The planet may not feel any different today, but there are now 7 billion people on it, according to the United Nations.
That number will continue to rise, of course, and global incomes are likely to rise as well. That means more cars and computers, and bigger homes: the kinds of things Americans take for granted. It's that rise in consumption that has population experts worried.
Klaus Lackner, a physicist at Columbia University's Earth Institute, says as economies improve in places like India and Africa — where populations are growing fastest — they're going to want to live more like we do.
"It's very hard to convince people to stop consumption," he says.
But maybe the world's next billion will be happy with Hondas instead of Hummers.
"I would expect consumption in the future gets larger, but we also learn how to do things more efficiently," Lackner says, "so the raw material consumption may well go down."
But Lackner says consumption will eventually go up again; you can only tighten your belt so much. Physicist Daniel Kammen at the University of California, Berkeley, says there just isn't much incentive for rich countries to do that anyway.
"In many parts of the world, energy — and I hate to say this — is simply too cheap," he says.
Kammen, the head of an energy laboratory at Berkeley, says cheap energy enables Western countries to live high on the hog. And people want to copy us.
"There's a huge impact of the decisions that we make," Kammen says, "and also we export a lot of technologies."
The problem is there just isn't enough cheap energy or water or land for 9 billion or 10 billion people to live the same way. So what if Americans set a different example? Consume less by living smaller? The Japanese do it. Can small be beautiful in the U.S.? Some people think so.
'Creative Ways To Use Space'
In an industrial park outside of Sacramento, Calif., there's a factory inside what looks like an old airplane hangar.
ZETA Communities builds modular homes here. Project manager Scott Wade says they're not like "stick-built" homes — "stick-built meaning they build it one piece at a time," Wade says, "whereas we build it an assembly at a time."
One "assembly" is the floor, with duct work; then the walls, the ceiling, and so on. Workers make and assemble the parts for one home — about 1,500 square feet — in a single day.
"It is a higher quality because we have more control over it," Wade explains. "And we don't have the weather delays getting in the way."
Everything from the caulk along the walls to the lumber is certified green or is from sustainable sources. The walls contain extra insulation and every hole is sealed to make the buildings energy efficient. ZETA says that in the right climate, rooftop solar panels could provide the entire home's power — a so-called "net zero" energy home.
ZETA founder and President Naomi Porat sees cities as her company's big market.
"The population all around the world is moving toward the cities," says the former real estate executive. "Land is a vital resource, there's not a lot remaining, so we need to think about creative ways to use space."
In cities, modules can be stacked to make a new generation of efficient buildings. At ZETA headquarters, architect Taeko Takagi rolls out a blueprints with one of ZETA's prototypes.
"It is a micro studio," she says. "The units are under 300 square feet."
That's truly micro: smaller than most suburban living rooms. Porat says there's a group that might find this alluring, though: "What I call the technocrati generation that uses the city as its living room and kitchen and goes to practically a dorm room to crash at the end of the day."
But how do you convince someone to live this small?
"The psychology of convincing someone is to provide very simple things, like enough storage," Takagi says. "I like to provide a large sink, so that the person who's using it doesn't feel like they're lacking or living smaller and everything is miniaturized."
Since buildings consume about 40 percent of the nation's energy, they're a logical target for more efficiency. But Berkeley's Kammen says living smaller isn't the ultimate solution. With 9 billion or 10 billion people, rising consumption will overwhelm any efficiency, as well as our current sources of energy. What's needed, he says, is renewable energy that's cheap and won't run out.
"And by essentially every measure," he concludes, "we are not moving fast enough."