Sein-Way Tan is a modest man with a global ambition - to make cities environmentally sustainable and economically vibrant.

From an early age, Tan has been captivated by all aspects of what makes a city; from a city's history to its social and economic development.

"I really can't put my finger on where or when my fascination with cities and urban development started," he says.

The founder and CEO of the Green World City Organisation, a global alliance of senior executives and experts involved in creating sustainable projects globally, has seen his interest in cities grow to become an all-consuming passion, and his advice is now sought by governments, politicians and business leaders alike.

Former US secretary of state Hillary Clinton is said to be an enthusiastic supporter, as are the former Indonesian president Susilo Bambang Yudhoyono, and Singapore's Prime Minister Lee Hsien Loong.

Tan has delivered his message at meetings of the Asia-Pacific Economic Cooperation forum, the Boao Forum for Asia and Oslo City Hall in Norway, home of the Nobel Peace Prize award ceremony.

He has advised dozens of governments around the world on creating green cities, including the Chinese government, and is also on the steering committee of the UN-Habitat World Urban Campaign, which aims to enhance the quality of life in cities throughout the world.

"A few years ago we passed the 50-50 mark - 50 percent of the planet is now urbanized," Tan says, adding that in 20 years' time it will be 60 percent.

"The question I keep asking myself is: What will cities of the future look like? Will they be sustainable and give those living in them a good quality of life?"

Born in Myanmar and raised in Singapore, Malaysia, Hong Kong and eventually Australia, Tan's family assumed he would pursue a career in medicine just like many of his family members. But it was not to be.

When he was 15, his parents decided to leave Hong Kong for the more laidback lifestyle of Australia.

Living in a modest suburb in Sydney was markedly different from the frenetic pace of Hong Kong and its towering apartment blocks and offices.

A bright student, Tan was drawn more to business and environmental issues rather than medicine. "I suppose medicine would have been the natural course for me. Both my parents were doctors, so was my grandfather, aunts, uncles and cousins," he says.

Growing up in so many different cities and cultures in Asia probably had an impact on his career path, he says.

"From an early age I was fascinated by cities ... the buildings and the people. Even today when I travel around the world advising governments, city mayors and the like, I often take a few days off to explore the city I am in."

Established in 2003, the Green World City Organisation is committed to creating sustainable cities and addressing pertinent issues such as rapid global urbanization, environmental pollution and climate change.

With a focus on cost-effective solutions, the organization boasts an advisory board made up of international experts who can advise governments on how to develop cities that are economically vibrant and more environmentally sustainable.

To that end, Tan says Green World City's framework adds significant value, minimizes risk and provides guidelines in many areas such as sustainable planning, green transport and green technologies.

A few years after Tan finished university he went into property investment and development, which felt like a natural step for him.

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