Our human population is increasing beyond anything Earth has experienced before. The rate of global population growth may now be slowing, but we are still expecting to top nine billion people by 2050, all of whom need to eat. It means we will have to produce more food in the next 40 years than we have had to for the past 8,000 years, when agriculture began. Ultimately all of our food – indeed, all life on Earth – relies on the conversion of carbon dioxide into sugars by photosynthesis, using the Sun‘s energy. Most plants, including most crops, use a chemical pathway for photosynthesis that binds three carbon atoms from the air. It‘s called the C3 pathway. But around 5% of plants have evolved a different pathway that binds four carbon atoms. This C4 pathway is not only more efficient at warmer temperatures, it also uses less nitrogen (fertiliser) and less water during photosynthesis and because the plants‘ pores, or stomata, need to be open for a shorter time compared with C3 plants to receive carbon dioxide. This means there is less opportunity for the leaves to leak water – C3 plants lose 97% of the water they take up through their roots to transpiration. So the C4 pathway is ideal for the hotter drought conditions that are increasingly prevalent owing to climate change. C4 plants are so successful, especially in tropical savannahs, that they are responsible for as much as 30% of all terrestrial carbon fixing, even though they make up a tiny percentage of plants. Scientists are now trying to genetically manipulate C3 crops to turn them into C4s.