The document discusses future energy sources for air travel in 2050. It notes that while biofuels offer potential as a drop-in replacement for jet fuel, second generation biofuels are preferable to avoid competing with food resources. Promising biofuel sources mentioned include algae, woodchip waste, camelina, and waste products. The aviation industry aims to achieve carbon neutral growth by 2020 and halve CO2 emissions by 2050 compared to 2005 through more efficient fuel use and alternative fuel development.