When climate researcher Dailson Bertassoli went to measure greenhouse gas emissions at the Belo Monte hydropower plant in Brazil, the first thing he noticed was the bubbles.
Developers have built hundreds of hydroelectric plants in the Amazon basin to take advantage of the allegedly "green" energy generated by its complex of rivers.
But climate researchers now know hydropower is not as good for the environment as once assumed. Though no fossil fuels are burned, the reservoirs release millions of tons of methane and carbon dioxide as vegetation decays underwater.