U.S. wind power slipped last year for the first time in a quarter-century due to weaker-than-normal Midwest breezes, underscoring the challenge of integrating volatile renewable energy sources into the grid.

Power produced by turbines slipped 2% in 2023, even after developers added 6.2 gigawatts of new capacity, according to a government report Tuesday. The capacity factor for the country’s wind fleet — how much energy it’s actually generating versus its maximum possible output — declined to an eight-year low of 33.5%. Most of that decline was driven by the central U.S., a region densely dotted with turbines.

Wind is a key component of the effort to cut carbon emissions, but the data highlights the downside of relying on intermittent energy sources tied to the effects of global weather. Last year’s low wind speeds came during El Niño, a warming of the equatorial Pacific that tends to weaken trade winds. La Niña, the Pacific cooling pattern that dominated in 2022 and is poised to return later this year, usually has the opposite effect.