Exponential growth occurs when system keeps growing a certain rate. For example, from 1900 to 2000, the per capita material output of the global economy grew at about 2.3% per year. As a rule of thumb one can calculate the doubling time of a given growth rate by dividing 72 by the growth rate. This means that per-capita output double more than three times during the twentieth century, an increase more than nine fold. Over the same period, the human population has increased from 1.6 billion to 6.1 billion, almost a fourfold increase total increase in material output has increased more than 36 times this century. Hoe many more times can our material output double?