Aviation emissions are a major clause of climate change, writes Valerie Brown – yet they remain unregulated. The gap between the best and worst performing airlines demonstrates ample opportunities for improvement – but is the political will there to impose effective regulation?The performance gap suggests the industry could reduce GHG emissions significantly if the least efficient airlines would emulate the most efficient.If commercial aviation were a country, it would rank seventh in global greenhouse gas emissions according to a recent report by the International Council on Clean Transportation(ICCT).