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Scientific American 60 Second Science. I'm Annie Sneed. Solar, wind hydropower.

These renewable energy sources are all much better for the climate than fossil fuels, right?

Well, yes for wind and solar.

But it turns out that the picture for hydropower is more complicated than we think.

A new study by the Environmental Defense Fund

analyzed the climate impacts of 1,500 hydropower facilities across the globe.

That accounts for about half of hydropower generation worldwide.

The researchers looked at whether a facility behaves as a greenhouse gas sink or as a source.

To figure this out, they investigated all the different components that help determine

a hydropower facility's greenhouse gas emissions. There are so many factors that contribute to greenhouse gas emissions from hydropower,

but essentially the majority of greenhouse gas emissions arise from the reservoir itself as vegetation and soils are submerged

under water in the dam that is used for the hydropower generation. Alyssa Acho, a senior climate scientist at the Environmental Defense Fund and co-author of the study.

As the submerged vegetation decays, it releases methane or CO2. The larger, the surface area of the

reservoir, the more greenhouse gases that are going to be emitted from that

reservoir.

Also, the temperature plays a role as well.

How warm the reservoir is will affect

how much greenhouse gases are produced and emitted from the reservoir.

Through their analysis, ACO and her co-author Stephen Hamburg, also with the Environmental Defense Fund, discovered that the climate impacts of hydropower run the gamut.

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