Late last month, EPA chief Lisa P. Jackson announced she will step down after almost four years as administrator. Under her leadership, the agency declared for the first time that carbon dioxide was a danger to human health and could be regulated under the Clean Air Act, leading the EPA to develop a new regulatory regime to limit carbon emissions.

Jackson, 50, is expected to leave her cabinet position after Obama's State of the Union address in early 2013. Click here for the full story.