RIGHT now, there are more than half a million people in the sky. Some 11 kilometres up, at the base of the stratosphere, the equivalent of a small city’s population is strapped into seats in pressurised tubes atop gigantic tanks of kerosene. It is an extraordinary thought.
It is also a worrying one. By some estimates, aviation is set to become the single biggest source of carbon dioxide. You may have switched to a green energy supplier, swapped your car for a bike, and maybe even stopped eating meat. But if you’re thinking about taking that holiday in the Mediterranean and don’t want to bust your carbon budget, you’re going to have to paddle there.
Yet we are addicted to flying. Few would willingly give up the freedom and opportunities it gives. So is there any way we can keep that city in the sky aloft without destroying the planet?
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One factor that makes it especially tough is the ever-increasing number of us up there. An average return plane ticket in 2017 was about 60 per cent cheaper in real terms than it was in 1995. That has driven an annual 5 per cent rise in passenger numbers. At the moment, 4 billion passenger seats are sold in civilian airliners each year, according to the International Air Transport Association. By 2036, that figure is predicted . That means annual passenger numbers in roughly 20 years will be a shade higher than Earth’s entire population today.
At the same time, aviation’s emissions of potent greenhouse gases like carbon dioxide and nitrogen oxides will increase. Flights currently account for about 2 per cent of anthropogenic CO2 emissions. By 2050, estimates suggest that the figure could be 10 per cent. It could even be 20 per cent or more if other sources of pollution like road transport cut their emissions, a prospect that looks likely as countries begin to set deadlines that mandate a shift to electric cars (see “Diagram”).
All this makes aviation a critical challenge in fighting global warming. Yet unlike road transport, international aviation has escaped the bite of environmental accords like the UN’s Paris Agreement. Domestic flights are covered by the Kyoto Protocol, but represent a tiny proportion of emissions. International flights, however, cross borders, which has made it difficult to agree who is responsible for cutting their emissions.
An agreement was finally reached in 2016, when the UN’s International Civil Aviation Organization brokered the first global deal to curb emissions. The linchpin is the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). This aims to keep the industry’s net emissions static beyond 2020 by requiring airlines to buy carbon credits, which pay for measures that reduce the amount of CO2 in the atmosphere, like planting trees.
At aircraft manufacturer Airbus in France, head of environmental affairs Hubert Mantel says he is encouraged by the programme’s uptake. Some 72 countries have signed up to the voluntary scheme, meaning it covers more than 75 per cent of today’s CO2 emissions from international aviation, he says. “It’s a remarkable start, and more countries are hopefully going to join.”
But plenty of people see it as not nearly radical enough. For instance, China, where major passenger growth is expected, hasn’t signed up, and last month the European Union formally objected to the scheme. The deal also allows airlines to emit as much as they like as long as they offset it with financial instruments. “It’s not driving innovation in emissions reduction in the aviation system as a whole,” says , who studies climate policy at the University of Manchester, UK. So what practical measures could we take and how effective might they be?
Fly in straight lines
One way to reduce emissions would be for planes to actually fly in straight lines. Air traffic travels along flight corridors that zig and zag to enter and exit nations’ airspaces at a few fixed points. This is a legacy from decades ago when planes flew from waypoint to waypoint so as to stay within range of radar.
Flight corridors are gradually being swept away by a new approach, called free route airspace, enabled by technologies like GPS and constant satellite tracking of planes. Each flight publishes its planned route in advance, then, as long as air traffic controllers monitor traffic, they can avoid clashes.
Morten Grandt, an engineer developing this system with German air traffic control, says the increase in route choice lowers aircraft density, meaning less noise under flight corridors and an even lower crash risk. It is as if the planes all take a different country lane rather pile onto the motorway.
The approach is already taking hold in Europe and the US. Eurocontrol, an intergovernmental organisation developing a that will see free route airspace in effect across Europe, says most member countries should be operating it by the end of 2019. The US Federal Aviation Administration (FAA) is working on a similar set-up called the . It says this will be part of the mix of measures that will help cap emissions beyond 2020.
How much carbon would flying straighter save? Eurocontrol calculations suggest free route airspace will prevent 150,000 tonnes of CO2 per year being emitted in Europe. That is the tiniest sliver of global aviation emissions, which were .
Freeing pilots to fly as they see fit more broadly, however, could make a big difference to emissions, says Craig Lawson, an aviation systems designer at Cranfield University, UK. “The optimum way to cruise for minimum fuel burn is what’s called a cruise climb,” he says. Aircraft get lighter as they consume fuel and so can stay aloft with the reduced lift provided in thinner air. Because that air provides less drag, higher cruise altitudes are more efficient too, so airliners should drift upwards as they fly, instead of ascending in steps (see “Diagram”). Lawson says cruise climbs could save aircraft 10 per cent of their fuel.
Make planes sleeker
Noodling with flight paths can cut some carbon, but we can probably save more by making planes slice through the air more efficiently. History certainly suggests so. The engines on a new Airbus A350 produce 15 per cent less CO2 for the same thrust than those of an Airbus A330 delivered in 2000, says Paul Stein, chief technology officer at Rolls-Royce Aero Engines, UK. That trend holds generally: for about the past decade, innovations have tended to make aeroplanes about 1 per cent more efficient every year.
There is every sign that will continue in the short term. For example, giving planes longer wings increases their lift and reduces the fuel they need to stay aloft. Boeing’s forthcoming 777X jet has wings that are so long they do not fit in standard airport gates, but their tips will fold up to get around this snag.
Unfortunately, innovations like this don’t add up to much. Just contrast that 1 per cent per year efficiency saving with the expected annual 5 per cent growth in passenger numbers. Plus, as Doug Parr, chief scientist at Greenpeace, points out, many airliners are in service for decades, so fuel-saving innovations aren’t used straightaway. Something more drastic and more immediate is needed.
The International Council on Clean Transportation, a non-profit organisation in Washington DC, has suggested that a return to propeller aircraft could be the thing. These have a much higher fuel economy. Aircraft like the Canadian Bombardier Q400 are the workhorses of short flights in many parts of the world, but they are slow. “It’s true, if you slow down you will burn less fuel,” says Lawson. “But making journey times longer, especially on long-haul flights, is not going to be welcome.”
Make carbon-neutral fuel
If we can’t get planes to use dramatically less fuel, maybe the stuff they burn could be greener. Most planes run on Jet A, which is basically kerosene, an oil-derived chemical composed of carbon and hydrogen. But make the stuff using carbon from greener sources, and you would be on to something.
at Delft University of Technology in the Netherlands began exploring this a decade ago. Kerosene from oil contains a range of low-level impurities that are thought to help the engines run smoothly, for example by swelling their rubber seals. In 2008, Melkert began testing whether synthetic fuels, which would contain fewer impurities, would still be safe. Soon afterwards, he .
That fuel was synthesised from natural gas, but, in principle, synthetic kerosene from any source would be safe. That prompted ASTM International, which sets the standards on jet fuel, to rule that blends of up to 50 per cent synthetic kerosene could be used on flights. That in turn got the airline industry developing synthetic kerosene from crops.
Biofuels have the potential to slash emissions, with some flights highly publicised to get the message across. In September 2018, an Airbus A320 was delivered from Mobile, Alabama, to the JetBlue airline in New York, flying with a 15 per cent blend of biofuel. But the overall impact has been modest. Sean Newsum, director of environmental strategy at Boeing Commercial Airplanes, says 143,000 passenger flights have used biofuel blends in the past decade – not many compared with the 39 million scheduled flights in 2018.
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Even if biofuels were widely used, there is concern over the impacts of growing the feedstocks. Would it displace food crops? And how much carbon is emitted generating the energy used to turn crops into fuel? Newsum says Boeing has a number of projects investigating the best sources of biofuel, including forestry waste in Canada, nicotine-free tobacco in South Africa and seawater irrigated plants in the United Arab Emirates. These all produce small amounts of fuel.
Better would be to make a synthetic fuel using carbon sucked from the air, so no net carbon is released when it is burned. This was first demonstrated by a company called Air Fuel Synthesis in 2014. In June 2018, a Canadian firm, Carbon Engineering, showed that it had got the cost of removing 1 tonne of CO2 from the atmosphere down to , at least a third cheaper than previous estimates. But the challenge is to emit less carbon making the fuel than you save by avoiding oil-derived kerosene. And that’s tough. Tellingly, Air Fuel Synthesis quietly folded in 2016.
Build electric planes
In June last year, Norway’s transport minister strapped himself into a small, white aircraft beside the CEO of Avinor, the state-owned company that runs the nation’s airports. They wobbled into the air, buffeted by winds, and took a few laps around Oslo airport. The stunt was to underline Norway’s pledge that all domestic flights will be fully electric by 2040.
Electrifying aircraft is the sort of step that really could make a serious dent in emissions. Were planes to be run on batteries charged with electricity from renewable sources, flight could be almost carbon neutral.
However, the plane flown in Norway was just a two-seater. No one has yet managed to power an airliner on electricity, though some are trying. One of the most ambitious schemes is the E-Fan X project from Airbus. The firm is working with electric motor-maker Siemens, and Rolls-Royce, on converting a four-engine jet into a hybrid-electric plane, where one engine is replaced by an electric fan. A small engine hidden inside the plane will charge the batteries that run the electric motor, “like a Toyota Prius in the sky”, says Stein. The plan is for a test flight in 2020.
Likewise, Boeing is investing in , a start-up aiming to build a 50-seat hybrid plane by 2022. The firm claims its plane will burn half the fuel of a similarly sized standard jet. The big drawback is that planes like this would have fairly meagre ranges, at least with today’s battery technology. Zunum reckons its planes will manage about 1100 kilometres on a single charge (see “Diagram”).
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However, if electric planes do take off, it could transform the way we travel. Electric aircraft wouldn’t just been green, they would be cheap and quiet. This could finally make personal aerial vehicles – otherwise known as flying cars – a reality. In the long term, these might even replace commuter trains, often considered relatively green.
It might come sooner than you think. German firm Volocopter has been developing an electric vertical take-off and landing (eVTOL) aircraft that has a cabin suspended beneath a ring of several rotors. The firm plans a test flight of its two-person machine in Singapore this year, the first in a built-up environment. Meanwhile, the EU has , while NASA and the FAA are hammering out similar regulations.
Large passenger planes that fully avoid kerosene are a long way off though, barring a huge tech breakthrough. Paul Peeters, a former aviation engineer now researching sustainable transport at the University of Breda in the Netherlands, has analysed the battery requirements of a 60-seater electric aircraft. “The battery, with current lithium technology, would have to be literally bigger than the whole aircraft,” says Peeters.
He thinks the only solution is to find a way of limiting the number of flights, perhaps through an international agreement that goes far beyond what the UN has brokered so far. “We cannot count on these measures,” he says. “It is way too late.”
This article appeared in print under the headline “Green sky thinking”
