The unintended consequences of water
When the Furnas Dam was completed on the Rio Grande in 1963, it was a major project of Brazilian engineering. At 127 meters high and 550 meters long, it created a vast reservoir with a surface area of 1,473 square kilometers, holding a volume of water seven times that of Guanabara Bay. This massive project was designed for hydroelectric power generation and flow control, becoming a central part of the regional electrical grid. But in the process, it also created a sprawling, human-made laboratory for studying greenhouse gases.
The flooding of such a large area of terrestrial area traps immense quantities of organic material—trees, soil, and leaf litter—underwater. In the deep, oxygen-poor (anoxic) parts of the reservoir, a specific group of microbes called methanogenic archaea gets to work. Through a process of anaerobic decomposition, these organisms break down the submerged carbon, producing methane (CH4) as a byproduct.
This methane, a potent greenhouse gas, reaches the atmosphere through two primary pathways. One is diffusion, where the gas dissolves in the water and escapes across the surface. The other, more dramatic and significant pathway, is ebullition—the bubbling of methane directly from the sediments to the surface. Studies have shown that ebullition can account for 65% or more of total methane flux from a reservoir. Research at Furnas and other tropical reservoirs was instrumental in demonstrating that these emissions were significant, forcing a global re-evaluation of hydropower's climate impact.
Rewriting the climate-impact equation
For decades, large-scale hydropower was widely considered a "zero-emission" energy source. The research conducted at tropical reservoirs like Furnas fundamentally challenged this idea. Scientists discovered that methane emissions, especially in the first few years after a reservoir is filled, can be substantial. Over a 100-year timescale, methane is about 27 times more potent than carbon dioxide at trapping heat in the atmosphere, according to the IPCC's Sixth Assessment Report. Over a 20-year period, its impact is more than 80 times greater.
Measurements from Brazilian reservoirs showed a huge range of emission intensities, with some tropical dams having a greater climate impact per kilowatt-hour than a modern natural gas power plant. The emissions are not constant; they fluctuate with temperature and the biological state of the water. The highest emissions often occur within the first decade after flooding, as the most accessible organic matter decomposes, but they can continue for the entire life of the dam.
This work had direct policy implications. The findings from Furnas and other sites provided critical data that informed how the Intergovernmental Panel on Climate Change (IPCC) accounts for emissions from human-made reservoirs. The simple picture of "clean" hydro-energy became a more complex equation involving biogeochemistry, reservoir age, and latitude, with tropical dams identified as major sources of methane production.