News Excerpt:

According to the study by Candelaria Bergero and colleagues, Aviation accounts for 2.5% of global CO₂ emissions, but it has contributed around 4% to global warming to date.

Key highlights of the study:

• To calculate carbon emissions from aviation, three metrics are required:
• Aviation demand: How many passenger and freight kilometers;
• Energy efficiency: How much energy is used per kilometer;
• Carbon intensity: What fuel is being used, which tells us the carbon emitted per unit of energy.
• Multiply these metrics together to get carbon dioxide (CO2) emissions.

• In the above chart, between 1990 and 2019, both passenger and freight demand has approximately quadrupled.
• In 2019, passengers travelled more than 8 trillion kilometres, about the same as a light year.
• At the same time, flying has become more than twice as energy efficient. Traveling one passenger-kilometer in 1990 used 2.9 megajoules (MJ) of energy. 
  • By 2019, this had more than halved to 1.3 MJ. 
  • This efficiency has come from improved design and technology, larger planes that can carry more passengers, and a higher ‘passenger load factor’. 
  • Empty seats are less common than in the past.
• Suppose flying has become more than twice as energy efficient, and the carbon emitted per unit of energy has not changed. In that case, it follows that the carbon efficiency of travelling one kilometre is also more than twice as high.
  • In 1990, one passenger-kilometer would emit 357 grams of CO2. By 2019, this had more than halved to 157 grams.
• Aviation accounts for 2.5% of global CO2 emissions.
• In 2019, aviation accounted for 2.5% of CO2 emissions from fossil sources and land use. 
• This share has fluctuated from 2% to 2.5% since the mid-1990s, with a marked increase since 2010.

How have the changes in demand and technology affected CO2 emissions?

• If demand has quadrupled, but aviation has become twice as efficient, then emissions will double. The gains in efficiency have partly counteracted the emissions from increased demand.
• In 1990, global aviation emitted around 0.5 billion tonnes. In 2019, that was around 1 billion.

Global CO2 emissions from aviation have quadrupled since the 1960s:

Non-CO2 climate impacts:

• While aviation accounts for around 2.5% of global CO2 emissions, its overall contribution to climate change is higher.
• Along with emitting CO2 from burning fuel, planes also affect the concentration of other atmospheric gases and pollutants. 
• They generate a short-term increase but a long-term decrease in ozone and methane and increased emissions of water vapour, soot, sulfur aerosols, and water contrails. 
• While some of these impacts result in warming, others induce a cooling effect. But overall, the warming effect is stronger.
• The authors estimate that aviation has accounted for approximately 3.5% of effective radiative forcing to date. Another study estimates that it has been responsible for 4% of global temperature rise since pre-industrial times.
• Radiative forcing measures the difference between incoming energy and the energy radiated back to space. If more energy is absorbed than radiated, the atmosphere becomes warmer.
• Although CO2 gets most of the attention, it accounts for less than half of this warming. Two-thirds come from non-CO2 forcings. 
• Contrails — water vapour from aircraft exhausts — account for the largest share. 
• This explains why aviation contributes 2.5% of annual CO2 emissions but more when it comes to its total impact on warming.

Aviation’s share of global emissions is likely to rise as other sectors decarbonize faster:

• Aviation is one of the hardest sectors to decarbonize. 
• Electricity can become low-carbon through the deployment of renewables and nuclear energy, and road transport and heating can be made low-carbon through electrification. 
• Even “hard-to-abate” industries such as cement and steel have emerging low-carbon alternatives.
• Aviation is, by comparison, behind the curve. Global demand will likely grow in the coming decades as populations get richer. 
• Therefore, the rise in emissions will be determined by whether aviation can maintain improvements in energy efficiency and switch to low-carbon fuels. So far, the sector has made almost no progress on the latter.
• While more efficient planes can dampen some of the growth in emissions, they can’t eliminate them completely. 
• To do that, the industry will need to move from jet fuel to electrification, biofuels, hydrogen, or a combination. 
• Until it makes this switch, aviation will make up an ever-increasing share of global emissions.