It looks that our planet is warming. Just 2010 was one of the hottest years, since meteorological data started to be tabulated. Leaf photosynthesis was measured under different CO₂ air concentrations. The measurements were performed on plants that were grown in pots and fertilized with 0, 60, 120 and 180 kg of nitrogen per hectare. The leaf measurements were performed at all the development phases of this plant. As the CO₂ concentrations increased also the leaf photosynthesis increased until a maximum was reached, the saturation point. Nitrogen fertilizations changed the saturation point, which was increased as the nitrogen applications increased. At the beginning of the plant cycle, leaf photosynthesis was the highest, and as it advanced the measurements decreased until leaves died. For a warm earth, with a higher CO₂ concentration the increase of leaf photosynthesis is probably going to be due to the amount of nitrogen fertilizations because it can change the saturation point of leaf photosynthesis. The amount of other resources, fertilizers, water, etc., is going to be critical to maintain crop production of C₃ plants in a warm earth.