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This story was originally published by Slate and is reproduced here as part of the Climate Desk collaboration.

In the rush to decarbonize the world’s economy, there’s one simple, surprising technology that’s more important than any other: water falling down a hill.

Huge dams fitted with hydroelectric power plants may seem very 20th century. Their basic technology — falling water turning a paddle wheel — hasn’t changed much in thousands of years. But hydropower is by far the world’s No. 1 renewable energy resource, and it’s going to stay that way for quite some time, despite growing questions surrounding its reliability.

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Over the next 25 years, the vast majority of the world’s newly installed renewable energy will come via hydroelectric dams, mostly in the developing world, according to a recent U.S. Energy Information Administration outlook. (To be fair, the EIA has a history of underestimating the growth rate of non-hydro-based sources of renewable energy, like solar and wind.)

Since making electricity by water requires a steady supply, the world’s increasing commitment to hydropower bakes in significant risk should weather patterns continue to become more erratic. In general, global warming will result in more intense rainfall events as well as more intense droughts and a loss of mountain glaciers that feed rivers in many parts of the world. For some places like the American West, Latin America, India, and Africa, that erratic energy future is already here. And nearly everywhere, less reliable hydropower could lead to dirtier energy use overall, at least in the short term.

“In a world in which the climate is changing, the value of hydro becomes more uncertain,” says Peter Gleick of the Oakland, California-based Pacific Institute. “We know that one of the worst impacts of climate change will be impacts on water — on droughts, on floods, on demand [via increased evaporation].” You can add energy to that list.

A recent report by Gleick showed that in the first three years of California’s current megadrought, residents of the state have faced an extra $1.4 billion in costs due to the shortfall in hydroelectricity generation and an associated increase in greenhouse gas emissions as natural gas power plants have taken up most of the slack.

That drought tends to increase carbon emissions in places like California isn’t immediately obvious. The reason is that, during droughts, natural gas power generation can be spun up quickly when it’s needed, unlike solar and wind. After all, you can’t just create more sunshine, or ask the breeze to pick up, whenever you need it.

“We had a big expansion in California in solar and wind over the last three years, but that would have occurred with or without the drought,” Gleick says. “We take 100 percent of the solar and wind we can generate … when we don’t have hydro, we’re not ramping up extra wind and solar, we’re ramping up extra natural gas.”

Most of the big dams in the American West were built decades ago. Despite repeated calls from farmers and other big water users, it’s likely the era of U.S. dam building is over. If anything, some particularly nasty dams are being removed to match an increased realization that in many cases, they’re a net negative on the environment.