“The avoidance of contrails would allow aircrew to make a direct contribution to the reduction of global warming”

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What is climate?

Climate is the basically the “average” weather, e.g. average annual sunshine, average annual rain fall, average wind, etc., where the average is based upon observations taken over many decades. Hence, climate change means that the average weather is changing with time.

What causes climate to change?

Climate is closely related to the global mean temperature. This is the average temperature of the planet, which is determined by the balance of incoming solar radiation and the outgoing Earth’s radiation.

This balance and, hence, the global mean temperature is affected by, amongst other things, the level of the “greenhouse” gases in the atmosphere and the level of cloud, i.e. the Earth’s albedo.

A warmer atmosphere means that the air can hold more water, more water means more weather and progressively more weather may mean climate change.

What contribution does aviation make?

At present, aviation burns kerosene and the products of combustion are released into the wake. The exhaust gases are hot and humid and contain a cocktail of gases, plus some soot.

The exhaust contains carbon dioxide, which is an important, i.e. a powerful, greenhouse gas. It also includes nitric oxide and nitrogen dioxide. These are not greenhouse gases but, through complex chemical reactions, they affect the levels of the pre-existing, green house gases methane and ozone.

Under certain metrological conditions, the water vapour in the exhaust condenses onto the soot particles and the droplets freezes to form ice crystals. The ice crystals reflect sunlight and a visible contrail appears.

What is the significance of contrails?

When the aircraft is forming a contrail, if the surrounding atmosphere is dry, the contrail disappears after a few kilometres as the ice becomes water vapour again. However, if the surrounding atmosphere is super-saturated with respect to ice, the water in the atmosphere also forms ice crystals and a “persistent” contrail forms.

These can be hundreds of kilometres long and tens of millions of tonnes of ice become trapped in the trailing vortices behind the aircraft. Persistent contrails can last for many hours and some develop into “contrail induced” cirrus cloud.

The ice crystals absorb, reflect and transmit both solar radiation and the radiation from the Earth’s surface and, consequently, make a direct contribution to global mean temperature.

What does all this do?

Both the carbon dioxide emission and the emission of the oxides of nitrogen lead to an increase in the global mean temperature.

However, the effect of contrails and contrail induced cirrus depends upon the time of day. In daylight, the reflection of incoming solar radiation back into space dominates, producing a cooling effect. At night, the absorption and reflection of the Earth’s radiation dominates, producing a warming effect.

In the current operational environment, the net effect of contrails and contrail cirrus is to increase the global mean temperature.

How big are the effects?

Comparing these different contributions is difficult. However, based upon the current level of understanding, aviation’s total contribution to an increasing global mean temperature is roughly 1/3 carbon dioxide, 1/6 oxides of nitrogen, 1/2 contrail and contrail cirrus.

What is being done ?

Currently, the primary focus has been placed upon carbon dioxide and its removal from the aircraft, but not necessarily from the environment, by the use of sustainable aviation fuels. However, this is a massive undertaking that will take many years and incur huge cost, requiring a massive industry that hardly exists today.

The current internationally agreed target is to limit mean global temperature to 1.5 degree Celsius above pre-industrial levels by 2050. However, to deliver any significant benefit the climate by 2050, all the carbon must have been removed from the global fleet (and from the environment) and, even the most optimistic projections, suggest that this is unlikely to be achieved.

However, nothing is being done about aviation’s largest contribution – the contrail and contrail induced cirrus!

What can be done and when?

The avoidance of contrails is a purely operational issue and this is something that would allow aircrew to make a personal and direct contribution to the reduction of global warming. It requires no new technology, it incurs only marginal cost, it is ethical, its effects are immediate, the benefit are substantial and the process could be initiated immediately.

In the first instance, eliminating all contrails would cut aviation’s contribution in half. However, if technology and operating techniques could be refined to identify accurately when individual cooling and individual warming contrails would be formed, the operational environment could be developed so that the warming contrails were supressed, but the cooling contrails would be allowed to form. This would be a major and unique contribution to the reduction of global warming that only aviation could make.

What can airline pilots’ professional associations do?

Raise awareness of the potential for the operational avoidance of contrails with employers and politicians at every opportunity. Determine and demonstrate how contrail avoidance would work in practise and develop policy. Develop the techniques, find the problem areas, suggest solutions and determine the costs.

Make the public aware of what could be achieved.

More information at the following link.

About the profesor the Sepla technical  department interviewed 

Ian Poll is Emeritus Professor of Aerospace Engineering at Cranfield University and a member of the Council of the Air League. He is a Past President of the Royal Aeronautical Society, a Past President of the International Council of the Aeronautical Sciences, a former member of the UK Natural Environment Research Council, a former Chairman of the UK Defence Scientific Advisory Council, Ministry of Defenceand a former Council member of the Royal Academy of Engineering.

A graduate of Imperial College and the College of Aeronautics (PhD), he has over 50 years experience in both academia and industry. He is an aerodynamicist and, for the past 20 years, his research has focused on the gaining of a better understanding of aviation’s interaction with the environment.

He is a Fellow of the Royal Academy of Engineering, aFellow of the City and Guilds Institute of London, an Honorary Fellow of the American Institute of Aeronautics and Astronautics, an Honorary Fellow of the International Council of the Aeronautical Sciences and a Fellow of the Royal Aeronautical Society. He was made an OBE in 2002.