• Article
  • 10 Nov 2021
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George Allen

Are Contrails the Next Aerospace Climate Issue?

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The aerospace industry has already introduced numerous measures to achieve carbon net neutrality by 2050. We discussed these in a post in late 2020 and published a follow-up piece earlier this year. Cutting emissions is the most obvious solution, yet one glaring issue has so far slipped through the net: contrails.

How do contrails affect the climate, and what changes could we make to lessen their impact?

What are Contrails?

Contrails are simply trails of condensation emitted by aircraft during flight. While emissions are (for now) an expected part of aircraft flight, contrails present an interesting issue.

They form when soot and particulate matter from aircraft engines act as a catalyst for water vapour to condense onto and form ice. It is essentially the same process as cloud seeding, except not as intentional.

However, contrails form under specific atmospheric conditions. The surrounding air must have high enough humidity (and the correct temperature) for water condensation to occur. The exhaust gases are a higher temperature than the surrounding air, which aids condensation. As the air then cools to its original ambient temperature, the water freezes and turns to ice.

Because contrails require certain conditions, we will not always see them forming in the same place. Ambient temperatures can fluctuate, meaning contrails can last for minutes, hours or days, or not form at all. That said, evidence of recent studies has highlighted areas where contrails form more consistently.

How Contrails Impact the Climate

Contrails’ impact on the climate is fairly straightforward. The NASA guidance linked above explains how contrails mix with naturally-occurring cirrus clouds, in turn increasing overall cloudiness in the sky.

But why does this matter? Simply put, contrails increase the amount of heat that is trapped and absorbed in the atmosphere. Normally, this heat should escape into space, but the presence of contrails prevents this from happening.

A study in Science Direct from January 2021 found that contrails could account for as much as 57 per cent of the aviation industry’s impact on climate change. Considering aviation is responsible for 2.4% of CO2 emissions globally, this is a staggering figure to have overlooked.

What Can Change?

The recent COP26 summit highlighted the need for rapid and revolutionary change to the way we live and produce and use energy. Although most media attention focused on large-scale practices, such as ending deforestation and curbing methane emissions, there are (comparatively) minor things that could make a considerable difference.

And that brings us back to contrails. A 2020 study found that adjusting less than two per cent of global flights could reduce the effect of contrails by 59 per cent. Based on the knowledge that contrails need certain atmospheric conditions, it simulated raising or lowering the altitude of flights by 2,000 feet. Doing so increased fuel consumption by less than a tenth of a per cent, but this was offset by the reduction in contrail formation.

Granted, the study was only a simulation on a small scale, but the information is certainly promising. Combining these results with more efficient combustion engines could reduce contrail damage by up to 90 per cent, the study also found.

Reaching Carbon Net Neutrality

As the industry looks for ways to become carbon net neutral, it is important to also focus on existing technology and practices. Contrails are one such example, as their impact on the climate is considerable but easily reduced.

We can only hope that this information is the beginning of an adjustment towards more efficient engines and flight paths.