Single-car drivers commuting in fossil-fuel burning cars, smog,
pollution, crime -- what other urban scourges can you think of?
Half of the world's population currently lives in urban areas;
yet these urban areas make up only 2 percent of the world's land
and spend three-quarters of the world's resources [source: MIT].
That's a lot of people in a very small space consuming a great
deal. Between now and the year 2050, urban growth will only
continue to rise: 89 million homes and 190 billion square feet
(about 17.5 billion square meters) of retail and other
nonresidential space will be built in the United States alone
[source: National Resources Defense Council]. And in conjunction
with that density, pollution is soaring. London, for instance,
released about 45 million tons (about 41 million metric tons) of
carbon dioxide, a greenhouse gas, into the atmosphere last year
[source: IEEE]. By greening cities and neighborhoods around the
world, we have the opportunity to make a positive impact on
global warming. How