Even though they are ultimately cleaner than fossil fuels, many hydroelectric dams absorb so much solar radiation that they could take several decades to provide a net climate benefit. 

In a study published Feb. 25 in Nature Energy, researchers from The University of Innsbruck in Austria and the Free University of Bolzano in Italy analyzed data on 724 hydroelectric dams around the world, examining their environment, their energy output and the albedo, or ability to reflect solar radiation, of each one's respective reservoir. They found that 19% of the dams analyzed would take 40 years to offset the solar radiation absorbed by their reservoir, and 13% could take as long as 80 years — roughly the entire lifespan of a hydroelectric dam. 

Albedo measures how much solar radiation a given surface reflects back into the atmosphere. For example, the albedo of grasslands and a coniferous forest is about 20% and 10% respectively. 

Water, meanwhile, reflects just 5% of solar radiation, absorbing the other 95%. When reservoirs are built, they create large surfaces of water where land had once been, leading to greater absorption of energy on the Earth's surface and offsetting some of the benefit of using hydropower over fossil fuel.

"If you eat more calories than you burn every day, you will gain weight. In terms of energy, that is what's happening if you change the albedo," said coauthor Georg Wohlfahrt, an associate professor of ecology at the University of Innsbruck. "The amount of energy that comes in from the sun is constant, but when less radiation goes out, the amount of energy on earth increases." 

While scientists knew the effects of albedo on various bodies of water, the phenomenon's impact on hydroelectric reservoirs relative to how much energy they generate had not been studied.

There are roughly 3,700 hydroelectric dams being built around the world. They serve as a popular form of renewable energy because, unlike solar or wind power, hydropower can be stored, and is less dependent on weather patterns to produce energy. 

According to Wohlfahrt, the most climate-friendly dams are built in deep, narrow gorges, where the surface area is smallest relative to the amount of electricity each dam produces. On the other hand, dams with large, shallow reservoirs produce less power but absorb more energy, because of their large surface area. This imbalance reduces their climate friendliness. 

Dams found at higher latitudes tended to "break even" faster than those at lower latitudes and in the tropics, according to the study. The southern hemisphere sees more solar radiation, and the motivation for building hydroelectric dams tends to differ in countries lower on the globe. 

For example, according to Wohlfahrt, it will take 100 years for the electricity produced by Ethiopia's recently built Grand Ethiopian Renaissance Dam to compensate for the radiation the dam absorbs, though he acknowledges its primary purpose is to bring power to millions, rather than be a climate-friendly source of power.

"In the first world I think the motivation for building a reservoir will be to reduce fossil fuel in order to help reach climate goals," Wohlfahrt said. "And if that is the goal, then there are certain hydropower plants that won't help us get there." 

In addition to their albedo penalty, hydroelectric dams can displace animals and people, so it is somewhat disheartening for Wohlfahrt to see so many in the works. 

"It hurts in a way, but we need to be realistic," Wohlfahrt said. "Decarbonizing society is a huge challenge, because by 2030 fossil fuel use needs to really go down, and we need to use all the technologies we have." 

Despite the fact that about one-third of the hydroelectric reservoirs Wohlfahrt studied will take 40 to 80 years to break even with the solar radiation they absorb, almost half will take fewer than four. Moving forward, Wohlfahrt believes that as long as there are as few dams that take 20 to 30 years to break even, hydropower can be a viable source of renewable energy to meet climate goals. 

In future studies, Wohlfahrt plans to analyze the climate friendliness of hydropower more holistically, examining not only the albedo of reservoirs, but how much methane and carbon dioxide they release from the decaying plant matter they contain. 

The study, "The albedo-climate penalty of hydropower reservoirs," published Feb. 25 in Nature Energy, was authored by Georg Wohlfahrt and Albin Hammerle, University of Innsbruck; and Enrico Tomelleri, Free University of Bolzano.

Correction: A previously published version of this article misstated the albedos of grasslands and coniferous forests. The error has been corrected.