Huge tracts of the planet are already experiencing water stress. The oil that fuels the world's tractors is a finite resource; cheap phosphorus fertiliser cannot be guaranteed indefinitely. So, as human numbers multiply, the food on the plate becomes harder to serve. According to the UN, 2013 could turn out very badly. Climate change, too, is likely to compound problems, with increasing extremes of flood, storm and heat. North American harvests were hit in 2012 by drought and record heatwaves. Global grain reserves are already low, and a second disastrous harvest in the northern hemisphere could condemn hundreds of millions of crowded and angry city dwellers to poverty and hunger, with increasing likelihood of food riots.