Eduardo Paes is the mayor of Rio de Janeiro, a sprawling, complicated, beautiful city of 6.5 million. He shares four big ideas about leading Rio -- and all cities -- into the future, including bold (and do-able) infrastructure upgrades and how to make a city "smarter."
Analysts of global environmental trends seek to topple the three pillars of sustainable development.
The Bullitt Center in Seattle is an experiment in sustainability, using recycled rainwater, solar panels and composting toilets.