I saw the headline “[a] plane fueled by fat and sugar has crossed the Atlantic ocean,” and I thought it was a yo mama joke, but it was actually about sustainable aviation fuel (SAF). SAF is a fun topic because it is commonly suggested as a solution for reducing aviation emissions. But does it really live up to the hype, or are there better options?

Air travel has grown rapidly and is expected to more than double by 2050. But as it has increased, so too have the industry’s greenhouse gas (GHG) emissions. In response to this, there is a big scramble to figure out a way to achieve cheap air travel—and make it climate friendly. The proposed solution is often low-carbon, liquid fuel, usually biofuels made from products like fat, sugar and corn. And because food and plant matter produce energy-dense liquid fuels (often even from food waste), they offer a means of attaining low-carbon air travel with existing infrastructure, hence the SAF terminology.

But there are a few wrinkles that have prevented SAF from solving the aviation-emission problem. The first is that SAF is really expensive. A ton of SAF costs $2,437, compared to $1,094 for jet fuel. These costs are expected to come down as the technology scales up, but the big unanswered question is scalability. Biofuels work well, but a major constraint is the amount of land required to produce fuel; in the United States, about 10 percent of the content of purchased gasoline is corn-based ethanol that is blended into the fuel, and this requires about 40 percent of the nation’s corn production. It’s not clear how practical it would be to make SAF a primary fuel for air travel if new feedstock sources (and cropland) are needed.

Another issue is that those estimates of 80 percent emission reduction for SAF compared to jet fuel are very rosy because of a methodology quirk (or mistake, if you prefer). Whenever you see an asterisk on an estimate indicating it used CORSIA methodology, it means that the GHG emissions of producing the food waste that turns into fuel are excluded. The assumption here is that this would otherwise be discarded, so it is the emission equivalent of finding it on the ground.

But that is not an accurate assumption. An estimated 72 percent of used cooking oil from the hospitality industry is recycled because it is used for animal feed, soaps, cosmetics and other biofuels. In other words, unless SAF producers are actively finding waste sources for feedstock that nobody else uses, then the emissions associated with producing feedstocks need to be included, and then the emission advantage is more in the 34 to 65 percent range.

A final question to consider is whether it might be better to simply offset or capture the GHG emissions from aviation instead of using SAF. Consider that the projected cost of SAF in 2050 is $5 per gallon ($1.32 per liter), and current conventional jet fuel prices are $2.71 per gallon. Jet fuel has a carbon content of 21.5 pounds per gallon, meaning that it takes 102 gallons of jet fuel to produce one metric ton of carbon dioxide.

Generously assuming that SAF has 80 percent lower emissions, it takes 128 gallons of SAF to avoid one metric ton of carbon dioxide. The cost of emission abatement is then the price difference for 128 gallons, which is $294 per ton. By contrast, carbon dioxide removal is expected to cost $200 per ton by 2050, with some hoping for $100 per ton. This means that in time, it might be cheaper for passengers to just pay for carbon dioxide removal than to use SAF.

Overall, the milestone of the first transatlantic SAF-only flight should be celebrated. It is good that such innovation is making it to the market, and it is also good that the SAF industry is growing and will hopefully be able to compete with conventional jet fuel (just as biofuels compete with gasoline and diesel). However, from a policy perspective, it’s crucial to remember that SAF faces many hurdles in terms of cost, scalability and emission accounting to abate aviation emissions to the extent some proponents claim it will. At the same time, it’s important to remain open to other cost-effective ways of reducing the carbon footprint of air travelers.

Every Friday we take a complicated energy policy idea and bring it to the 101 level.