Jeffrey Sachs
author, economist, and director of Columbia University's Earth Institute

A big thinker, Sachs could move the political conversation to a higher plane.

In The End of Poverty (Penguin, 2005), Sachs spelled out a hopeful plan to improve the lives of the world's billion-plus poor people. The Guardian newspaper said Sachs's latest tome, Common Wealth: Economics for a Crowded Planet (Penguin, 2008), "is suffused with what can only be described as irrepressible optimism" that humans can live well and avert climate-change catastrophe.

Upside | Having U2's Bono rhapsodize about him in a 2005 Time magazine bio could cinch the middle-aged rocker-wonk vote.

Downside | Some enviros think he's Pollyannaish.