Human populations are presently following a J-curve model and are in an exponential or “log” phase of growth, passing 6 billion persons in 1999. In the diagram above, note especially the large breaks in the time scale and how quickly our numbers have increased. The world population doubled from 1800-1930 and then again by 1975. Currently, r (the slope of the curve) is about 1.4 for humans worldwide, which means we can expect the next doubling of the human population in ~50 years (if the rate stays constant).

Environmental scientists, as well as sociologists, ethicists, and others wonder whether there is a K for humans that will shift us to an S-shaped curve, leaving us at a stable, sustainable population size. Some wonder if even our current population size is sustainable, if everyone had even a lower-middle class (U.S.) standard of living.