The 2008 TIME 100

I've long argued that one of our most critical environmental issues is the challenge of making our cities attractive, enriching and safe places to live. The best cure for destructive sprawl is to build cities people don't want to abandon, places where they can live healthy, fulfilling lives in densities that don't devour our landscapes, pave our wilderness and pollute our watersheds, air and wildlife. To achieve this, we need to invest in urban schools, transportation, parks, health care, police protection, and infrastructure that makes cities great magnets with gravity sufficient to draw back the creeping suburbs.

There is a moral as well as an environmental imperative to attend to landscapes that are home to so many. For more than 8 million New York City residents, the environment is not a Rocky Mountain meadow with pronghorns grazing beside an alpine stream. It's their transit system and office buildings, the parks where their children play.

No one understands this better than New York City's mayor, Michael Bloomberg, 66, who has not only worked to make his city livable but has also promised to make it a global model of sustainability. Mayor Bloomberg realizes that a better future for New York will not be constructed on jobs or housing alone. It must also include cleaner air, safer drinking water, more green spaces and a healthy, accessible Hudson River.

In addition to protecting the local environment, he has promised to make New York a paradigm in the fight against global warming. His visionary PlaNYC commits New York to plant 1 million trees, slash greenhouse gases 30% by 2030 and achieve the cleanest air of any big city on the continent. Mayor Bloomberg has stepped into the breach left by a Federal Government that has abdicated all leadership on global warming. With his pragmatism and boundless energy, he has shown that a city can be both great and green. If that idea can make it here, it can make it anywhere.

Kennedy is senior attorney for the Natural Resources Defense Council