He calls her 'one of our planet's greatest advocates'
                                
                                
                                    (Newser) -
                                    Swedish climate activist Greta Thunberg met with a president during her visit to Washington, DC, on Monday—but not the one currently in office. Instead, she met with former President Obama, who later praised her in a  tweet , the  Hill  reports. At 16, Greta is "already one of our...