France’s president, Nicolas Sarkozy, is one of the few world leaders to focus so much attention on making his capital city bigger and better. His project for Le Grand Paris is hugely ambitious. Over the next 20 years, he plans to expand the city so that its suburbs are no longer cut off from its heart while also making it the greenest metropolis in the world.

Architects have put forward plans that include moving the Élysée Palace to the grim north-eastern suburbs. Another involves stretching the city out along the Seine right up to the port at Le Havre. The president himself has also made clear that he is not averse to skyscrapers changing the Paris skyline – as long as they are beautiful, that is.

The French president is increasingly criticised for running a so-called “egocracy” and his plans for Paris may have something to do with his desire to leave as much of a mark on the capital as Napoleon III did.

But they also show an understanding of the significance of a capital city. Make yours the most ambitious, connected and green in the world and both your own people and your world counterparts will be impressed. The capital is a showcase for a country as a whole. And Sarkozy’s ambitions for Paris match his ambitions for France on the world stage. No modesty, no hesitation. Only bold plans to lead and not to follow.

Dickson Despommier’s Vertical Farm Project (he wants to create hi-tech indoor farms in towers in the hearts of cities) has a beautiful poetic appeal: local food production in an architectural package that perfectly expresses the idea of sustainable urbanism. A professor of environmental health sciences at Columbia University, Despommier has 96 students researching aspects of the experiment, which the US government is considering funding.

“We could ask questions like ‘Can I make a vertical farm for a restaurant, a school or a hospital?’” he says.

Italian architect Mario Cucinella wants to revolutionise the prefab with his Casa100k, a sustainable, low-cost housing scheme built around 100 sq m apartments that cost €100,000 each. Residents can personalise their homes by subdividing the space with sliding partitions and picking doors, windows and their flooring.

To keep CO2 emissions low, smog-eating cement, rainwater collection and photovoltaic panels are used; ideally, residents will generate more electricity than they consume. Tenants will also share a vegetable garden, launderette and tool shed.

“We wanted to combine three themes into one: affordable housing, the environment and sociability,” explains Cucinella. The first apartments go up next year outside Turin.

Greensburg, Kansas, seemed doomed in 2007, when 95 per cent of it was destroyed by a tornado. Instead, the 1,500-person town is now America’s greenest, relying solely on renewable energy, with over 30 LEED (Leadership in Energy and Environmental Design) certified buildings and hundreds of new, energy-efficient homes.

That tally dwarfs even the greenest big cities. As a result, Greensburg is now a template for green design and a destination for urban policy makers and property developers from across the US.

Daniel Wallach, founder of a non-profit environmental advocacy group, Greensburg Green Town, is the brains that put the shattered town at the cutting edge of US green thinking. Tasked by the city with organising $69m (€50m) of incoming government aid, he has also taught sceptical townsfolk about green building techniques – convincing them how to spend the $153m (€112m) in insurance claims that also flowed into the town. “We showed them that their ancestors respected the land, and we should too,” he says. “Losing everything frees your vision.”

Sweden is investing €40bn on transport over the next 10 years. Professor of transport analysis at the Royal Institute of Technology (KTH), Jan Eliasson, is helping make sure that money is spent in the right places. As head of the Centre for Transport Studies, set up at KTH in Stockholm in 2007, he looks after a group of researchers, transport agencies and consultants working on efficient, sustainable, transport solutions for roads and railways.

“The most cost effective and innovative transport systems are in developing countries. The combination of very big needs with very little money forces people to look for fresh solutions. We’re working on getting people here to adopt that thinking,” says Eliasson.

In 2005, Nic Marks, a statistician by trade, founded the Centre for Wellbeing at the London-based New Economics Foundation – a think tank looking at how to create a socially just low-carbon future. He looks at how happy or unhappy British people are and lobbies government to make policy changes where they fail to meet people’s wellbeing expectations.

There is a strong urban element to his work. “The type of system change that we need is to localise the economy. People make that out as being something bad but it’s not. Knowing the people around you, living embedded in communities is where you feel a strong sense of belonging and trust. It’ll be a great future.”

They call him the Metro Man, in honour of the €1.6bn New Delhi subway system he finished in December 2005, within budget and nearly three years ahead of schedule. Elattuvalapil Sreedharan, 77, is that rare combination of bureaucrat and engineer, who has the ability to build bridges, both literally and figuratively.

Delhi Metro’s trains are clean, punctual, safe and fast. Every day, they transport millions of commuters from the mosques and bylanes of Chandni Chowk to leafy Delhi University, Kashmere Gate and beyond. In an emergency, stations can be evacuated in four minutes.

Sreedharan achieved all this by demanding a remarkable degree of autonomy. In red-tape riddled India, he was able to get the system operational without kickbacks or a million permissions. A disciplined boss, he wakes before dawn, practises yoga, reads the Gita and other Hindu spiritual books and clocks in ahead of his workers.

Lahore has come calling as have more Indian cities and the UAE who want him to build their metros. What does Sreedharan want? Retirement, he says, with a twinkle in his eye.

Terry Schwarz, senior planner at the Cleveland Urban Design Collaborative and founder of the Shrinking Cities Institute, is an expert in transforming urban areas struggling with depopulation. The Institute’s Land Lab has been promoting temporary uses for abandoned sites in Cleveland, such as turning a petrol station into a community space, as well as piloting up to 100 projects to test how the city can make its water usage greener.

“A lot of the strategies we’re putting forth are kind of untested,” she says, “but the bigger risk is not taking risks, because if we keep going the way we have been, the long-term implications for Cleveland are not very good.”

Beijingers are breathing easier in their polluted city, thanks in part to the work of CS Kiang. This world-renowned air quality expert returned to China in 2001 after retiring from the Georgia Institute of Technology, becoming founding dean of Peking University’s new College of Environmental Science and advising senior government officials on cleaning up Beijing’s noxious air ahead of the 2008 Olympics.

Though Dr Kiang says that China has made great strides in the past decade, its work is far from finished. “In the last 30 years they have been all about manufacturing. In the next 30 years they need to respect intellectual property and the rule of law,” he says. “You need innovation for renewable energy and technology.”

Still, Beijing has retained many of its Olympics anti-pollution measures, so the scientist and lobbyist has now shifted his focus to working on a climate-change agreement between China and the US. “We need a tenable resolution to the dual challenges of the global economic crisis and of climate change,” he says. “We want to establish standards that can be utilised for the whole world.