Reuters: President Barack Obama’s negotiators make their debut at U.N. climate talks on Sunday but U.S. promises of tougher action are unlikely to brighten prospects for strong treaty now overshadowed by recession. Up to 190 nations meet in Bonn from March 29-April 8 to work on plugging huge gaps in a pact due to be agreed in December. Some industrial nations — Japan, Russia and Ukraine — have not even set goals for key 2020 cuts in greenhouse gas emissions. The U.N.’s climate chief …