President Barack Obama ’s plan to curb greenhouse-gas emissions alongside China sets the stage for lengthy confrontations with other nations resistant to making cuts, Republicans who want to roll back regulations and environmentalists pushing for bigger reductions. In a surprise announcement Wednesday in Beijing, Mr. Obama and President Xi Jinpingof China, the biggest emitter of carbon dioxide, released their targets for cutting greenhouse-gas emissions more than a decade down the road. Under the proposals, Chinese emissions of carbon dioxide would peak by around 2030, while the U.S. would cut emissions by more than a quarter between 2005 and 2025. Groups concerned about climate change praised the White House for bringing China into the fold of industrialized economies eager to curb emissions. China’s move to lower reliance on fossil fuels to 80% of its energy raised the prospect of a huge expansion in the market for alternative energy.