As it stands, the people of the planet seem to be leaning toward a peak in population followed by a gradual decline—a 21st-century world of the aged, which can be seen today in Japan or parts of Europe—but there is still a good chance of continued growth in our numbers. The U.N. had more recently predicted a leveling off at nine billion but now says we will reach the 10.1 billion figure in 2100—and potentially keep climbing as birth rates have not fallen as far or as fast as previously anticipated. If the choices we make are a little different, there could be as many as 16 billion of us by the end of this century—and that number may prove more than the planet can bear if our lifestyles don't keep pace with our numbers.