COPENHAGEN -- The political script for a big climate-change conference in this Danish city has U.S. President Barack Obama and other world leaders flying in later this week to christen a new era of global environmental cooperation. In reality, the summit is shaping up as a pivotal economic showdown between the U.S. and China.

The International Energy Agency projects that nearly all the growth in global greenhouse-gas emissions over the next two decades will come from developing countries -- and that fully half of that total...