Imagine the commute of the future: "The Jetsons," or more "Blade Runner"?

Will family sedans crisscross in blue skies, or will machines crawl past craggy skylines in perpetual night?

Such visions are staples of the entertainment business but deservedly scarce in universities, government and industry. None have the crystal ball that shows which future technologies and behaviors -- some of which would surely astonish today's city slicker -- have become routine.

In the climate world, such a crystal ball would be useful. Some transportation experts, looking at population and economic trends, see a grand collision course: Hundreds of millions of people, mainly in the developing world, are moving to cities. Their energy and resource demands -- and the way they will get to work -- represent a huge variable in global emissions.

These experts say that unless someone guides cities' development, they could lock in a high-carbon infrastructure that makes it far tougher to fight climate change.

Urban areas already account for about two-thirds of world energy use, and they'll hit 73 percent by 2030, according to the International Energy Agency. The cities of the future will be bigger, and there will be more of them. According to the United Nations, there were 21 "megacities" in 2009 -- urban agglomerations whose population exceeds 10 million. By 2025, there will be 29 -- and they'll hold one-tenth of humanity.

And if fossil fuels do get locked in? According to Ivana Gazibara, senior adviser on "futures" at Forum for the Future, a British think tank, cities will still adjust to that reality. It just won't be pretty.

From 'Sprawlville' to 'Renewabad'
"There are going to be some pretty profound changes in the pipeline anyway that either force us to change in a kind of reactive way, or that we choose to anticipate today and pre-empt through innovation and solutions that we start thinking through now," she said.

Forum for the Future recently released "Megacities on the Move," a thought exercise about where the world's cities are headed -- toward infinite sprawl and traffic, or hyper-computerized commutes marching by each other in a synchronized flow? Will they adjust by cutting carbon and pollution, or could these become permanent scourges of urban life?

To get at these questions, the forum consulted researchers and city planners, but also companies like Vodafone, General Motors and Tata Motors. These companies are already strategizing for the cities of the future, because they want to make money there.

Depending on what happens, that money could come in very different ways. In the "Megacities" report, there are four possibilities for the city of the future, each with a basic organizing principle, each with its benefits and drawbacks.

"Sprawlville," for example, represents the ultimate triumph of the personal car: Cities widen and lengthen roads as quick as they can, but they still lose the battle to traffic jams, which crunch into a metropolitan heart attack.

In "Planned-opolis," by contrast, fossil fuels are too pricey to take for granted, so the government plans and computerizes its citizens' lives to make the city work. This allows many to work from home; others can call for a self-guided car that glides into a network of synchronized commutes.

Then there are two low-carbon scenarios, "Renewabad" and "Communi-city." In the first, megacities clean up by breaking free of national control and building dense development and public transit. In the second, power devolves even further: City centers disappear, and a person's entire life fits in a neighborhood. People build their own vehicles based on open-source technologies, such as electric cars, bikes and scooters. Some brew biofuel at home.

Can countries skip sprawl?
Forum for the Future doesn't predict which of these will take hold in tomorrow's megacities. But its report makes clear that these cities can take the low-carbon road or the high-carbon one.

The choice will be different in developed and developing countries. Consider Phoenix, one of the United States' fastest-growing metropolitan regions, and one of its most car-oriented. In 2005, 3.7 million called it home, but in 2030, 6 million will.

To cope, the region will spend $40 billion between now and 2030 to expand roads and add 89 new miles. It will also spend $16 billion on new transit, including a bus "supergrid"; much of that money comes from a decision seven years ago to devote more money to transit rather than roads.

That's not exactly Sprawl-ville, but transit is so late to the game that it won't be a major form of transport anytime soon. "Out here in the West, the orientation from the beginning has been to the auto," said Roger Herzog, senior project manager with the Maricopa Association of Governments, the group that put together the regional plan. He said when MAG made projections for the region, even with high investment in transit, it didn't expect transit to carry more than 5 percent of travel.

In Asia, the challenge is different: Much of the relevant infrastructure doesn't exist yet.

"The question is, how do you skip some of the rungs in that ladder that we climbed in the United States?" said Deron Lovaas, federal transportation policy director at the Natural Resources Defense Council. "How do you skip Sprawl-ville?"

Cities face growing pains with cars as the 'variable'
Ahmedabad, India, is one city trying to answer that question. Under imperial Britain, its clutch of textile plants earned it the nickname "Manchester of the East." These plants never strayed too far from the city center, so development grew densely. Even today, the average Ahmedabadi still travels about 5 or 6 kilometers (3.1 to 3.7 miles) to work, said Madhav Pai, director of the India Program for EMBARQ, a transport project run by the World Resources Institute.

Like other Indian cities, Ahmedabad expects prodigious growth. Its population of 4.5 million would make it the second-largest U.S. city. By 2030, Pai said, it's on track to grow to 10 million -- megacity scale.

To prevent the crippling traffic and pollution seen in many other Indian cities, Pai said, Ahmedabad is building 50 kilometers of bus rapid transit, or BRT, an approach that mimics trains but costs far less. The city is also requiring new development to occur along these corridors, so future residents won't have to have a car.

In Bangalore, by contrast, Sprawlville may be on its way. Pai said the economy is largely driven by information technology parks at the city outskirts. Suburbs billow around these parks, making car trips of 12 to 15 kilometers (7.5 to 9.3 miles) typical. A March article in the Indian newspaper The Hindu called Bangalore's roads "some of the most chaotic in the country."

Pai believes sooner or later, Bangalore will have to make a painful transition away from personal cars.

"Eventually, all cities will have to do this, because you're just otherwise going to be gridlocked. ... I can foresee a city like Bangalore becoming a Bangkok," he said. "You'll grow, and you'll grow with all the pain of a city stuck in gridlock."

To Pai and other smart growth advocates, cars are the variable that determine how spread out or compact a city is. Sam Staley, director of urban and land-use policy at the Reason Foundation, disagrees: He thinks Asian cities can have their cars and cut carbon, too.

Staley is researching mobility in China, where he believes cars are a reality of the future; "personalized travel" is simply too quick and inexpensive for transit to dethrone it.

Trying to drive toward a cleaner future
But he also thinks this can be a sustainable future, if the cars can be rid of their carbon emissions. In the United States, he said, almost all car pollution except greenhouse gases has been eliminated by technological advances. Electric cars and hybrids now present a way to do this for greenhouse gases. "We can decarbonize without having to shift travel modes. It's technically feasible," he said.

Staley sees a future China that has to throw both solutions -- roads and transit -- at cities that are modernizing and hungering for mobility. He says this is evident in the many cities setting their goals for travel modes. In Beijing, for example, transit's target is 27 percent, walking 20 percent, biking 23 percent, and cars 29 percent.

Most cities have transit targets of 40 to 50 percent. But cities aren't trying to get these riders out of cars -- they're trying to get them off of bikes and sidewalks. They're trying to move people farther, more quickly. Indeed, the shift to transit may even cause a net growth in carbon emissions.

The "Megacities on the Move" report agrees that people seek more "mobility" in this sense, but it also proposes that in the future, mobility can take new forms. In the super-automated "Planned-opolis," for example, companies have developed ever-better ways to convey video, audio and other data, so telecommuting has actually become a major climate- and energy-saving strategy.

In a short cartoon portraying "Planned-opolis," a character named Vee explains that her husband works from home: He's a "virtual engineer" controlling robots at a desalination plant, but he uses a massive switchboard at his house.

Vee herself goes to work in a car that drives itself -- an idea that made an impression in the transportation world when it showed up in the 2002 movie "Minority Report."

"It makes so much sense, doesn't it?" Vee says. "Switch off brain, and go to work. With this many people around, I'm glad that a megacomputer's in charge."

But do we want to live there?
Here, Vee speaks to changes that go beyond commuting and carbon footprints. "Megacities" proposes that urban organization changes the very fabric of society. Do elites wield more power? Does religion gain popularity? Is government centralized or decentralized?

Will the city even be a worthwhile place to live? Gazibara, one of the report's authors, said "Planned-opolis," as an example, has its merits -- a hyper-efficient use of resources and energy.

But "if you overrely on technological solutions and you overrely on central command," she said, "you might end up with a world that works, but you're not necessarily going to end up in a world that's very pleasant."

In this sense, "Megacities on the Move" ponders more than the vehicles we'll ride in, or the appearance and layout of the metropolis. There's a common thread to its four visions: that there's no way for tomorrow's cities to resemble today's -- if only because they can't.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500