The aviation industry has announced what it claims is “a historic agreement” to reduce greenhouse gas emissions, but industry experts and environmentalists say the agreement is vague and lacks the enforcement mechanisms necessary to give it teeth.

At a meeting last week of the International Air Transport Association (I.A.T.A.), an industry group of more than 200 airlines representing 84 percent of the world’s air travel, the assembled airlines agreed on a plan to improve fuel efficiency by 1.5 percent annually until 2020, cap their net carbon dioxide emissions after 2020, and cut emissions in half by 2050 compared with a 2005 baseline.