DURBAN, South Africa—My share of the carbon dioxide my flight to Johannesburg emitted over 15 hours amounted to 1,391.3 kilograms, according to the helpful information provided by South African Airlines. Add a dollop of 53.8 kilograms of CO2 for the jet jaunt to Durban and you can see that the aviation industry—and the Durban climate talks—have an emissions problem.

In fact, flying now accounts for some 2 percent—and growing fast—of global greenhouse gas emissions, although the industry has pledged to stop that growth by 2020. According to the aviation industry, a full 80 percent of the roughly 650 million metric tons of CO2 annually emitted by aircraft are from those flying more than 1,500 kilometers (like my trip from New York City to Durban) for which there is no alternative mode of practical transport. And, given the energy density of kerosene, there really is no alternative to liquid fuel either—with the exception of lightweight solar-powered drones, electric planes cannot get off the ground. As for hydrogen, it is hard to carry enough of it and still have space for passengers, too.

That's why the U.S. military, a slew of airline companies, Boeing and others have invested heavily in jet fuel made from plants—the oils provided by weedy camelina or hardy jatropha shrubs or even algae. The fuels have successfully passed all trials—even delivering more thrust per gallon—and have now entered regular commercial use in the U.S. and Europe, promising to cut CO2 emissions by 80 percent, albeit at a premium price. The U.S. Federal Aviation Administration is giving out $7.7 million in contracts to such jet biofuel–makers.

"Everybody wants a solution to oil," says Jigar Shah, CEO of the Carbon War Room, an organization founded by Virgin Airlines founder Sir Richard Branson and others to combat climate change. "Aviation is where it's going to come first."

Unfortunately, there's a problem. As much as ethanol from corn turns out to be a bad biofuel idea, the climate-friendly value of these bio-jet fuels depends largely on how they are produced. A fuel made from palm oil turns out to be worse for greenhouse gas levels in the atmosphere than jet fuel refined from petroleum because it involves clearing rainforest or peatland. To help solve this problem, the Carbon War Room has launched, which ranks all biofuel companies on sustainability, among other criteria.

"We need to ensure that these fuels are made in a way that doesn't put pressure on ecosystems that are already stressed," says Suzanne Hunt, who is leading the Carbon War Room's aviation effort. "They must not put pressure on food security and we must make sure the greenhouse gas reductions are real and verified."

Per the Carbon War Room's main criteria of scalability (Can it be made in bulk?); sustainability (Can it be made with minimal environmental damage?); and economic viability (Can it be made at a profit?), the top five producers include: Lanzatech,  SG Biofuels, AltAir, Solazyme and Sapphire—all of which have already provided biofuels to fly jets.

All of these fuels cost more than petroleum-based jet fuel. "For airlines, a third or more of the operating costs are fuel," Hunt notes, arguing that locking in bio–jet fuel at a consistent price will help airlines hedge that cost. "The E.U. including aviation under the cap [of its emissions trading program] is a major incentive."

But each would-be jet biofuel–maker faces other challenges as well. Solazyme can make lots of oil from its stressed algae grown in the dark and fed industrial-grade sugar, but the source of that sugar makes the ultimate fuels less sustainable. Sapphire wants to grow its algae in ponds, which will make it more sustainable but also much more costly to produce. AltAir does better by sourcing its bio–jet fuel from oil seed–bearing plants, like camelina, but that limits the amount that can be planted in rotation with food crops like wheat given constraints on the amount of land available for the latter.

SG Biofuels gets high marks in all respects from the Carbon War Room and relies on jatropha grown in Guatemala and other parts of Central and South America. This hardy plant seems ideal, given that it can resist droughts and grow on marginal lands, producing oily seeds. But the biofuel crop has already come in for criticism both because it is displacing cereals in other places where it is grown, such as Kenya and Tanzania, as well as requiring fertilizers to get good oil yields.

That leaves Lanzatech, which has a technology to turn synthesis gas derived from almost anything composed of hydrogen and carbon into fuels and chemicals. But, of course, that technology doesn't have to use sustainably grown plants as a starting point. They can just as easily use other alternatives, like alcohol or coal. In fact, that is exactly what Lanzatech is attempting with Chinese coal mining company, the Yankuang Group.

Turning coal to motive fuel is something that South Africa's SASOL has been doing for a long time. For years SASOL has been making jet fuel (and diesel for trucks and buses) out of coal. It is actually chemically the same fuel as that which is made from plants—synthetic paraffinic kerosene. Unfortunately, jet fuel derived from coal results in even more CO2 emissions, which makes it no alternative at all if the goal is to combat climate change.