The 21st-century equivalent of the cavalry has come charging in to rescue cities in China and South Asia in their battles against air pollution and global warming. And it's also beginning to help out on the traffic-choked streets in London, New York, São Paulo and Los Angeles.

This is the electric bicycle, or "e-bike," a technology that blends the simplicity and mobility of a traditional bicycle with the speed of a moped or motorized scooter, but without the internal combustion engine.

Transportation experts say e-bikes -- along with electric cars, light-rail trains and more pedestrian-friendly cities -- could become one of the primary drivers of cleaner air and reduced global greenhouse emissions across much of the urbanized world, with China, India and Southeast Asia leading the fight to clear the air.

"The bicycle is an enormously efficient vehicle," said Ed Benjamin, managing director of eCycleElectric, a consulting firm to the light electric vehicle industry with offices in the United States, China and Taiwan. "The rolling resistance is minimal. They cost very little in terms of materials and the energy needed to build them compared to other vehicles. They don't require gasoline and can be parked almost anywhere."

"The problem," Benjamin added, "is we could say a bicycle is only good for healthy, strong people who are willing to get out in the weather. And there are large populations around the world that don't fall into that category."

Still, e-bikes -- defined as two-wheeled vehicles equipped with a traditional bicycle drivetrain but enhanced with an electric motor capable of propelling a bike as fast as 20 mph -- have solved the mobility problem for hundreds of millions around the world.

In China alone, more than 100 million e-bikes have been sold over the past decade, accounting for "the single largest adoption of alternative fuel vehicles in history," said Christopher Cherry, a University of Tennessee engineering professor and leading scholar on e-bikes.

In addition to being light and relatively inexpensive, e-bikes are also more climate-friendly than other modes of transportation, including gasoline- or diesel-powered cars and buses, and even electric passenger vehicles. In fact, carbon dioxide emissions for a Chinese e-bike are about one-tenth of what is emitted by a conventional electric car, when factoring in the electricity source needed to power the car's much larger batteries, according to research published recently by Cherry and colleague Shuguang Ji.

Yet despite their many positive attributes, e-bikes have been slow to win favor with consumers outside Asia, and they represent a tiny fraction of total U.S. bicycle sales. In the United States last year, official tallies show e-bike sales of 80,000 units, according to data compiled by Electric Bikes Worldwide Reports. That compares to 30 million e-bikes sold last year in China, 1.4 million in India, 400,000 in Europe and 300,000 to 350,000 in Japan.

Pedal-prone people grow older
While no two countries or regions have the same set of factors driving the adoption of e-bikes, most share a few things in common -- namely urban congestion, a lack of sufficient parking for cars and a cultural acceptance of two-wheeled vehicles as a viable form of daily transportation.

Throughout much of Europe, where the modern bicycle was invented in Germany around 1818, "the population is generally getting older and less mobile, but these are people who have ridden bicycles their entire lives," said Benjamin. "So the idea of being able to go longer distance at faster speeds, but still doing it on a bike, has broad appeal."

China's e-bike explosion dates to the mid-1990s, when large cities like Beijing and Shanghai adopted strict anti-pollution measures to alleviate some of the world's worst urban air quality. The country has its own cultural bond to two-wheeled transit.

In the 1860s, bicycles ridden mostly by foreign businessmen, students and missionaries began plying the streets of China's port cities, eventually spreading to interior cities and provincial capitals. But only after 1949, with the founding of the People's Republic of China, did bicycling see the kind of explosion that made for the popular image of Chinese thoroughfares jammed curb to curb with bikes.

According to bicycle historian Amir Moghaddass Esfehani, China's Communist Party leaders embraced the bicycle industry by merging small manufacturers into national firms, giving the industry preferential allowances of rationed materials and providing subsidies to Chinese workers to purchase bikes.

Cherry, the University of Tennessee engineering professor, said that while traditional bicycles remain part of the transportation mix, Chinese consumers have embraced the e-bike with revolutionary zeal. On recent trips to cities like Shanghai and Beijing, Cherry said, thoroughfares that used to carry a mix pedestrians and bicycles have become "veritable rivers of e-bikes."

'As clean as it gets'?
That transition, experts say, has netted huge benefits for China's urban air quality and has helped rein in greenhouse gas emissions from the country's transportation sector.

Recent research published by Cherry and colleagues in the journal Environmental Science & Technology identified e-bikes as the least-polluting form of motorized transportation in China, with significantly lower emissions of fine particulates (PM 2.5) and greenhouse gases than even conventional electric vehicles (EVs), when accounting for the source of electricity used to charge the cars' larger batteries.

While China's electricity sector is not uniform -- with some regions relying more on hydropower than on fossil fuels -- it remains, on the whole, dominated by coal plants that emit millions of tons of pollutants per year.

According to Cherry's recent study comparing vehicle emissions in 34 Chinese cities, carbon dioxide emissions from both internal combustion vehicles and EVs were an order of magnitude higher than those for e-bikes, which average just 250 watts and can be charged overnight using a standard wall plug. Conventional vehicle emissions came directly from tailpipes, while those associated with EVs come from power plant smokestacks, often located outside cities.

The result, he said, is that some urban areas are experiencing cleaner air conditions because the emissions-free electric vehicles are moving pollutants from city streets to power plants, but that the country's overall pollution budget remains unchanged or even slightly higher due to the additional generation by coal plants.

"Hands down, electric bikes are about as clean as it gets in terms of addressing these primary pollutants," he said.

To be sure, there are some downsides. Critics have noted that 95-plus percent of e-bike motors produced in China today rely on lead-acid batteries, which until recently were discarded as trash when they wore out, according to Benjamin. A gradual shift toward lithium-ion batteries, combined with an aggressive campaign by Chinese government officials to encourage battery recycling, has helped, but concerns about lead pollution persist in some areas.

And in some Chinese cities, where e-bike riders are vying for lane space with cars, traditional bikes and pedestrians, sometimes there are disastrous results. According to Chinese government data, the death toll from accidents involving e-bikes reached more than 3,600 in 2009, compared with 2,500 in 2007.

While some cities -- including Beijing and Shenzhen -- have responded to public safety concerns with e-bike bans, most have been poorly enforced or scrapped after the measures had little effect on e-bike ridership, Benjamin said.

Last March, the Chinese national government began enforcing decade-old standards requiring that e-bikes weigh no more than 40 kilograms (88 pounds) with top speeds of no more than 20 kilometers per hour (12.4 mph). Any e-bikes that exceeded those limits would be deemed substandard and be subject to confiscation.

U.S. consumers catch on, but slowly
In the United States, where e-bikes have captured a tiny niche within the larger bicycle industry, the challenge is convincing bicycle purists about e-bikes and converting users from strictly recreational riders into commuters for work or personal business.

Larry Pizzi, president of Chatsworth, Calif.-based Currie Technologies, the largest e-bike distributor in the United States, said the e-bike industry saw a sizable boost in 2008, when gasoline prices in the United States soared to nearly $4 per gallon. That trend repeated itself in 2011 as gas prices soared again, and could happen again this summer if pump prices go up.

"I think there is a growing interest, particularly among urban dwellers, that is directly linked to the cost of gasoline," Pizzi said. At the same time, he said, urbanites are seeking more convenient ways to navigate city streets and sidewalks without the hassle and cost of parking a motorcycle or full-size vehicle.

Benjamin said urban planners and developers are increasingly discussing how to efficiently move people to their workplaces, shopping districts, parks and other places that are within commuting distance but are not walkable due to terrain, weather or other obstacles.

"This is what transportation planners now call the 'first-kilometer vehicle,'" Benjamin said. "We used to drive those kinds of distances, and we touted the idea of 'inexpensive automobile transportation.' But those three words don't fit together anymore."

Not surprisingly, experts say the early adopters of e-bikes in the United States are people living in high-density areas such as New York, Los Angeles and Chicago, as well as in bicycle-friendly cities with hilly terrain such as San Francisco and Seattle.

E-bikes are also being deployed by police departments -- Los Angeles has police officers on 26 e-bikes -- and on traffic-choked college campuses such as the University of Tennessee, where Cherry and his associates have piloted a program to get students and faculty out of their cars and onto motorized bikes.

Pizzi said the U.S. market for e-bikes also includes many older Generation Xers and Baby Boomers who seek a recreational bicycle for short trips and mild exercise. "Their initial intent is to buy the e-bike to have fun recreationally, but what inevitably happens is they seed how easy and practical it is to use it for other things," Pizzi said. "They begin to use it for neighborhood transportation."

Cost remains a factor for e-bike adopters in the United States. An entry-level e-bike sold by Currie Technologies -- with most of its components coming from China -- runs around $1,000, roughly three times more than a conventional bicycle of equal quality, Pizzi said. Higher-end e-bikes, such as those used by the Los Angeles Police Department, can fetch as much as $5,000 per unit.

Even so, "almost every nation in the world has been exploring the idea of electric bikes," said Benjamin. And as Chinese production continues to scale up, with exports to as many as 200 countries last year, there's little reason to think the technology won't take hold elsewhere, including the United States.

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