The disparity in time frames also applies to climate change. On the face of it, the likelihood of longer droughts and more intense floods as a result of climate change seems to undermine the case for dams: They must be built larger to accommodate massive floods but smaller to justify their construction expense during droughts. Dam managers face conflicting imperatives: Reservoir water levels must be kept low so that no water needs to be released during floods, but low reservoir levels hamper electricity generation, the chief purpose of most large dams.
Dam advocates seem inclined to dismiss massive floods and droughts as infrequent occurrences that can be overcome with good engineering — even though, for example, the Amazon basin has undergone three unprecedented droughts and three extreme flooding years since 2005. Indeed, the website of the International Hydropower Association claims that the ostensibly steady flow of electricity generated by hydropower — so-called “base load” energy — is needed to offset the intermittency of electricity from wind and solar plants.
But events like the southern Africa drought that crippled Kariba Dam have struck at the idea of hydropower’s reliability, and an alternative idea — that dams ought to be used as supplements for wind and solar plants — is spreading. One of the fastest-growing segments of the hydropower industry is pumped hydro, which involves pumping water upstream into dams when electricity rates are low, usually at night, and then releasing it in the afternoon, when rates are high— in essence, it provides backup, not base-load, power.
The IHA touts dams as a clean technology, but that’s not quite true: Many reservoirs emit substantial amounts of methane, a potent greenhouse gas released by decomposing vegetation and other organic matter that collect in oxygen-poor reservoirs. A 2016 study in BioScience found that methane emissions from reservoirs constitute 1.3 percent all of global human-caused greenhouse gas emissions, and the highest-emitting reservoirs rival coal-fired power plants. It is commonly assumed that methane emissions occur chiefly in shallow, tropical reservoirs, as if it’s a problem for only a small number of dam projects. But according to John Harrison, a professor at Washington State University’s School of the Environment and one of the study’s authors, “There is strong and growing evidence that temperate reservoirs can produce methane at rates comparable to those reported from tropical reservoirs.”
Even so, the Intergovernmental Panel on Climate Change, which sets standards for measuring nations’ greenhouse gas emissions, doesn’t include reservoir emissions in its calculations; the IPCC is considering changing that policy next year. Growing understanding of the factors causing reservoir-generated methane could at least guide decisions about siting dams, avoiding places certain to produce high emissions.
The IHA acknowledges that many reservoirs emit methane, but it puts a different spin on the phenomenon. It cites its own study — cosponsored by the United Nations Educational, Scientific, and Cultural Organization, but not peer-reviewed — that “indicates that hydropower is one of the cleanest energy sources.” According to the study, only wind and nuclear energy emit less methane than reservoirs, and coal-fired power plants emit more than 40 times more. Therefore, the IHA concludes, countries that are reliant on coal should switch to hydropower.
But it is perhaps a measure of hydropower’s more modest standing in the world that the industry no longer presents itself strictly as the producer of stand-alone monuments of perpetual electricity generation, the way people once thought of Hoover Dam. Instead, the industry increasingly is offering to produce “smart dams” that complement other renewable electricity sources. The combination could, for example, address seasonal variations in precipitation by relying on solar power during the dry season and hydropower during the rainy season, when clouds impede solar power. Some reservoirs, most notably in China, now feature floating solar panels that simultaneously avoid occupying valuable land, reduce reservoir evaporation, and take advantage of existing power lines that distribute electricity from the dams.
“The existing hydropower fleet represents a tremendous opportunity to enable other renewable technologies” — wind, solar, and biomass — “to prosper,” Taylor, IHA’s executive director, said. “I think that’s starting to be understood.”
