5. Lisa Jackson
After eight years of a do-nothing, industry-cozy Environmental Protection Agency, green activists had high hopes for the new EPA administrator, appointed by a president who put climate policy near the top of his agenda. They soon had to lower their expectations, as Jackson took a hesitant and incremental approach to rolling back the highly controversial — and destructive — practice of mountaintop coal mining. But for all its wavering, the EPA took a truly historic step on Dec. 7 when it issued its long-awaited endangerment finding, which concluded that rising greenhouse gas levels posed a public health threat and enabled the agency to regulate these gases. Few expect Jackson to bear down on carbon emissions with a heavy hand, but her action served two important purposes: It showed international negotiators at the climate talks in Copenhagen that the United States was serious about cutting its emissions; and, more importantly, it put pressure on Congress to pass a comprehensive climate bill on its own terms.