British Airways and Air-France-KLM have proposed a worldwide emissions trading system for all airlines to help combat global warming. The system, which was presented at the United Nations climate-change discussions in Bonn this week, would limit every carrier’s greenhouse gas output and would calculate emissions based on the company’s annual purchases of fuel. Companies that don’t meet their targets would then have to get permits from other businesses that have fewer emissions. The proposal is an effort to include the airline industry in the global climate treaty, due to be signed in December in Copenhagen.