Agence France-Presse: US President Barack Obama on Tuesday said it was “imperative to redouble our efforts” to combat global warming, as European leaders pressed Washington to take action on climate change ahead of next month’s summit. Obama met top European leaders for an EU-US summit here, shortly after German Chancellor Angela Merkel offered a heart-felt appeal for a climate protocol in a rare address to a joint session of the US Congress. “All of us agreed that it is imperative for us to …