Humankind is changing the earth's environment at an unprecedented rate, with an ever-increasing population consuming more and more resources. Most scientists also now agree that the planet is getting warmer.

But what can be done? Are solar and wind power the answer? Some argue that dramatic change is needed, and fast.

Wangari Maathai, a long-standing Kenyan campaigner on the environment, sustainable development and human rights The fourth Principal Voices debate hears how carbon trading can -- perhaps -- save the world.
Amory Lovins, alternative energy expert, co-founded the Rocky Mountain Institute in 1982. Is the debate on global warming over?
David Hales, is president of the ecology-centered College of the Atlantic. Read white paper

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The third Principal Voices debate hears that alternative energy is here -- and competitive Read