WASHINGTON — It was almost unthinkable. The president of the United States walked into a meeting of fellow world leaders and there wasn't a chair for him, a sure sign he was not expected, maybe not even wanted. Barack Obama didn't pause, however. "I'm going to sit by my friend Lula," he said, moving toward Brazilian President Luiz Inacio Lula da Silva. A Brazilian aide gave the U.S. president his chair, and Obama spent the next 80 minutes helping craft new requirements for disclosing efforts to fight global warming. Along with India, South Africa and Brazil, the key member in...