Climate change and flying: what share of global CO2 emissions come from aviation?

Flying is a highly controversial topic in climate debates. There are a few reasons for this. 

The first is the disconnect between its role in our personal and collective carbon emissions. Air travel dominates a frequent traveller’s individual contribution to climate change. Yet aviation overall accounts for only 2.5% of global carbon dioxide (CO₂) emissions. This is because there are large inequalities in how much people fly – many do not, or cannot afford to, fly at all [best estimates put this figure at around 80% of the world population – we will look at this in more detail in an upcoming article].

The second is how aviation emissions are attributed to countries. CO₂ emissions from domestic flights are counted in a country’s emission accounts. International flights are not – instead they are counted as their own category: ‘bunker fuels’. The fact that they don’t count towards the emissions of any country means there are few incentives for countries to reduce them.

It’s also important to note that unlike the most common greenhouse gases – carbon dioxide, methane or nitrous oxide – non-CO₂ forcings from aviation are not included in the Paris Agreement. This means they could be easily overlooked – especially since international aviation is not counted within any country’s emissions inventories or targets.

How much of a role does aviation play in global emissions and climate change? In this article we take a look at the key numbers that are useful to know.

Global aviation (including domestic and international; passenger and freight) accounts for:

• 1.9% of greenhouse gas emissions (which includes all greenhouse gases, not only CO₂)
• 2.5% of CO₂ emissions
• 3.5% of ‘effective radiative forcing’ – a closer measure of its impact on warming.

The latter two numbers refer to 2018, and the first to 2016, the latest year for which such data are available.

Aviation accounts for 2.5% of global CO₂ emissions

As we will see later in this article, there are a number of processes by which aviation contributes to climate change. But the one that gets the most attention is its contribution via CO₂ emissions. Most flights are powered by jet gasoline – although some partially run on biofuels – which is converted to CO₂ when burned.  

In a recent paper, researchers – David Lee and colleagues – reconstructed annual CO₂ emissions from global aviation dating back to 1940.¹ This was calculated based on fuel consumption data from the International Energy Agency (IEA), and earlier estimates from Robert Sausen and Ulrich Schumann (2000).²

The time series of global emissions from aviation since 1940 is shown in the accompanying chart. In 2018, it’s estimated that global aviation – which includes both passenger and freight – emitted 1.04 billion tonnes of CO₂.

This represented 2.5% of total CO₂ emissions in 2018.^3,4

Aviation emissions have doubled since the mid-1980s. But, they’ve been growing at a similar rate as total CO₂ emissions – this means its share of global emissions has been relatively stable: in the range of 2% to 2.5%.⁵

Non-CO₂ climate impacts mean aviation accounts for 3.5% of global warming

Aviation accounts for around 2.5% of global CO₂ emissions, but it’s overall contribution to climate change is higher. This is because air travel does not only emit CO₂: it affects the climate in a number of more complex ways.

As well as emitting CO₂ from burning fuel, planes affect the concentration of other gases and pollutants in the atmosphere. They result in a short-term increase, but long-term decrease in ozone (O₃); a decrease in methane (CH₄); emissions of water vapour; soot; sulfur aerosols; and water contrails. While some of these impacts result in warming, others induce a cooling effect. Overall, the warming effect is stronger.

David Lee et al. (2020) quantified the overall effect of aviation on global warming when all of these impacts were included.⁶ To do this they calculated the so-called ‘Radiative Forcing’. 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. 

In this chart we see their estimates for the radiative forcing of the different elements. When we combine them, aviation accounts for approximately 3.5% of effective radiative forcing: that is, 3.5% of warming.

Although CO₂  gets most of the attention, it accounts for less than half of this warming. Two-thirds (66%) comes from non-CO₂ forcings. Contrails – water vapor trails from aircraft exhausts – account for the largest share.

We don’t yet have the technologies to decarbonize air travel

Aviation’s contribution to climate change – 3.5% of warming, or 2.5% of CO₂ emissions – is often less than people think. It’s currently a relatively small chunk of emissions compared to other sectors. 

The key challenge is that it is particularly hard to decarbonize. We have solutions to reduce emissions for many of the largest emitters – such as power or road transport – and it’s now a matter of scaling them. We can deploy renewable and nuclear energy technologies, and transition to electric cars. But we don’t have proven solutions to tackle aviation yet.

There are some design concepts emerging – Airbus, for example, have announced plans to have the first zero-emission aircraft by 2035, using hydrogen fuel cells. Electric planes may be a viable concept, but are likely to be limited to very small aircraft due to the limitations of battery technologies and capacity. 

Innovative solutions may be on the horizon, but they’re likely to be far in the distance.

Towards zero-carbon transport: how can we expect the sector’s CO₂ emissions to change in the future?