WASHINGTON — With energy legislation shelved in the United States and little hope for a global climate change agreement this year, some policy experts are proposing a novel approach to curbing global warming: including greenhouse gases under an existing and highly successful international treaty ratified more than 20 years ago. The treaty, the Montreal Protocol, was adopted in 1987 for a completely different purpose, to eliminate aerosols and other chemicals that were blowing a hole in the Earth’s protective ozone layer. But as the signers of the protocol convened the 22nd annual meeting in Bangkok on Monday, negotiators are considering...