Transportation is now the biggest single source of U.S. greenhouse gas emissions, having recently passed electricity generation. Worse, zero-carbon transportation technology is only in its early stages — especially for air travel and shipping, which accounts for large and growing share of emissions.

Steampunk fans and climate hawks alike want to know: what about airships? After investigating the subject for a time, I've come to a tentative conclusion that airships could indeed play an important role in a zero-carbon transportation infrastructure — but probably not in the form of romantic luxury travel. Big and weird cargo shipping might just be where the airship does best.

Airships are, of course, aircraft which use a large envelope of lighter-than-air gas, typically hydrogen or helium, to provide most or all of their lift. There are three basic types: non-rigid (a blimp), or semi-rigid (with a partial supporting structure), or rigid (with a complete supporting structure). There is also the hybrid airship, which is slightly heavier than air and uses traditional wings or rotors to provide lift and control.

Airships have several important advantages. First is that with the envelope (that is, the ballon-like structure where the gas goes) providing lift, there is no need to expend fuel to maintain flight. Second is low-speed maneuverability, meaning they don't require as much infrastructure as airplanes — no runway for instance, or only a short one in the case of hybrids. Third is high potential lifting power — the largest new designs could theoretically carry 500 tons or more, in the same league as the largest cargo planes. Fourth is very high cargo volume — a heavy lift airship would have to be very large, and so would be able to accommodate a correspondingly huge cargo bay.

If you need something to just hover in one place for a long time for cheap, there is basically nothing better than an airship — and indeed, there is already a moderate-sized industry providing airships for surveillance, video, advertising, weather data collection, and so forth.

However, airships have large downsides as well. The biggest one by far is drag — with an envelope many times larger than even the biggest planes, and drag increasing with the square of velocity, the amount of power required to move an airship will quickly eat into its efficiency advantage from floating — even at highway speeds (Germany's famous Graf Zeppelin had a top speed of 128 mph, but generally cruised at about 70 when it traversed the globe in the 1920s and '30s). Size itself is a problem as well. Airships need to be big to be useful, and wrangling lots of them in an airport or port will be tricky.

As big, relatively fragile objects, they are also more vulnerable to weather than airplanes — though that should be less of an issue with modern weather prediction.

The choice of gas is a particularly thorny question. As a technical matter helium is objectively superior, providing almost as much lift as hydrogen while being non-flammable. However, helium is much, much more expensive than hydrogen, and as a non-renewable resource arguably ought to be reserved for scientific research. (You need a lot of helium to fill up a big airship.)

The spectacular destruction of the Hindenburg (whose hydrogen envelope caught fire) basically ended civilian airship travel at a stroke. However, this was more about saturation coverage and mass media than it was about objective danger. The eye-popping newsreels, photographs, and heart-rending live radio broadcast obscured the fact that airplane travel was also quite dangerous at the time (not to mention the fact that most of the Hindenburg passengers actually survived). There were at the time, and would continue to be, mass casualty disasters from civilian airplane flights, often due to fuel catching fire and burning everyone alive after a crash. But they weren't covered in the same way, and so did not create an impression of extreme danger. Meanwhile, as government regulators, airplane manufacturers, and airlines got extremely good at flying, the rate of crashes was brought down to nearly zero, and the public came to correctly believe that the risk of flying was small.

Hydrogen is the riskier but cheaper choice, but ultimately it should almost certainly be possible to develop airship technology to the point where it posed no real threat. If airplanes can fly around carrying tens of thousands of gallons of highly flammable fuel, then we ought to be able to work out how to fly hydrogen airships.

At any rate, all this points to a particular use case for the airship: moving heavy and/or bulky cargo directly from point to point, at a speed faster than cargo ships but slower than planes, using sustainable biofuels, fuel cells, or electric motors (perhaps even substantially solar-powered in the latter case — if they ever figure out how to produce painted solar panels, a big envelope could generate a lot of power).

Airships can't go nearly as fast as airplanes; they could never carry truly time-sensitive cargo or compete with passenger flights. They are probably best at about 30-70 miles per hour, to keep drag down. But that's a lot faster than container ships — indeed, shipping companies have themselves taken to slowing their big ships down to 12-14 miles per hour to save on fuel. They would be particularly useful for medium-urgent goods that need to be shipped across the ocean and then flown to their final destination, since they could skip the port bottleneck.

They would probably be best of all at taking very large or awkward cargoes directly from the place of manufacture to their destinations, especially for remote locations. For instance, logging and mining companies that use very large pieces of equipment typically have to disassemble the machine after testing at the factory, ship the (still big) individual pieces part of the way on trains and giant trucks, and then put it all back together out in the field. The expense, time, labor, and resources required are immense. But a heavy lift airship could pick up the assembled item, drop it off directly where needed, and then pick it up again when it's done. Imagine also taking a big bundle of wind turbine blades directly to the wind farm construction site, instead of having to carefully manhandle them down the interstate one by one.

Lockheed Martin is developing a line of hybrid airships and selling them on exactly these grounds, and has reportedly secured a $480 million letter of intent to sell 12 of them.

Quite a considerable portion of some shipping routes could theoretically be taken up in this manner. A study of shipping between Hong Kong and the U.S. and Europe found they could replace up to half of the "dedicated cargo aircraft" capacity. And since cargo planes are generally the oldest and least efficient models in the sky, replacing them with zero-carbon airships would be a particular benefit.

On paper, this all sounds pretty good. But it will require sustained investment and policy attention. New technologies always have to climb the development and implementation ladder, meaning lots of mistakes and setbacks as best practices are ironed out and supply chains built. We were once quite a ways up that ladder — witness the Graf Zeppelin, which traveled over a million miles, including an around-the-world trip, before being scrapped — but much progress has been lost since then. At a minimum, that will mean lots of government R&D grants to spin up the new industry. But if that happens, the results could be a very big, very quiet success.