Aviation is a significant contributor to global CO2 emissions and climate change. As passenger numbers grow, aviation's environmental impact is expected to increase. However, replacing fossil fuels with sustainable alternatives is extremely challenging.

Alternative fuels and technologies

Engineers are exploring various ways to reduce emissions from aviation:

• Sustainable Aviation Fuels (SAF): SAF is produced from biomass or synthetically by reacting hydrogen with CO2. Its composition is similar to kerosene, so no major changes to aircraft design or infrastructure are required. The European Union requires SAF to make up an increasing proportion of fuel supplied to airports.
• Biofuels: Biofuels are produced from vegetable oils, crops or waste from forestry and agriculture. Their sustainability depends on the source of biomass. The use of food crops or crops grown on quality agricultural land can limit food supplies and cause deforestation.
• Synthetic fuels (e-fuels): Synthetic fuels are made from hydrogen and CO2, which is extracted from the atmosphere. The hydrogen must be "green", i.e. produced using renewable electricity. Producing green hydrogen and capturing CO2 requires large amounts of renewable electricity.
• Hydrogen: Hydrogen has a high energy density, but its low density makes it difficult to store in aircraft. Aircraft would have to carry large, heavy tanks. Liquefied hydrogen would require well-insulated, durable tanks.
• Flight route modification: Changing flight altitude can move aircraft to warmer or less humid areas where permanent contrails do not form.

Challenges and obstacles

The introduction of sustainable fuels and technologies faces several challenges:

• Costs: The production and use of SAF and hydrogen is currently more expensive than the use of fossil fuels.
• Infrastructure: The transition to hydrogen would require extensive investment in new infrastructure, such as pipelines, liquefaction facilities, and refueling systems.
• Availability of resources: Producing enough biomass or green hydrogen to meet air travel demand could compete with other industries and strain resources.
• Safety: The use of hydrogen in aviation requires new safety procedures and design solutions.
• Timeframe: Implementing sustainable solutions in air transport will take many years.

The drive for sustainable aviation is complex and demanding. It requires investment, innovation and political support. Reducing aviation emissions will require a combination of approaches, including the use of SAF, the development of new technologies and reducing demand for air transport. Spring

Glossary of key terms:

• SAF (Sustainable Aviation Fuel): Sustainable aviation fuel; a fuel that has similar properties to conventional aviation fuel but has a significantly lower carbon impact over its life cycle.
• E-fuels (Electrofuels): Synthetic fuels made from CO2 and hydrogen, which are produced using renewable energy.
• Lignocellulose: A component of plant biomass (e.g. wood, straw) that can be used to produce biofuels.
• Pyrolysis: Thermochemical decomposition of organic materials at high temperatures in the absence of oxygen.
• Contrails: Visible trails created by aircraft, consisting of water vapor that has frozen into ice crystals. They may contribute to global warming.
• Exajoule: A unit of energy equal to 10^18 joules.
• Green Hydrogen: Hydrogen, which is produced by electrolysis of water using electricity from renewable energy sources such as the sun or wind.
• Carbon Removals: Technologies and processes that remove CO2 from the atmosphere and store it long-term.
• Life-Cycle CO2 Emissions: The total amount of CO2 emitted throughout the entire life cycle of a product or fuel, from its production to its use and disposal.
• Blended Wing Body: An aircraft design concept that integrates the wings into the fuselage, improving aerodynamic efficiency.