But the increased use of biofuel for aviation — manufactured from animal fat and vegetable oil — could create new problems, according to a new study funded by the Brussels-based Transport and Environment, a nonprofit organization campaigning for “a zero-emission mobility system.”

There might not be enough dead animals to meet the demand for biofuel, the study says. And killing more of them doesn’t seem to be the right path, it adds.

Lower quality animal fats have already been long used for producing pet food, soap, candles and combustion material for boilers. This means soaring demand from the aviation industry could crowd out these manufacturers, forcing them to turn to alternatives such as palm oil, which would result in increased net carbon emissions globally, according to the study.

“In the worst case, moving animal fats out of existing uses could cause a large increase in net emissions rather than any reduction,” the study’s author, Chris Malins, wrote.

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Palm oil in theory creates less climate-changing emissions than traditional fuel. But to produce it, farmers plow land that was once rainforests and swamp, reducing habitats for endangered species and undermining the Earth’s ability to absorb existing greenhouse gases.

Slaughtering more animals doesn’t appear to be a viable alternative.

More than 70 billion animals are killed every year, the study says, primarily to provide meat for human consumption. This demand incentivizes meat producers to convert natural lands for meat production, creating more methane, a gas that can have an impact 34 times greater than carbon dioxide over a 100-year period. Meat production was behind more than half the greenhouse gas attributable to agriculture from 2018 to 2020, according to the United Nations.

Competition for the raw material needed to produce sustainable fuel will “come to a head” over the next five years, according to Malin’s study, which cited the International Energy Agency, a group of 31 industrialized countries. Demand will exhaust the supply of used cooking oil and animal fats over that period, the IEA report says.

This growing dilemma between using animal fat for existing purposes and turning more of it into green fuel puts into question the climate benefits of biofuel, Malin writes. In addition to rising prices for biofuel, cats “all over Europe may end up finding their dinner a little less appetizing,” Malin writes, as more animal fat is diverted for green fuel than pet food.