In the current issue of Global Biochemical Cycles (vol 13, p 503), the team reports that the methane will cause 85 per cent of the “greenhouse effect” from the reservoir. In terms of global warming, the total emissions from the reservoir over 20 years will be equivalent to 66 million tonnes of CO2, says Delmas. Shared out among French Guiana’s current population of 157 000, this amounts to 21 tonnes of CO2—or 5.7 tonnes of carbon—per person each year. This output is more than three times the total per capita emissions from burning fossil fuels in France and twice that in Britain. The findings show for the first time that greenhouse gas emissions from tropical nations can be dominated by the generation of supposedly “clean” hydroelectricity, rather than from burning fossil fuels. They also suggest that emissions of methane and CO2from hydroelectric reservoirs built in rainforests could be much higher than previously supposed (“Trouble bubbles for hydropower”, New Scientist, 4 May 1996, p 28). Delmas points out that reservoirs cover half a million square kilometres of the Earth—an area roughly the size of France. “Almost nothing is known about them as sources of greenhouse gases,” he says. “Field studies and especially gaseous measurements in tropical reservoirs are very scarce.” Concern is greatest for reservoirs that flood tropical rainforests, where high rates of decomposition will favour the formation of methane. Previous, less detailed studies of emissions from rainforest reservoirs have been criticised by electricity generators, but this study—whose authors include a researcher from the French electricity industry—will be harder to dismiss. According to Delmas, Petit Saut will produce more greenhouse gas than an equivalent fossil fuel plant during its first twenty years of operation. After that, the average annual emissions from the reservoir will fall,