In the spring, when the relentless Korean winter finally loosens its grip on the country, many of the 10 million residents living in South Korea’s capital city of Seoul head to the numerous hiking and biking trails that follow circuitous routes through the hills and mountains in the city or parallel the Han River in level paths that are perfect for families with children.

In the past, these millions of Seoulites — outfitted with colorful backpacks, hiking shoes, walking sticks, wide-brimmed hats or face scarves for sun protection, some on bicycles of every shape and size — would be exercising while breathing in the polluted air of a congested city. Like many other heavily populated Asian cities where emerging economies resulted in millions of cars and trucks clogging downtown and suburban streets and industrial neighborhoods that had little concern for wastewater treatment, Seoul’s air and water quality was abysmal.

These days, however, being outdoors in Seoul in any season is no longer bad for your health. In fact, Seoul is now one of the world’s leading “green” cities, as the municipal government, along with Korea’s major technology corporations, have managed to reverse the decades-long degradation of the city’s air and water quality, transforming a noisy concrete city into a softer, quieter and much greener urban fabric.

Transforming Seoul into a legitimate “green” city has not been easy. During the 1960s and ’70s, as South Korea’s industrial boom lifted the country from being the second-poorest nation in the United Nations to one of the richest and millions of Koreans moved from rural villages to Seoul for employment opportunities, the city’s environment suffered. Even in the present decade, increased prosperity continues to present new challenges. In just two years, from 2003 to 2005, car ownership in Korea increased from 215 automobiles to 319 for every 1,000 persons, leading to massive traffic jams in Seoul and the resulting air pollution. In 2005, Korea was the 16th-largest greenhouse gas emitter in the world, with 538 million tons of CO2 emitted annually.

The Seoul Metropolitan Government, under the leadership of Mayor Oh Se-hoon, has been very aggressive in dealing with the city’s environmental concerns, including the even larger issue of global warming. Using public and private funding sources, the city government has been able to greatly enhance air and water quality in Seoul. At the end of 2010, the SMG announced that the quality of Seoul’s air was the highest since 1995, when the city began measuring air quality. During Mayor Oh’s term, he reduced fares on Seoul’s extensive subway and bus system and made plans to construct seven new rail lines from the suburbs into the city center, with the goal of reducing commuter traffic. In 2008, the city converted its 72,000 taxis to run on liquefied petroleum gas instead of regular petrol; and in 2010, about 86 percent of Seoul’s bus engines were converted to compressed natural gas from diesel. At the end of 2010, Seoul became the first city in the world to put full-size electric buses into commercial operation when the Mt. Namsan route started using these high-tech vehicles. The buses, built by Korean firms Hyundai Heavy Industries and Hankuk Fiber, use high-capacity, lithium-ion batteries. All these measures are part of the SMG’s “Clean and Green Seoul” program, one of the city’s five ongoing core projects.

Seoul, one of the world’s most populated cities, is a major component of South Korea’s “Master Plan for Low Carbon Green Growth” initiative, which extends to 2030. The city’s goal for its public transport system calls for every bus, taxi and other public conveyance to be an eco-friendly, high-efficiency hybrid or electric vehicle. “More than 100,000 charging stations will be available in Seoul by 2020,” said Mayor Oh. Several Korean corporations have already taken a world leadership role in developing the emerging electric vehicle market. Last year, the mayor signed a memorandum of agreement with his counterpart in Los Angeles, Mayor Antonio Villaraigosa, to supply Korean-made electric vehicles to that U.S. city.

Seoul’s plans include reducing energy consumption and greenhouse gases; creating 1 million “green” jobs in 10 major green technology industries including hydrogen fuel cells, solar cells and waste management; transforming 10,000 buildings larger than 2,000 square meters into “green” buildings; creating 125 miles of bike-only lanes parallel to main roads to increase bike ridership; replacing city light sources with LED; expanding the use of high-speed KTX trains from Seoul Station; and increasing research and development funds in the new areas of nanotechnology and biotechnology.

Air pollution had once been one of Seoul’s major environmental headaches, literally and figuratively. When the city’s population and industrial base expanded during the last 40 years of the 20th century, millions of diesel cars and trucks began to clog Seoul’s streets, spewing out dirty blue smoke. Factories were constructed in the nearby suburbs when few, if any, environmental regulations were in place; and westerly winds brought collateral pollution from China’s emerging economy. The air above Seoul was often heavy with smog. In 2002, between January and August, the amount of particulate matter in the air reached 83 micrograms per cubic meter; in 2003, only 130 days had clean air quality.

The city’s efforts to clean its own air has paid off, however. An innovative campaign begun in 2006 allows car owners to choose which days during the month they will not drive into downtown Seoul. To encourage participation, car owners receive discounts for public parking lots and tunnel tolls on the days they do use their cars. About 40 percent of registered vehicles take part. Last year, only 55 micrograms of particulate matter per cubic meter was registered, and 225 days were classified as having clean air quality.

Two of Seoul’s most successful environmental projects concern waterways. The Hangang, or Han River, 308 miles long, is South Korea’s fourth-largest river, and part of it runs right through the middle of Seoul. The other waterway, called Cheonggyecheon, or the Cheonggye stream, is just 3.7 miles long, but both waterway redevelopment and enhancement projects have been stunning in their conception and for their contribution to the quality of life for residents of the city.

In 2007, the SMG started the “Hangang Renaissance” project, slated for completion in 2030. The water quality of the river, and the natural surroundings along its banks, was seriously compromised during Seoul’s modernization and industrialization period; but the ongoing recovery work has significantly changed, in a good way, how residents use the river. Eight new riverside parks have been created, high-tech fountains have been constructed (including the whimsical Moonlight Rainbow Fountain Bridge, which plays Vivaldi as it shoots out rainbow-colored sprays of water), and bicycle and jogging paths with lovely textured surfaces parallel the Hangang as it flows through the city.

With improved wastewater treatment plants and heavy fines for illegally dumping trash into the river, fish and plant life has returned. More than 70 species of freshwater fish, including sawbelly, mullet, carp and golden Mandarin fish, now live in the river; a fish observation platform, located on the bank of the river at Jamsil Park, allows visitors to view some of the larger fish species climb an artificial “fish ladder” as they move upriver. Five years ago, no one in Seoul would have predicted that water taxis, sightseeing boats, water skiers and jet-skis would be on the once-polluted Hangang; but today these are common sights on the river.

Along other segments of the riverbank, encompassing part of the Seoul Forest, nature trails wind through a willow forest and freshwater reed field; and a new observatory overlooks a bird and waterfowl area where increasing populations of gray herons, egrets, spot-billed and Mandarin ducks, mallards and moorhens come to eat and bask in the sun. Ridding the shores of harmful invasive plants and letting natural species like bulrushes, water parsley and water hyacinths propagate resulted in riverbanks now flourishing with birds, insects and amphibians. And while it is not possible yet to actually drink from the Hangang, the SMG was delighted to find little silver fish and planaria insects living in the Paldang Reservoir, the main source of Seoul’s pretreated drinking water. Both species can survive only in the cleanest of waters.

Perhaps an even more impressive revitalization project than the Hangang Renaissance was the redevelopment of Cheonggyecheon, a 600-year-old downtown stream that was part of Seoul since 1394, when King Taejo, the founder of the Joseon Dynasty, moved his capital to Hanyang (now Seoul). The stream, heavily polluted and prone to flooding, was paved over and covered during the city’s post-war urban renewal projects in the 1950s and ’60s; an elevated roadway was constructed on top of the paved-over stream in the 1970s. The elevated roadway, instead of alleviating downtown traffic, became a slow-moving, congested funnel for more than 160,000 heavy trucks and cars a day entering that portion of the city, severely adding to Seoul’s air pollution.

In 2003, South Korean President Lee Myung-bak, then the mayor of Seoul, decided that the elevated roadway needed to come down. More than $281 million was spent to remove the concrete pillars supporting the elevated road, exposing the covered stream bed, and to landscape the entire 3.7-mile-long Cheonggyecheon into a beautifully designed public park in downtown Seoul.

The restored stream, opened in 2005, is now filled with clean, fresh water through an ingenious use of underground city pumps and gates. The stream cools downtown temperatures during Seoul’s hot and steamy summers, and air pollution in the affected neighborhoods was significantly reduced when the elevated roadway was demolished. The stream is bordered on both sides by walking paths, native grasses, plants and flowers, as well as custom-designed benches and lighting. Although most of the pedestrian and auto bridges that cross the stream are stunning, Postmodern structures, the park’s designers were also able to reconstruct two beautiful, 19th-century stone bridges from salvaged blocks of the original stone. All of the bridges are now illuminated at night, creating a necklace of sculptured jewels along the course of the stream.

Since South Korea and Japan hosted the soccer World Cup in 2002, Seoul city planners have not only redesigned the capital to reflect new environmental landscaping techniques, but new “green” buildings with glass, wood and steel façades, designed by noted Korean and international architects, have helped Seoul achieve the prestigious title of World Design Capital 2010. Although much work remains to be done, when visitors now walk or bike on the new trails along the Han River under a clear blue sky, surrounded by thousands of slender, newly planted ginkgo trees, they will surely be impressed by the progress Seoul has made in becoming a clean, green and global city.