Speaking of mass starvation, did you know there's a theoretical limit to how many people the planet can support? It's mainly limited by available solar radiation, but there are other limits we would reach well before that one.
In the 18th century, economist Thomas Malthus famously worried that the population was growing much faster than the food supply. Many scholars shrug off his warning today, but near the turn of the 20th century, a food crisis loomed because of the lack of nitrates and ammonia. German chemists Fritz Haber and Carl Bosch bought some time by devising a nitrogen fixation process that takes gas from the air and turns it into fertilizer.
Today a different nutrient shortfall looms — a shortage of phosphorus. Our bodies need phosphorus to move energy around and to build cells and DNA. But our demand will likely outstrip our known supply within 30 to 40 years [source: Clabby]. The push for biofuel options will only deepen the crisis.
Currently, a large amount of phosphorus is lost in human and animal waste. Much of what remains ends up in the trash or washes away as farm runoff. Reclaiming these sources and finding new ones could buy some time, but everything has its limit — even the bounty of the earth.