The debate over the renewable status of hydro power has hit another hurdle with the revelation that reservoirs can emit large quantities of greenhouse gases (GHG).

A report prepared for the World Commis-sion on Dams and presented at a workshop as part of WCD’s thematic review process, discussed the role of reservoirs as emitters of greenhouse gases (mainly CO2 and CH4). The report stated that greenhouse gases are emitted for decades from all reservoirs in the boreal and tropic regions for which measurements have been made. This is in contrast to the widespread assumption that such emissions are zero.

A number of factors influence GHG emissions from reservoirs, including the inflow of carbon from the surrounding catchment area, but GHG emissions cannot be directly explained by the volume of submerged biomass or its carbon content.

The report disclosed that a dam intervenes in, and modifies, the natural carbon cycle within the catchment. The dam serves to store this carbon and interrupt its natural flow to the sea or to the atmosphere.

However, the workshop recognised that while dams may produce net greenhouse gas emissions, the timescale over which gases are stored and emitted and the quantity of gas released varies considerably for each reservoir. The WCD also appreciated that the release of GHGs should not detract from how dams compare to the other options available to any given country.

Luc Gagnon, whose research went into the report and is the senior advisor on climate change for Hydro-Quebec in Canada, says that despite these findings hydro power is still a powerful tool to reduce GHG emissions. The long term goal of the research is to predict accurately GHG emission from reservoirs.

WCD will release its final report, including research on this subject, in November 2000. (See article on pp44-5 of this issue for more information).