Wheat and maize are among the world’s most important crops, covering nearly 400 million ha of Earth’s land surface (an area roughly twice the size of Mexico) and playing a major role in the diets and livelihoods of millions of people. Leaf characteristics (or leaf traits) of these crops—including photosynthetic rates—are widely used in models that predict how wheat and maize will respond to environmental change. Using a large global database, we evaluate how leaf traits of wheat and maize differ across cultivars, and link this variability to climate conditions. In doing so, we assess how leaf traits can be expected to vary under climate change, and in turn, influence predictions of future food security.