New farming technology—from selective breeding to chemical fertilizers—is assumed to be behind increasing U.S. corn yields over the past 3 decades. But another key contributor turns out to be something more basic: sunlight. Satellite measurements and a model of how efficiently maize converts that light to mass, reveal that solar brightening, an increase in the sunlight penetrating the atmosphere and reaching Earth, accounted for 27% of the yield increase U.S. Corn Belt farmers have observed between 1984 and 2013, researchers report today in Nature Climate Change. Many suspect crops in industrialized Western countries have been getting more light since the 1980s thanks to clean air regulations that brought down emissions of aerosols, which scatter and absorb solar radiation. But if the decrease in aerosols is driving the brightening trend and the yield increase, there’s a limit to how low those emissions can get. That means existing climate change models predicting the effects of rising temperatures and heat stress on maize may be counting on yield boosts that aren’t coming, and overestimating how much our corn fields will yield in the future.