Ladybower Reservoir in Derbyshire. Credit: justfluff via Flickr.

We only said a brief note of thanks for the arrival of the new version of what used to be Conservation magazine, one of our most-cited sites. Now, after some days of observing Anthropocene, we are even more grateful for their effort at overhaul:

Dam greenhouse gas emissions really add up

by Sarah DeWeerdt

For all their ecological faults, hydropower dams are usually thought of as a source of green, carbon-neutral energy. But it turns out that the reservoirs behind dams release a significant amount of greenhouse gases that, until now, have gone unaccounted for in global carbon budgets.

A study in 2000 first suggested that reservoir emissions might be a problem. But that study was based on measurements from only 22 reservoirs, and in the succeeding years it has been difficult to quantify these emissions at a global scale.

Now, an international team of researchers has produced such a comprehensive take by compiling data on reservoir releases of the greenhouse gases carbon dioxide, methane, and nitrous oxide from dozens of other individual studies.

The new analysis covers reservoirs behind hydroelectric dams as well as those constructed for other purposes such as flood control, irrigation, navigation, or recreation. It has better coverage of reservoirs outside the tropics than previous efforts. And it provides the first global estimate of reservoir releases of nitrous oxide, a greenhouse gas nearly 300 times more potent than carbon dioxide.

The researchers estimate that reservoirs cover nearly 306,000 square kilometers worldwide. The study includes data from 267 reservoirs with a surface area of over 77,287 square kilometers, about one-quarter of the global total. However, the team didn’t have measurements of all three greenhouse gases from all reservoirs.

Greenhouse gas emissions from reservoirs total 0.8 gigatons of carbon dioxide-equivalent yearly, the researchers report in BioScience. That’s roughly 1.3% of all human-caused greenhouse gas emissions as estimated by the Intergovernmental Panel on Climate Change (IPCC)…

…In the meantime, greenhouse gas emissions from existing reservoirs are substantial enough that they need to be taken seriously, the researchers argue. “We argue for inclusion of [greenhouse gas] fluxes from reservoir surfaces in future IPCC budgets and other inventories of anthropogenic [greenhouse gas] emissions,” they write.

Source: Deemer BR et al. “Greenhouse gas emissions from reservoir water surfaces: A new global synthesis.” BioScience. 2016.

Read the summary in its entirety here.