Agence France-Presse: As marathon talks open in Copenhagen on climate change, US President Barack Obama is investing heavily in their success but is walking a tightrope as he weathers criticism both at home and abroad. Obama abruptly changed his plans and will head to the Danish capital in the finale of the December 7-18 conference, hoping to add his prestige to push through a deal to fight what he sees as a top threat to the planet. But the US Congress has not finished legislation to cut emissions …