While some delegates at last week's COP27 climate conference were criticized for arriving by private jet, others arrived in Egypt aboard flights that were described as being emission-free, as they were powered fully by sustainable aviation fuel. The flights were part of a program called "Greenliner," run by Etihad Airways, the national carrier of the United Arab Emirates, as a testbed for sustainable air travel solutions. Commercial aviation accounted for just over 2% of global CO2 emissions in 2021, which suggests it may be a modest contributor to climate change. But the number doesn't tell the whole story, as air travel affects the climate in more complex ways than just carbon emissions, and its impact is predicted to rise in the future, as more and more people will want to fly.<br/>Whereas renewable energy and electric vehicles offer clear pathways to decarbonize sectors like power and road transport, it's less straightforward for the aviation industry. As a result, the industry is not on track to meet its net zero target by 2050, and is banking on two thirds of that transition coming from sustainable aviation fuel (SAF). It's made from waste products and can curb emissions by 80% on average — but it currently accounts for just 1% of global jet fuel usage. Something needs to change, fast. That's why SAF is one of the key elements of the "Greenliner" program. "It's essentially a call to action," says Mariam AlQubaisi, head of sustainability at Etihad. "The idea came about in late 2019 to launch a message to the industry: let's try everything possible to decarbonize." The program is based around the Boeing 787 Dreamliner, but Etihad has a similar initiative, called Sustainable50, dedicated to the Airbus A350. Since its launch, Greenliner has championed the adoption of SAF and tackled issues such as plastic waste and inefficient flight routes.<br/>