Global aviation accounts for 2 percent of man-made carbon emissions. Scientists at the Australian National University are trying to shrink that number by growing jet fuel on trees -- specifically, Australia's eucalyptus trees, also called gum trees.

Jets and missiles require a high-energy fuel, which makes finding a suitable alternative to fossil fuel challenging.

"Renewable ethanol and biodiesel might be okay for the family SUV, but they just don't have a high enough energy density to be used in the aviation industry," lead researcher Carsten Kulheim said in a news release. "Eucalyptus oils contains compounds called monoterpenes that can be converted into a very high energy fuel, and this high energy fuel can actually fly jets and even tactical missiles."

Unfortunately, not all eucalyptus oils are created equally. The challenge for researchers is to find a way to breed and harvest trees with high concentrations of monoterpenes like pinene and limonene.

Kulheim and his colleagues recently laid out a path forward for scaling up eucalyptus oil production in the journal Trends in Biotechnology.

Kulheim says scientists must identify the proper eucalyptus species with ideal monoterpene-producing genes. With the proper feedstock, researchers will be able to genetically engineer gum trees to yield high concentrations of high-energy oils. Finally, scientists must develop more efficient processing and catalyst processes -- to convert oils into fuel while using as little energy as possible.

Researcher David Kainer, a PhD candidate at the ANU Research School of Biology, is working on finding the right combination of genetic traits among gum tree species.

"We're looking for species that have the right type of oil and in addition to that, since the oil is in the leaves, they need to grow a lot of leaves in a short amount of time," Kainer said.

Kainer hopes to double or triple oil yields through new breeding techniques and genetic engineering.

Though eucalyptus-derived fuel will initially be more expensive, its adoption could offset most of the aviation industry's carbon footprint.

"It has minimal ecological impact," Kainer said. "We can plant these trees on marginal lands that have low rainfall, and we can also plant them in agricultural systems that have salinity problems and help them defeat that problem."