The 2009 TIME 100

Whenever I feel overwhelmed by the challenges we face to reinvent the energy economy and to stop global warming, talking to someone like MIT chemist Daniel Nocera does me good. Nocera, 51, is working to invent a way out of this disaster. He's developing a new method for making hydrogen fuel from water, taking his cue from plants' photosynthetic ability to split water using sunlight. His discovery makes it conceivable that by midcentury we could satisfy our global energy needs by splitting — each second — just a third of the water in MIT's swimming pool. That's all we'd need to power the world.

Scientists have long known how to split water into hydrogen and oxygen, but unlike plants, they've needed high temperatures and harsh solutions, or rare and expensive catalysts like platinum. Nocera's catalyst is the first made from cheap, abundant materials (cobalt and phosphate) that works in benign conditions: a glass of water at room temperature.

The crucial insight that makes this possible came to Nocera in 2004, when biologists figured out plants' water-splitting machine. They learned that the machine falls apart regularly, requiring the leaf to rebuild it from scratch. Nocera realized that the decades scientists had spent trying to make catalysts stable had been misguided. They too could let the catalyst break down, then use a small amount of solar energy to reconstruct it, again and again. Allowing what's worn out to die and then building something stronger shows up everywhere in nature. It's a great model too for how we can replace a burned-out energy strategy with a clean new one.

Krupp, a co-author of Earth: The Sequel — The Race to Reinvent Energy and Stop Global Warming, is president of the Environmental Defense Fund