By any standard, the Solar Impulse 2 is a marvel of engineering. This solar-powered plane — currently being flown around the world by Bertrand Piccard and André Borschberg — didn't use a drop of kerosene on its epic trip across the Pacific Ocean. It's a real testament to how far solar technology has advanced.

Unfortunately, for anyone hoping that we'll all be puttering around in solar planes soon — well, that's pretty unlikely.

Consider: The Solar Impulse 2 features 17,000 solar cells crammed onto its jumbo jet–size wings, along with four lithium-polymer batteries to store electricity for nighttime. Yet that's still only enough power to carry 2 tons of weight, including a single passenger, at a top speed of just 43 miles per hour.

By contrast, a Boeing 747-400 running on jet fuel can transport some 400 people at a time, at top speeds of 570 miles per hour. Unless we see some truly shocking advances in module efficiency, it'll be impossible to cram enough solar panels onto a 747's wings to lift that much weight — some 370 tons in all.

Nor is it enough to load up on batteries charged by solar on the ground, since that would add even more weight to the plane, vastly increasing the energy needed for takeoff. A gallon of jet fuel packs about 15 to 30 times as much energy as a lithium-ion battery of similar weight. That fundamental difference in energy density is a big reason we're unlikely to see large commercial airliners powered by batteries fill the skies.

That said, the idea behind the solar plane is basically correct: We do need to rethink the way we fly. After all, burning jet fuel carries huge environmental costs. Aviation now accounts for 3 percent of humanity's global warming footprint, and it's one of the fastest-growing sources. The aviation industry is currently facing heavy pressure to reduce its CO2 emissions after 2020 in order to mitigate climate change.

So if not solar planes, then what? How can we make air travel greener? Cleaning up aviation is even trickier than cleaning up our cars and trucks, but there are some really neat ideas out there. Some are fairly unsexy, like slimming down the planes or streamlining air traffic operations. But more radical changes are afoot, too, like replacing jet fuel with biofuels, futuristic wing designs, or even fuel cells. Let's take a closer look at the options.

Here's how we'll (probably) make air travel greener in the next 30 years

One of the clearest and most comprehensive road maps I've seen for how commercial air travel might get greener is this 2015 Nature Climate Change paper by Andreas Schäfer, Antony Evans, Tom Reynolds, and Lynnette Dray.

They start with the fact that commercial aviation is already getting considerably more fuel-efficient over time. Jet fuel is one of airlines' biggest costs, and they're always looking to save on it. So manufacturers keep finding ways to bolster engine efficiency and make planes more aerodynamic. Boeing's newest 787 Dreamliner, for instance, is 20 percent more fuel-efficient than its predecessor, the 767. Airlines have also gotten better at juggling demand so that there are fewer empty seats on flights (which are a waste of fuel).

These things add up: Today, the US commercial aircraft fleet emits just one-third the CO2 per passenger-mile that it did back in 1970, as seen in the black line below. But how much more efficient can it get in the future? Here the authors lay out five different plausible scenarios, shown in the colored lines:

Let's run through these scenarios:

The gray line, (1), simply assumes that older planes retire over time and are replaced with the most efficient planes currently on the market. This is the baseline, the bare minimum we can expect. Not very exciting.

The blue line, (2), assumes the airlines take a bunch of additional steps to further improve plane efficiency using methods expected to be viable in the near future. Airports would optimize air traffic management to avoid wasting fuel. Manufacturers would widely adopt technologies such as blended winglets that reduce drag and electric taxiing to cut fuel use on the runway, and drastically reduce cabin weight by, for instance, making the seats even lighter.

In the red line, (3), airlines take this efficiency strategy even further by aggressively retrofitting their existing planes rather than waiting for them to retire in due course. This scenario also assumes that next-generation, vastly more efficient aircraft are introduced by 2035 that feature technologies such as "open rotor engines, an all-carbon fibre airframe, and the structural advantage resulting from non-swept wings which are made possible by a slight reduction of cruise speed."

Finally, the last two scenarios, (4) and (5), assume that airplanes also start using low-carbon biofuels to replace up to 30 percent of the jet fuel they use by 2050. Note that in the most drastic scenario, (5), the carbon intensity of air travel would be about one-third what it is today by mid-century.

If you're curious about the specific technologies, the authors lay them all out in this detailed PDF and array them on the chart below. The horizontal axis shows how much CO2 each of these measures could potentially save. The vertical axis shows how much they cost. Note that about three-fourths of these measures have negative cost — they're expected to pay for themselves so long as oil remains above $50 per barrel:

So take No. 6 above, "inefficiency reduction during cruise." The authors note that there are lots of opportunities to save fuel when planes are cruising at altitude. Often, due to inefficiencies in air traffic control, planes don't fly the most direct routes. Tweaking these things could save a lot of CO2 — and save money in the process.

By contrast, look at No. 16, in which airlines aggressively retire older aircraft before their 25-year life spans are up. That can save a lot of CO2, but it's not likely to make financial sense for most airlines unless either the price of oil soars past $100 per barrel or there's some sort of policy pushing airlines to take these steps — like, say, a carbon tax or cap-and-trade system. More on that in a second.

Biofuels are the biggest question mark here

When I called Schäfer to chat about his paper, he pointed out that biofuels represent a big chunk of the reductions in the most optimistic decarbonization scenario. But, he cautioned, it's still not certain that low-carbon biofuels will actually materialize in such large quantities.

Ideally, we'd want biofuels that produce far fewer greenhouse gas emissions than jet fuel does and don't conflict with food supplies the way corn-based ethanol does. So companies like Boeing and Airbus are looking into making fuel from cellulosic biomass (i.e., grasses or the inedible parts of plants) or algae. They've even flown a few planes on experimental jet fuel made from these green sources.

But right now, these biofuels are still quite expensive. Marie Caujolle, a spokesperson for Airbus, told me that current estimates suggest aviation biofuels are about three to four times as expensive as jet fuel. The hope is that the cost will come down over time as production ramps up. But, points out Schäfer, "there is currently no commercial-scale plant operating" that produces synthetic jet fuel made from, say, cellulosic biomass. "These processes may work well in the laboratory, but scaling it up has proven challenging." (See here for more on the challenges.)

There's also the question of whether there will even be enough biomass available to supply vast quantities of jet biofuels. The United States and Europe already have laws and programs to ramp up cellulosic ethanol for cars and trucks. And some climate plans call for a massive ramp-up of "negative emissions" electricity involving cellulosic biomass. There's only so much plant matter to go around for our energy needs — so who gets priority?

If biofuels don't pan out, could fuel cells or new wing designs be the answer?

If biofuels don't pan out, there might still be a few other options for decarbonizing air travel, though they're also fairly uncertain.

NASA and Boeing, for instance, are working on a radical new wing design that could cut fuel use by a whopping 50 percent compared with current aircraft. The wings are much longer and thinner than existing models and are supported by a truss. They perform stunningly well in wind tunnel experiments:

But, Schäfer says, aircraft manufacturers aren't just going to rush to adopt these designs tomorrow. "There's an enormous amount of risk here," he says. "A radically different design can cost tens of billions of dollars, and if you get it wrong, you're out of business." As such, manufacturers prefer to adopt steadier, incremental improvements to wing designs.

Beyond that, there are fuel cells — devices that convert hydrogen into electricity. Compressed hydrogen is vastly more energy-dense than batteries, so it could be suitable for air travel. But this is also a much newer technology, and there are a lot of kinks to work out. Caujolle says that Airbus is looking into fuel cells as a way to provide emergency power onboard aircraft, though the company says this isn't likely to hit the market before 2025, and broader applications are even further off.

Growth in air travel could swamp technology improvements. So what then?

The other question we've danced around so far is how much aviation emissions actually need to fall to avoid drastic climate change. This is a little trickier.

Remember, the most optimistic scenario that Schäfer and his co-authors looked at envisioned the carbon intensity of aviation — that is, CO2 per passenger-mile — falling 2.6 percent per year between now and 2050.

But here's the catch: If the total amount of air travel grows by more than 2.6 percent a year, then overall aviation emissions would rise. And right now, Schäfer notes, rapid growth in air travel seems quite plausible. As countries like China and India get richer and see their middle classes expand, more and more people are getting on airplanes and flying places. The expansion of travel alternatives like high-speed rail could help slow the growth in air travel demand, but it's unlikely to halt it altogether.

Here's one way to visualize this dilemma, courtesy of the environmental group FlightPath 1.5°C. In order to avoid more than 1.5°C of global warming, they say, aviation emissions will need to stabilize by 2020 and then start falling. (The industry has been discussing a 2020 target for years.) But right now, most projections see overall aviation emissions going up between now and 2040 as the increased volume of air travel outpaces any improvements in efficiency:

So how on earth can the aviation industry hit that 2020 target if technology alone isn't enough? I called Annie Petsonk, an aviation expert at the Environmental Defense Fund who is working with FlightPath 1.5°C. She argues that the airline industry will likely have to resort to carbon markets to make up that gap.

The idea is that governments would come together and agree to a cap on global aviation emissions — say, they'd pledge to stay carbon-neutral after 2020. If airlines went beyond that cap, they would have to purchase carbon credits that essentially paid for reductions in CO2 emissions elsewhere in the world. Maybe that means buying renewable electricity for airports, or paying people to avoid cutting down forests.

"We think if it's done well, and the markets are designed with integrity, this can be an important piece of the puzzle" for cleaning up air travel, Petsonk says.

As it happens, the International Civil Aviation Organization (ICAO) is hosting a high-level meeting in Montreal this month to discuss what those carbon markets might look like exactly. Petsonk explains that there are all sorts of thorny questions to resolve in designing a cap: What countries are covered? Do developing countries with greater projected growth in air travel (like China) get more leeway than countries where air travel isn't expected to grow that much (like the United States)? Do domestic flights get treated the same as international flights? And, crucially, how do you verify that the "carbon credits" being purchased by airlines actually lead to real reductions?

That's all easier said than done, and ICAO is expected to take months to work through these questions. It's not nearly as fun to think about as solar-powered airplanes. But sorting through these questions could prove even more crucial for figuring out how to make flying sustainable.

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