Carbon trading is a business arrangement in which someone who produces CO2 pays a fee, which in turn is used to support a project that reduces CO2. The carbon trading market is a measurable and deliverable means to offset emissions for which a person is responsible. If all airline passengers paid a "carbon tax", for example, it could have a broad impact on carbon reduction projects around the world.

Portland is a city that successfully implemented a plan for transit-oriented development. Its urban planners made the public domain more appealing and accessible by emphasizing density and pedestrian travel. These transit policies are credited with spurring urban rejuvenation in Portland and many cities, while slowing the postwar dominance of the suburbs.

Cities are population magnets, and soon two thirds of the world's population will be urbanized. People often move to cities for economic opportunities, and this requires that the public domain be more than a place of automobile passage, but an interface for commerce and human interaction. Seoul is one of those cities, set apart by its focus on creating a positive public environment.

Today, food travels an average of 1,500 miles from its production source to plate. Now, however, the end of cheap, abundant fuel is leading to the end of cheap food. Unpredictable fossil fuel costs and federal ethanol policy are both reshaping the global food economy.

Paris' citywide bike-sharing plan, Velib', has been so successful that the mayor wants to create a similar system for cars. Autolib', as it would be called, could offer a reliable and affordable alternative to driving personal vehicles in the city's congested streets. Some, however, feel that giving people the option to use a car in lieu of public transport is a step backward.

The majority of the world's population is now urban, making cities major sources of greenhouse gas emissions. In response, leaders from 40 cities met in London to tackle the challenge of making urban areas less polluting. The result is a worldwide network of cities - named C40 - that shares innovative strategies and advances climate change policy on the political agenda.

The Make It Right Foundation - founded by celebrity activist Brad Pitt - made history in storm-ravaged New Orleans by placing 150 pink symbolic homes in the city's hardest hit neighborhood, the Lower Ninth Ward. Each of Pitt's pink houses represents a sustainable home that will eventually be built in its place as money is raised by the foundation. Thirteen architects from around the world were recruited by Make It Right to design innovative, affordable, sustainable housing, which would also ensure the strongest protection possible against volatile weather.

The storm surge of Hurricane Katrina could have been mitigated had New Orleans' wetlands remained undeveloped. Now, citizens of the city's Lower Ninth Ward have begun the restoration of Bayou Bienvenue, which runs along the devastated neighborhood's northern border.

Melbourne, Australia's Council House 2, or CH2, has set a new standard for sustainable building design. While it employs established sustainable technologies, like energy and water conservation, the design includes many features that enhance the quality of the indoor environment.

His Highness the Aga Khan, spiritual leader of the Ismaili Muslims, has deep ancestral roots in the city of Cairo. His sponsorship of numerous social and economic development projects is grounded in the principle that people should be active participants in their communities and cultures.