Looking up all you will have seen is ominous heaps of clouds with darkening bases threatening to yet again dump their watery loads upon us. You may muse that these cloud formations are very similar to those seen by our forebears but you’d be wrong. There is one type of cloud that has only appeared in the skies since the run up to the First World War when pilots discovered extremely fast ribbons of winds high in the troposphere. These high-speed ‘jet-streams’ have since been used by aircraft to dramatically reduce flight times and more importantly fuel consumption, well at least on one leg of the journey anyway. However the by-product of this discovery is the appearance of the condensation trail or ‘contrail’ that forms in the wake of long haul aircraft criss-crossing our skies.
Clouds have a huge if somewhat contradictory effect on ground surface temperature. Most clouds that block out sunlight have an overall cooling effect at the Earth’s surface. However this is not true for clouds that form at higher altitudes where they are thin enough to let much of the sunlight through; clouds like the ‘contrail’. By acting as condensation and icing nuclei, the tiny particles in the aircraft exhaust encourages the water vapour naturally present in the atmosphere to form into droplets or crystals. They effectively seed the saturated atmosphere and act as catalysts that encourage the formation of thin cirriform clouds. These clouds, like the invisible water vapour and other greenhouse gases, absorbsome of the Earth’s heat and re-radiate a proportion of it back down. In this way they actually reverse the cooling effect at the Earth’s surface.
A number of scientific reports have been published backing up the claim that man-made ‘contrails’ do in fact contribute to ‘global warming’. One report1 concluded that warming attributable to ‘contrails’ over Europe was ten times greater than that expected from aviation CO2 emissions. Another2 claimed that the rise in temperature observed across the US during the last 25 years could be entirely attributed to the increase in cirriform clouds. There is other evidence too, during the three days following 9/11 when all flights over US airspace were cancelled there was a recorded 1.1C increase in the difference between day and night temperatures.
It doesn’t take much blue sky thinking to realise that just by reducing the cruising altitude of long-haul aircraft we can prevent ‘contrails’ from forming and have an immediate impact on one aspect of ‘global warming’ that we do appear to have some control over. But this plan obviously takes courage to implement.
Whilst there would be an overall increase in CO2 emissions due to longer journey times this would be ‘offset’ by the reduction in ‘contrail’ formation; estimated at 4 per cent versus 70 per cent. Apart from the additional costs associated with lower flying altitudes, increases in air traffic congestion and noise levels would we all be prepared to pay the additional price that would be undoubtedly be passed onto us?
There are conveniently too many unknown factors for existing research to be conclusive on the impact of ‘contrails’ on ‘global warming’ – enough doubt to justify government and commercial inaction and perhaps our own consciences.
Notes:
1 Mannstein & Schumann: ‘Observations of contrails and Cirrus over Europe’. Proceedings of the AAC Conference, 30 June – 3 July 2003 in Friedrichshafen, Germany
2. Minnis, Ayres, Palikonda and Phan: ‘Contrails, Cirrus Trends and Climate’ 2004, Journal of Climate, 17.
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