Jackson had an up-and-down four years leading the EPA (Creative Commons).

Jackson's tenure included passing new, stricter emissions standards for cars and small trucks after deeming carbon dioxide and five other gases pollutants under the rules of the Clean Air Act. This was the first time the federal government limited global warming pollution.

Jackson's four years was not without its problems and frustrations. A cap-and-trade bill in 2009 that planned to focus on climate change and change how the U.S. makes and uses energy stalled in the Senate, and the Obama administration has since shifted its focus away from the environment and towards the economy. 

Jackson also drew controversy for using an alias under a private email address for communication with her E.P.A. colleagues. 

Read the full story on CNN.com here.

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