The fat of dead pigs, cattle and chickens is being used to make greener jet fuel, but a new study warns it will end up being worse for the planet.
Animal fats are considered waste, so aviation fuel made from the material has a much lower carbon footprint.
Demand for fuel made from animal by-products is expected to triple by 2030, with airlines leading the charge.
But experts fear scarcity will force other industries to use more palm oil – a huge generator of carbon emissions.
Airlines are under pressure to rein in their huge carbon emissions, which mainly come from burning fossil-based kerosene in aircraft engines.