Smart building: Freiburg’s energy efficiency standards are above the German norm. Photograph: ADEUPa de Brest

If you wanted a vision of how human beings might live in a green future, you could do worse than a trip to Vauban, Germany.

A neighbourhood of Freiburg, Vauban was once a military base that played host to the Nazis and then the French army. But in the 1990s, town leaders razed most of the barracks and re-developed the site as a green mecca.

Vauban boasts futuristic houses whose roof tiles have been replaced by solar panels. Buildings are crowded together to share energy. And cars, the mainstay of the modern, high-energy life, are virtually non-existent. Town planners laid out the road network to intentionally frustrate drivers while at the same time pouring resources into a robust public transport system.

In carbon terms, those efforts have yielded impressive results: Thanks to Vauban, Freiburg’s greenhouse gas emissions fell 14 percent between 1993 and 2007 – in spite of a 10 percent increase in population. That means the city is well on its way to surpassing the EU’s overall goal of reducing emissions by 20 percent from 1990 levels by 2020.

“We’re very proud of this,” said Andreas Markowsky, executive director of Okstrom, a local company that produces wind, solar and hydroelectric energy, and is based in one of Vauban’s signature buildings.

Vauban is one of a handful of European cities at the forefront of the fight against global warming. The successes they have achieved are a tribute to their citizens’ green zeal. But more than that, they offer possible clues to other cities across Europe – and indeed the world – trying to solve one of the most vexing policy challenges in that battle: how to improve energy efficiency?

Among climate researchers and pundits, energy efficiency is commonly referred to as “the low-hanging fruit”. An oft-cited study by McKinsey & Co, the management consultancy, identified it as the most cost-effective way to reduce carbon emissions.

Unlike carbon capture and storage or next-generation nuclear reactors, the technology for energy efficiency already exists and is on the shelves. McKinsey found that investments in this area tend to return billions of euros in cost savings over time. “In terms of the things we should be doing, this is a no-brainer,” says one official from the European Investment Bank.

As Europe gears up to improve its......