By ELISABETH ROSENTHAL


BOGOTA, Colombia - Like most thoroughfares in booming cities of the developing world, Bogota’s Seventh Avenue resembles a noisy, exhaust-coated parking lot - a tangle of cars and minibuses that have long provided transportation for the masses.

But a few blocks away, sleek red vehicles full of commuters speed down the four center lanes of Avenida de las Americas. The long, segmented, lowemission buses are part of a novel public transportation system called bus rapid transit, or B.R.T. It is more like an above-ground subway than a collection of bus routes, with seven intersecting lines, enclosed stations that are entered through turnstiles with the swipe of a fare card and coaches that feel like trams inside.

Versions of these systems are being planned or built in dozens of developing cities around the world - Mexico City, Cape Town, Jakarta, Indonesia, and Ahmedabad, India, to name a few - providing public transportation that improves traffic flow and reduces smog at a fraction of the cost of building a subway.

But the rapid transit systems have another benefit: they may hold a key to combating climate change. Emissions from cars, trucks, buses and other vehicles in the booming cities of Asia, Africa and Latin America account for a rapidly growing component of heattrapping gases linked to global warming. While emissions from industry are decreasing, those related to transportation are expected to rise more than 50 percent by 2030 in industrialized and poorer nations. And 80 percent of that growth will be in the developing world, according to data presented in May at an international conference in Bellagio, Italy, sponsored by the Asian Development Bank and the Clean Air Institute.

To be effective, a new international climate treaty that will be negotiated in Copenhagen in December must include “a policy response to the CO2 emissions from transport in the developing world,’’ the Bellagio conference statement concluded.

Bus rapid transit systems like Bogota’s, called TransMilenio, might hold an answer. Now used for an average of 1.6 million trips each day, TransMilenio has allowed the city to remove 7,000 small private buses from its roads, reducing the use of bus fuel - and associated emissions - by more than 59 percent since it opened its first line in 2001, according to city officials.

TransMilenio last year became the only large transportation project approved by the United Nations to generate and sell carbon credits. Developed countries that exceed their emissions limits under the Kyoto Protocol can buy credits from TransMilenio to balance their emissions budgets, bringing Bogota an estimated $100 million to $300 million so far, analysts say.

Indeed, the city has provided a model of how international programs to combat climate change can help expanding cities pay for transit systems that would otherwise be unaffordable.

“Bogota was huge and messy and poor, so people said, ‘If Bogota can do it, why can’t we-’ ‘’ said Enrique Penalosa, an economist and a former mayor of the city who took Trans- Milenio from a concept to its initial opening in 2001 and is now advising other cities. In 2008, Mexico City opened a second successful bus rapid transit line that has already reduced carbon dioxide emissions there, according to Lee Schipper, a transportation expert at Stanford University in California, and the city has applied to sell carbon credits as well.

But bus rapid transit systems are not the answer for every city. In the United States, some cities, like Los Angeles, have built B.R.T.’s, but they tend to serve as an addition to existing rail networks.

In some sprawling cities in India, where a tradition of scooter use may make bus rapid transit more difficult to create, researchers are working to develop a new model of tuk-tuk, or motorized cab, that is cheap and will run on alternative fuels or with a highly efficient engine. “There are three million auto rickshaws in India alone, and the smoke is astonishing,” said Stef van Dongen, director of Enviu, an environmental network group in Rotterdam, the Netherlands, that is sponsoring the research. “So this could have a huge impact.”