In common with many other corporate lobby groups, the International Air Transport Association publicly proclaims their commitment to achieving net-zero carbon emissions by 2050.1
Yet the evidence that such an achievement is likely, or even possible, is thin … to put it charitably. Unless, that is, major airlines simply shut down.
As a 2021 Nova documentary put it, aviation “is the high-hanging fruit – one of the hardest climate challenges of all.”2 That difficulty is due to the very essence of the airline business.
What has made aviation so attractive to the relatively affluent people who buy most tickets is that commercial flights maintain great speed over long distances. Aviation would have little appeal if airplanes were no faster than other means of transportation, or if they could be used only for relatively short distances. These characteristics come with rigorous energy demands.
A basic challenge for high-speed transportation – whether that’s pedaling a bike fast, powering a car fast, or propelling an airplane fast – is that the resistance from the air goes up with speed, not linearly but exponentially. As speed doubles, air resistance quadruples; as speed triples, air resistance increases by a factor of nine; and so forth.
That is one fundamental reason why no high-speed means of transportation came into use until the fossil fuel era. The physics of wind resistance become particularly important when a vehicle accelerates up to several hundred kilometers per hour or more.
Contemporary long-haul aircraft accommodate the physics in part by flying at “cruising altitude” – typically about 10,000 meters above sea level. At that elevation the atmosphere is thin enough to cause significantly less friction, while still rich enough in oxygen for combustion of the fuel. Climbing to that altitude, of course, means first fighting gravity to lift a huge machine and its passengers a very long way off the ground.
A long-haul aircraft, then, needs a high-powered engine for climbing, plus a large store of energy-dense fuel to last through all the hours of the flight. That represents a tremendous challenge for inventors hoping to design aircraft that are not powered by fossil fuels.
In Nova’s “The Great Electric Airplane Race”, the inherent problem is illustrated with this graphic:
graphic from Nova, “The Great Electric Airplane Race,” 26 May 2021
A Boeing 737 can carry up to 40,000 pounds of jet fuel. For the same energy content, the airliner would require 1.2 million pounds of batteries (at least several times the maximum take-off weight of any 737 model3). Getting that weight off the ground, and propelling it thousands of miles through the air, is obviously not going to work.
A wide variety of approaches are being tried to get around the drastic energy discrepancy between fossil fuels and batteries. We will consider several such strategies later in this article. First, though, we’ll take a brief look at the strategies touted by major airlines as important short-term possibilities.
“Sustainable fuel” and offsets
The International Air Transport Association gives the following roadmap for its commitment to net-zero by 2050. Anticipated emissions reductions will come in four categories:
3% – Infrastructure and operational efficiencies
13% – New technology, electric and hydrogen
19% – Offsets and carbon capture
65% – Sustainable Aviation Fuel
The tiny improvement predicted for “Infrastructure and operational efficiencies” reflects the fact that airlines have already spent more than half a century trying to wring the most efficiency out of their most costly input – fuel.
The modest emission reductions predicted to come from battery power and hydrogen reflects a recognition that these technologies, for all their possible strengths, still appear to be a poor fit for long-haul aviation.
That leaves two categories of emission reductions, “Offsets and carbon capture”, and “Sustainable Aviation Fuel”.
So-called Sustainable Aviation Fuel (SAF) is compatible with current jet engines and can provide the same lift-off power and long-distance range as fossil-derived aviation fuel. SAF is typically made from biofuel feedstocks such as vegetable oils and used cooking oils. SAF is already on the market, which might give rise to the idea that a new age of clean flight is just around the corner. (No further away, say, than 2050.)
Yet as a Comment piece in Nature* notes, only .05% of fuel currently used meets the definition of SAF.4 Trying to scale that up to meet most of the industry’s need for fuel would clearly result in competition for agricultural land. Since growing enough food to feed all the people on the ground is an increasingly difficult challenge, devoting a big share of agricultural production to flying a privileged minority of people through the skies is a terrible idea.5
In addition, it’s important to note that the burning of SAF still produces carbon emissions and climate-impacting contrails. The use of SAF is only termed “carbon neutral” because of the assumption that the biofuels are renewable, plant-based products that would decay and emit carbon anyway. That’s a dubious assumption, when there’s tremendous pressure to clear more forests, plant more hectares into monocultures, and mine soils in a rush to produce not only more food for people, but also more fuel for wood-fired electric generating stations, more ethanol to blend with gasoline, more biofuel diesel, and now biofuel SAF too. When SAF is scaled up, there’s nothing “sustainable” about it.
What about offsets? My take on carbon offsets is this: Somebody does a good thing by planting some trees. And then, on the off chance that these trees will survive to maturity and will someday sequester significant amounts of carbon, somebody offsets those trees preemptively by emitting an equivalent amount of carbon today.
Kallbekken and Victor’s more diplomatic judgement on offsets is this:
“The vast majority of offsets today and in the expected future come from forest-protection and regrowth projects. The track record of reliable accounting in these industries is poor …. These problems are essentially unfixable. Evidence is mounting that offsetting as a strategy for reaching net zero is a dead end.”6 (emphasis mine)
Summarizing the heavy reliance on offsetting and SAF in the aviation lobby’s net-zero plan, Kallbekken and Victor write “It is no coincidence that these ideas are also the least disruptive to how the industry operates today.” The IATA “commitment to net-zero”, basically, amounts to hoping to get to net-zero by carrying on with Business As Usual.
Contestants, start your batteries!
Articles appear in newspapers, magazines and websites on an almost daily basis, discussing new efforts to operate aircraft on battery power. Is this a realistic prospect? A particularly enthusiastic answer comes in an article from the Aeronautical Business School: “Electric aviation, with its promise of zero-emission flights, is right around the corner with many commercial projects already launched. …”7
Yet the electric aircraft now on the market or in prototyping are aimed at very short-haul trips. That reflects the reality that, in spite of intensive research and development in battery technology through recent decades, batteries are not remotely capable of meeting the energy and power requirements of large, long-haul aircraft.
The International Council on Clean Transportation (ICCT) recently published a paper on electric aircraft which shows why most flights are not in line to be electrified any time soon. Jayant Mukhopadhaya, one of the report’s co-authors, discusses the energy requirements of aircraft for four segments of the market. The following chart presents these findings:
Table from Jayant Mukhopadhaya, “What to expect when expecting electric airplanes”, ICCT, July 14, 2022.
The chart shows the specific energy (“eg”, in Watt-hours per kilogram) and energy density (“vb”, in Watt-hours per liter) available in batteries today, plus the corresponding values that would be required to power aircraft in the four major market segments. Even powering a commuter aircraft, carrying 19 passengers up to 450 km, would require a 3-time improvement in specific energy of batteries.
Larger aircraft on longer flights won’t be powered by batteries alone unless there is a completely new, far more effective type of battery invented and commercialized:
“Replacing regional, narrowbody, and widebody aircraft would require roughly 6x, 9x, and 20x improvements in the specific energy of the battery pack. In the 25 years from 1991 to 2015, the specific energy and energy density of lithium-ion batteries improved by a factor of 3.”8
If the current rate of battery improvement were to continue for another 25 years, then, commuter aircraft carrying up to 19 passengers could be powered by batteries alone. That would constitute one very small step toward net-zero aviation – by the year 2047.
This perspective helps explain why most start-ups hoping to bring electric aircraft to market are targeting very short flights – from several hundred kilometers down to as little as 30 kilometers – and very small payloads – from one to five passengers, or freight loads of no more than a few hundred kilograms.
The Nova documentary “The Great Electric Airplane Race” took an upbeat tone, but most of the companies profiled, even if successful, would have no major impact on aviation’s carbon emissions.
Joby Aviation is touted as “the current leader in the race to fill the world with electric air taxis.” Their vehicles, which they were aiming to have certified by 2023, would carry a pilot and 4 passengers. A company called KittyHawk wanted to build an Electrical Vertical Take-Off and Landing (EVTOL) which they said could put an end to traffic congestion. The Chinese company Ehang was already offering unpiloted tourism flights, for two people and lasting no more than 10 minutes.
Electric air taxis, if they became a reality after 50 years of speculation, would result in no reductions in the emissions from the current aviation industry. They would simply be an additional form of energy-intensive mobility coming onto the market.
Other companies discussed in the Nova program were working on hybrid configurations. Elroy’s cargo delivery vehicle, for example, would have batteries plus a combustion engine, allowing it to carry a few hundred kilograms up to 500 km.
H2Fly, based in Stuttgart, was working on a battery/hydrogen hybrid. H2Fly spokesperson Joseph Kallo explained that “The energy can’t flow out of the [hydrogen fuel] cell as fast as it can from a fossil fuel engine or a battery. So there’s less power available for take-off. But it offers much more range.”
By using batteries for take-off, and hydrogen fuel cells at cruising altitude, Kallo said this technology could eventually work for an aircraft carrying up to 100 passengers with a range of 3500 km – though as of November 2020 they were working on “validating a range of nearly 500 miles”.
To summarize: electric and hybrid aviation technologies could soon power a few segments of the industry. As long as the new aircraft are replacing internal combustion engine aircraft, and not merely adding new vehicles on new routes for new markets, they could result in a small reduction in overall aviation emissions.
Yet this is a small part of the aviation picture. As Jayant Mukhopadhaya told treehugger.com in September,
“2.8% of departures in 2019 were for [flights with] less than 30 passengers going less than 200 km. This increases to 3.8% if you increase the range to 400 km. The third number they quote is 800 km for 25 passengers, which would then cover 4.1% of global departures.”9
This is roughly 3–4% of departures – but it’s important to recognize this does not represent 3–4% of global passenger km or global aviation emissions. When you consider that the other 96% of departures are made by much bigger planes, carrying perhaps 10 times as many passengers and traveling up to 10 times as far, it is clear that small-plane, short-hop aviation represents just a small sliver of both the revenue base and the carbon footprint of the airline industry.
Short-haul flights are exactly the kind of flights that can and should be replaced in many cases by good rail or bus options. (True, there are niche cases where a short flight over a fjord or other impassable landscape can save many hours of travel – but that represents a very small share of air passenger km.)
If we are really serious about a drastic reduction in aviation emissions, by 2030 or even by 2050, there is just one currently realistic route to that goal: we need a drastic reduction in flights.
* * *
Postscript: At the beginning of October a Washington Post article asked “If a Google billionaire can’t make flying cars happen, can anyone?” The article reported that KittyHawk, the EVTOL air taxi startup highlighted by Nova in 2021 and funded by Google co-founder Larry Page, is shutting down. The article quoted Peter Rez, from Arizona State University, explaining that lithium-ion batteries “output energy at a 50 times less efficient rate than their gasoline counterparts, requiring more to be on board, adding to cost and flying car and plane weight.” This story underscores, said the Post, “how difficult it will be to get electric-powered flying cars and planes.”
*Correction: The original version of this article attributed quotes from the Nature Comment article simply to “Nature”. Authors’ names have been added to indicate this is a signed opinion article and does not reflect an official editorial position of Nature.
1 IATA, “Our Commitment to Fly Net Zero by 2050”.
2 Nova, “The Great Electric Airplane Race” – 26 May 2021.
3 “The Difference In Weight Between The Boeing 737 Family’s Many Variants”, by Mark Finlay, April 24, 2022.
4 Steffen Kallbekken and David G. Victor, Nature, “A cleaner future for flight — aviation needs a radical redesign”, 16 September 2022.
5 Dan Rutherford writes, “US soy production contributes to global vegetable oil markets, and prices have spiked in recent years in part due to biofuel mandates. Diverting soy oil to jet fuel would put airlines directly in competition with food at a time when consumers are being hammered by historically high food prices.” In “Zero cheers for the supersoynic renaissance”, July 11, 2022.
6 Kallbekken and Victor, Nature, “A cleaner future for flight — aviation needs a radical redesign”, 16 September 2022.
7 “The path towards an environmentally sustainable aviation”, by Óscar Castro, March 23, 2022.
8 Jayant Mukhopadhaya, “What to expect when expecting electric airplanes”, ICCT, July 14, 2022.
9 “Air Canada Electrifies Its Lineup With Hybrid Planes”, by Lloyd Alter, September 20, 2022.
Photo at top of page: “Nice line up at Tom Bradley International Terminal, Los Angeles, November 10, 2013,” photo by wilco737, Creative Commons 2.0 license, on flickr.