When it comes to reducing humanity's carbon emissions, one particularly tough challenge is going to be air travel. And the airlines know it. Earlier this month, Delta Airlines said it would spend $1 billion over the next decade to zero-out its carbon emissions — the biggest such pledge from any airline to date.

But can Delta, or any airline, actually live up to such a promise? Unfortunately, for the moment, the answer is almost certainly no.

The core problem is this: It's not obvious how we can do air travel without carbon emissions. For a lot of other economic activities — driving cars and trucks, producing electricity, heating homes — the technology is here, it's ready to go, and we just have to make big one-time investments to switch society over to the new way of doing things. Hence the growing calls for a Green New Deal. But for other areas of economic activity, such as air travel and heavy industry, green technology is much less mature, and it's not certain exactly how much carbon the technology can ultimately cut.

"We will continue to use jet fuel for as far as the eye can see," Delta's CEO Ed Bastian explained. "We'll be investing in technologies to reduce the impact of jet fuel. But I don't ever see a future where we eliminate jet fuel from our footprint."

And that's a problem, since flying is one of the most carbon-intensive forms of travel there is. All the airplane trips we take around the world only amount to about 2.5 percent of global carbon emissions right now, but global demand for air travel is also rising fast. Assuming we eliminate carbon emissions from all those other easier sectors first, air travel will quickly become a much larger chunk of all the carbon humanity can afford to release, crowding out other activities. On top of that, we need to eliminate our net carbon emissions completely by the middle of this century. Eventually, humanity will have to figure out a way to make air travel completely green, or completely offset its emissions, or just fly a whole lot less than we do now.

So. What can Delta do?

The most obvious route is to use that $1 billion to buy a whole lot of carbon offsets. This is a practice where, if you do something that releases some CO2 into the atmosphere, you pay someone else to do something that removes CO2 from the atmosphere. Delta will reportedly focus on planting trees and rebuilding wetlands — obviously the most common form of offset, but there are others.

Unfortunately, while it might be a bit harsh to dismiss carbon offsets as a scam, it's not too far off. The first problem is, did the money actually incentivize activity that really wouldn't have happened at all otherwise, or did it simply give someone a bonus for something they'd have done anyway? That's crucial to whether your carbon offset actually zeroed out your emissions. The second problem is simple verification: Did the people actually do the thing your carbon offset paid them to do, and did it work? The European Commission recently determined that 85 percent of the carbon offsets in the United Nations' Clean Development Mechanism didn't produce "real, measurable, and additional" carbon reductions.

Setting all that aside, planting trees comes with lots of other problems, from the scale of the reforestation necessary, to the fact that trees take decades to reach their full carbon-reducing potential — and we don't have decades.

You could imagine Delta investing in electric air travel technology. But a battery that can deliver the same amount of energy as a gallon of jet fuel weighs a whole lot more than a gallon of fuel. Barring huge advances in battery technology, it's a poor solution for air travel. And even if it wasn't, using electrical propellor-driven planes for small local and regional flights is one thing; using them to replace the long-haul flights that our jets do now is quite another. At that point, you've basically accepted that modern air travel as we know it is going away and not coming back.

That brings us to biofuel: Basically, jet fuel made from plants and other forms of biological material. If you use fossil fuels, you're digging up carbon and releasing it into the climate system. But if your fuel comes from plants and such, the carbon you're releasing was already in the climate system — a net zero addition.

Airlines are already experimenting with production lines that deliver biofuel that can be mixed right in with their regular jet fuel supplies, thus reducing total emissions. But, as you probably guessed, there are big problems too. Production of biofuels is so small and limited that they still cost considerably more than regular fuels — a problem that can only be solved by a massive and rapid increase in the industry's scale. Next, most biofuel feedstocks are agricultural products like corn. Not only does using those crops for biofuel compete with food supplies, the increased demand for biofuels also incentivizes people to turn more land around the world into agricultural land. That destroys natural habitats and ecosystems, and cropland actually tends to pull less carbon out of the atmosphere than natural land. Not good, if taming global warming is your goal.

There are ways around these challenges. Biofuel can be made from agricultural waste — the plant parts not used for food — so it doesn't compete with food supplies, and doesn't drive more agriculture than is already occurring. Or we could get wild and produce biofuel from algae grown in giant tanks. There are even projects trying to produce fuel from carbon dioxide and hydrogen that's literally sucked right out of the air. It's just that all these efforts are still pretty preliminary, and there's no guarantee they could ever produce enough fuel to save humanity from having to cut its air travel way back.

Ultimately, air travel is one problem where the strengths of carbon taxes and cap-and-trade schemes actually come to the fore. Including the climate costs in the price of traditional jet fuel would drive industry efforts to bring those alternative technologies to maturity. (Obviously, big public investments in research would help too.) And at the end of the day, if we can't make the technology work, pricing in the climate costs would force everyone to make do with less air travel.

The hard truth is that Delta's announcement is probably just a publicity stunt. As CNN noted, spending $1 billion over 10 years would reduce the airline's annual profits by all of two percent. Delta is hardly going out on a limb here. If humanity's search to find a green way to fly is to have any chance of succeeding, we'll have to make a much larger and more sustained effort.

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