Bloomberg: China is at the forefront of major developing nations that must help reduce greenhouse-gas emissions in a new treaty to stem global warming, U.S. climate envoy Todd Stern said. The Asian nation, the world’s biggest producer of heat- trapping gases, will need to make commitments in the worldwide agreement planned under United Nations leadership this year, the top U.S. treaty official said, without naming specific actions. The U.S. and China together produce more than 40 percent …