The typical landscape might resemble the Netherlands: a crowded, monotonous tableau in which no aspect of nature is free from human manipulation, says environmentalist D. Meadows.

The state of the environment in the latter part of the next century will be determined largely by one factor: human population. If the species doubles its numbers by 2050, to nearly 11 billion, humanity may complete the devastation that accelerated so steeply in this century. Such unabated expansion in…