Reuters: World leaders at the G20 summit disappointed environmental groups on Thursday who said their commitment to fight climate change had been vague. The leaders reaffirmed a previous commitment to sign a U.N. climate deal this year, a step the U.N. climate-change chief said was useful, though action would be better. The London summit had focused on averting a financial meltdown, pledging a $1 trillion package to save the world economy and boost fragile consumer and business …