Solar-powered buses. Carbon neutral buildings. Motion-sensitive lights and water faucets.

Sounds like something out of ‘The Jetsons.”

But cutting-edge technologies like these are already the norm in some of the world’s greenest cities, where the environment takes precedence over industry and the debate over sustainable living has long been decided in favor of it.

As the world celebrates Earth Day, GlobalPost takes a look at six cities that are among the most environmentally friendly based on their energy sources, transportation options, sustainable planning and other factors:

Oslo, Norway

The Norwegian capital has always been one of the cleanest cities in the world, and has one of the lowest carbon emission levels among European cities (per capita).

Nearly 140 city buses will soon run on biofuel from household food waste, and municipal buildings have nearly phased out heating oil in favor of renewable energy.

The city hopes to phase out fossil fuels entirely by 2020.

Copenhagen, Denmark

Currently the title-holder for European Green Capital, Copenhagen saw nearly a third of its residents using bicycles to commute in 2010.

City officials hope to raise that number to 50 percent by 2015 and have set a goal for being carbon neutral by 2025.

Copenhagen boasts that North Harbour – already under construction – will […]