Bloomb: A new international accord to fight global warming must extend greenhouse-gas limits to poorer nations that have fought caps for years, British economist Nicholas Stern said in a proposal today. All countries must pitch in to reduce emissions by a combined 50 percent and help avoid dangerous temperature increases, Stern said in a report released in the U.K. Currently a professor at the London School of Economics, Stern published a widely cited study of climate-change economics in …