At the dawning of the 20th century, there were 1.6 billion people on Earth. Now, at century's end, there are nearly 6 billion. The phenomenal population growth has renewed a longstanding debate about how many people Earth can support. Thomas Robert Malthus launched the debate 200 years ago, predicting that global population would eventually overwhelm food supplies. Technological advances thus far have enabled agricultural productivity to outpace population growth. But the rekindled debate over mankind's survival is about more than food supplies: Population growth causes environmental problems from water shortages to global climate change.