Jackson's tenure marked by big fights with Republicans
                                
                                
                                    (Newser) -
                                    The Obama administration's chief environmental watchdog, EPA Administrator Lisa Jackson, is stepping down after a nearly four-year tenure marked by high-profile brawls over global warming pollution, the Keystone XL oil pipeline, new controls on coal-fired plants, and several other hot-button issues that affect the nation's economy and people'...