The airline industry is pushing to support a United Nations proposal to limit pollution from international flights even though the measure may eventually cost companies $24 billion annually. The airlines are pushing nations to join the agreement, which would require companies to offset their emissions growth by funding environmental initiatives. The deal has backing from at least 60 nations, including the U.S. and most of Europe. Yet, developing nations led by China and India are still aiming to change key elements of the proposal.