Dam Reservoirs May Emit Double the Carbon They Sequester

Dams are one of the most impressive engineering feats – from the almost mythical way they produce energy, to the ways they vary around the world. These mammoth structures may protect us from flooding, but unfortunately, a new study could put a damper on our moment of appreciation. 

It turns out these marvels of engineering may not be the eco-friendly wonders we once believed them to be. “Carbon emissions from dams had been significantly underestimated,” Matthias Koschorreck, a biogeochemist, and the study's lead author, said in a statement. Koschorreck even goes so far as to suggest dams should be considered a source of emissions. The researchers estimate that the reservoirs can emit double the carbon they sequester.

This isn't entirely new data. Back in 2005, it was reported that hydroelectric power was actually producing large amounts of carbon emissions and methane. “Everyone thinks hydro is very clean, but this is not the case,” Éric Duchemin, a consultant for the Intergovernmental Panel on Climate Change (IPCC), told New Scientist. 

But this new data brings more than 30 years of research to the topic. It all has to do with the dams' reservoirs that supposedly capture a lot of carbon by accumulating plants from upstream that sink to the bottom of the water.

By trapping sediment in their reservoirs, dams supposedly also trap a lot of carbon. But more and more research is indicating that these reservoirs have increasingly been drying out. Without the water, there is no way to trap the carbon from the rotting plants and debris, meaning it is actually released into the atmosphere. According to the new study that used analysis of satellite data, an average of 15 percent of global reservoir areas were dry between 1985 and 2015.

"Taking into account drawdown areas, the ratio between carbon emissions and carbon burial in sediments is 2.02. This suggests that reservoirs emit more carbon than they bury, challenging the current understanding that reservoirs are net carbon sinks. Thus, consideration of drawdown areas overturns our conception of the role of reservoirs in the carbon cycle," the authors write in their study published in the journal Nature Geoscience.

The question then becomes: what can we do to make these reservoirs more eco-friendly? Since we cannot forgo the energy or services that dams provide, another solution must be found.