Seven billion. It was fewer than 1 billion in 1800, 3 billion in 1960, and 6 billion as recently 1999.
The number keeps growing. The planet does not. Thomas Malthus famously predicted in 1798 that at some point, it would all be too much: starvation and disease would kill people more quickly than we can replace them.
But is this year's milestone cause for worry? Or just the opposite -- celebration?
"With the population still growing by about 80 million each year, it's hard not to be alarmed," writes Robert Kunzig, the author of National Geographic magazine's January cover story, "7 Billion." "Right now on Earth, water tables are falling, soil is eroding, glaciers are melting, and fish stocks are vanishing. Close to a billion people go hungry each day."
A funny thing has happened on the way to the apocalypse, though. Yes, there is poverty more dire than most of us in America can imagine, but the growing population has not caused the world to collapse. Instead, we've conquered infectious diseases, learned how to grow more food, provided clean water even in crowded cities.
The much-discussed "population bomb" may yet go off, says Kunzig -- but so far it has not.
"Why did the population boom happen?" he asked in a telephone interview. "We conquered death. We saved a lot of children from dying."
What's more, though the world population continues to rise, the rate appears to be slowing. It is not because people are dying off; instead, it is because people have fewer children as they become wealthier. If babies are not dying of smallpox or cholera, their parents will need fewer just to keep the family farm going.
Already, in parts of Europe and East Asia, they worry about the opposite of a population explosion: not enough young people to support the growing number of retirees. Demographers say the average couple needs to have 2.1 children to keep the population steady. In western Europe, the actual number had dropped to 1.4 by the late 1990s.
Kunzig quotes Hania Zlotnik, director of the UN Population Division: "We still don't understand why fertility has gone down so fast in so many societies, so many cultures and religions. It's just mind-boggling," she told him. "The population as a whole is on a path toward nonexplosion -- which is good news."
Still, the educated guess among researchers is that the world population will not level off until mid-century, at about 9 billion.
"How we're going to feed 9 billion people by 2050 is a daunting question," said Kunzig, "but it's not 2050 yet."
There is a lot of time for civilization to solve daunting problems, he said, as it has in the past.
So we come back to our opening question: Is the population boom really a looming disaster or not? Kunzin said he concluded that population growth makes other problems larger -- from food to disease to climate to pollution -- but it is not the fundamental problem itself.
"Whether you think we're falling off a cliff has a lot to do with your personality," Kunzig said. "You have to walk right on the knife's edge between sounding too much like a Pollyanna on the one hand, and a Cassandra on the other."