Hydropower has a far greater climate impact than thought. Reservoirs—including those created by hydropower dams—contributed nearly 29 million metric tons of carbon emissions in 2021 alone. Hydropower makes up at least 1.3 percent of total global carbon emissions. As organic matter, including vegetation, dead animals and even fertilizer runoff, gets carried downstream, it piles up in large quantities behind dams and decomposes in the reservoirs. Normally, that decaying organic matter would eventually reach the ocean, where chemical reactions would convert the methane into CO2 and other compounds. But in the oxygen-depleted waters of a dam reservoir, that transformation often can’t occur, resulting in reservoirs having a far larger climate impact than lakes and other natural bodies of water.
