Decarbonizing by 2050: Optimists, pessimists and realists

April 2021

The air transportation sector once believed that the best it could do globally speaking would be to reduce its net-carbon output by 50% below 2005 levels by 2050. That view placed the sector out of sync with what the United Nations has called a “global rallying cry” and “race” toward a net-zero economy by 2050. Now, a shift is underway that could sync up the sector with those embracing the more ambitious goal.

BY ASTERIS APOSTOLIDIS

For every kilogram of kerosene an aircraft engine consumes, approximately 3 kilograms of carbon dioxide are emitted, according to the International Civil Aviation Organization’s Carbon Emissions Calculator. A large jet engine consumes a kilogram every single second during cruise, so by multiplying this figure with the number of aircraft flying at any given moment, the total emissions escalate rapidly to hundreds of millions of tons per year. During the last decades, carbon that was stored underground for millions of years has been released in vast amounts into the atmosphere, and of that all forms of air, land and sea transport combined account for around one-fifth of global carbon dioxide emissions.

Increasingly, the need for aggressive emissions reductions is being recognized by airlines, airports and others. Europe’s aviation sector unveiled in February its flagship sustainability initiative, Destination 2050. According to this plan, all flights within or departing from the European Free Trade Association, the European Union and the United Kingdom must be CO2 net-zero by 2050, a dramatic step beyond the sector’s global target of a 50% reduction by that year, a goal set by the International Air Transport Association in 2009 and at the time one of the first global commitments by a sector. The catch for the new, more aggressive goal is that flights bound for Europe are excluded for now, but of course this is something that could change if similar commitments are made by the countries of departure. In the United States, the new administration has set the ambitious target of a net-zero economy by 2050, in line with the global climate action under the Paris Agreement. However, it is not clear whether this will be reflected in introduction of even stricter emission goals for air transport in the U.S., similar to the ones in Europe. The stakes are enormous, because it’s far from clear that a 50% reduction in emissions would be enough to meet the climate goal of the Paris Agreement, which seeks as a worst case to hold the temperature rise to below 2 degrees Celsius, while pursuing efforts to limit the increase to 1.5 degrees.

For sure, decarbonizing the whole transportation industry will be an enormous undertaking, including for the air transportation sector. Let’s look at the various paths and trade-offs for reaching net-zero in our sector by 2050.

Grappling with weight and range

In aviation, weight is critical. No matter how effective a decarbonization technology is expected to be, it cannot be implemented if it imparts a nonacceptable weight penalty on the aircraft. In other words, this is a firm design constraint. Batteries are an example of a technology that works great in road vehicles, because weight is of less concern. However, in the context of commercial aviation, battery-electric flying shouldn’t be expected anytime soon, with the exception of small, regional aircraft. That’s because the energy that batteries can store per kilogram (known as specific energy) is still far lower than the specific energy figure for kerosene. Also, the weight of batteries stays permanently with the aircraft, affecting negatively its performance throughout a trip and requiring reinforced undercarriages for landings. On the other hand, fuel is exhausted during the flight, making aircraft lighter with flight time and allowing for fuel dumping in case of emergencies and avoiding overweight landings.

Range is also an important performance metric, since current long-haul aircraft can fly nonstop almost everywhere in the world. This convenience comes with a significant fuel penalty, however, as airplanes must lug fuel that will be used only after many hours of the flight. Research has shown that short refueling stops in long-haul flights could improve fuel efficiency, something that could be required in the future by regulators in order to curb emissions. Range, in contrast to weight, can be considered a soft constraint, meaning that some passengers will not be happy if their flight includes stopping to refuel for the sake of reducing emissions, or if they have to switch to a charged electric aircraft for the final leg. This is also the case with electric cars, as their drivers have to take longer breaks to recharge them during long trips, while refueling in comparison only takes a few minutes.

Batteries can’t power long-haul flying

Something important to also take into consideration is that the vast majority of emissions from commercial aviation can be attributed to long-haul flying. According to EUROCONTROL, the European organization for the safety of air navigation, approximately half of air transport CO2 emissions come from long-haul flights, which represent only 6.2% of total departing flights in Europe. In other words, many of the technical solutions under development that aim to reduce the carbon dioxide emissions of flying focus on regional and short-haul flying, while the most important gains would come from long-haul. Of course, most technologies need to be implemented in smaller aircraft before being scaled up to bigger ones, but we need to keep in mind that wide-body aircraft must also be part of the solution, independently of how aggressive the different countries will finally be in their 2050 aviation emissions targets.

Focusing on the different options for long-haul flying, electrification of planes does not seem applicable in the Paris Agreement time frame. Again, here the main impediment is the increased weight and volume of batteries, unless some step changes in technology take place. However, electrification can realistically be expected at the second half of this century, according to the analysis, “Technological, economic and environmental prospects of all-electric aircraft,” in the journal Nature Energy.

In addition, the business model of airlines will need to be adapted to the operational particularities of electrical components, since batteries will have to be recharged, a process that needs much more time than refueling with jet fuel. Also, the electricity production needs would skyrocket under a shift to an electrified global fleet, requiring significant investments in renewable energy for aircraft to be recharged.

Hydrogen shows promise

Batteries do not seem anywhere close to widespread application, which is why the industry has begun placing so much emphasis on hydrogen as an option. Hydrogen can either be held in fuel cells, producing electricity and acting as an alternative to batteries, or directly combusted in gas turbines, in a similar way with conventional fuels. Both options have various advantages and disadvantages, but combustion seems more feasible due to its proximity to current technologies. This is the case with Airbus. The company’s flagship sustainability project, ZEROe promises a midsized, hydrogen-powered aircraft in service by 2035. Hydrogen can be treated to some extent as a conventional fuel, with the challenge of very low storage temperatures so it can remain liquid. In general, scaling up hydrogen production and the whole supply chain will be very challenging, but hydrogen is three times lighter than kerosene for the same energy content. It also occupies approximately four times more volume, which means that fuel tanks will need to be much bigger for the same range.

Sustainable fuels are here now

The most promising development, particularly for long-haul flying, is the shift that we’re just starting to see toward the use of sustainable aviation fuels that require no changes to engines and only minimal adjustments to the supply chain, if any. SAFs can be produced by a variety of methods, but in order not to compete with food crops, companies are focusing on production from waste. The main downside is the current volume of production, which is still thousands of times lower than the actual fuel needs of the industry. There is a long way to go before SAFs become available on a large scale. Environmentally speaking, SAFs result in minimum 80% reduction of life cycle CO2 emissions, since their production recycles carbon dioxide that was emitted previously and subsequently absorbed from the atmosphere during biomass production. However, there is a remainder of 20% that is not recycled during production and remains in the atmosphere.

A single path may not be necessary

A consideration worth mentioning is that all these different technologies are being developed in parallel, while a major question for airlines is what kind of aircraft they will be called to operate in 15 to 20 years from now. Is there going to be a single, winning technology that will cover all market segments? Or will we see a parallel deployment of different aircraft architectures for the different flight length segments? This question is difficult to answer now. There is no obvious winning technology, and technical consolidation is not expected to happen soon. However, the announcement of national hydrogen strategies during the past year by France, Germany, Japan, the UK and others might favor this type of energy transition for aviation applications.

Another important aspect of this changing landscape is the role of the different governments. The coronavirus crisis acted in some cases as a catalyst for stricter environmental requirements for airlines. For example, the French government supported the national flag carrier Air France with loans during the pandemic but required Air France to scrap all flights competing with the high-speed TGV trains on routes under 2.5 hours. As a result, we might see similar interventions in the future with the aim of accelerating a green transition. What remains to be seen is the impact in the operating models of airlines, since the effect of the different disruptions in aircraft technology, regulations and legislation has the potential of changing the way we travel, favoring multimodality (a combination of different modes of transport) and influencing the way airlines approach ticketing and revenue management.

Taking into consideration the above points, the target of a net-zero output in aviation seems extremely challenging by 2050. The good news is that many in the aeronautical and air transport industry are showing determination to meet the challenges with fundamental changes to their long-established practices. They are supported or in some cases pushed by governments for a solid commitment toward the net-zero 2050 goal. The majority of the technical building blocks that could make zero- or low-emission aircraft feasible for commercial flights are still in low technology readiness levels, sparking a battle against time to improve their maturity. We can never eliminate the risk that not everything will be in place within the intended time frame. Consequently, either a net-zero result or even a 50% reduction by midcentury relies on numerous assumptions. Another important element here is the term “net-zero” itself. It implies that emissions can still be positive but be counterbalanced by carbon offsetting or other carbon removal methods that are being developed. Therefore, a net-zero economy is still achievable with some emissions in place.

Now that the United States and the EU are aligned in their target, it’s going to be critical to join forces and share technology and knowledge and form some kind of common vision about the future of the planet. This will allow everyone to work at the same speed in technology development and inspire other nations along the way. The most important objective for every party is to work toward satisfying the Paris Agreement temperature criterion, and there are different paths that can lead to this outcome.