Strategies for reducing aviation’s climate change impact tend to focus on engine technologies to improve fuel economy and air traffic control schemes to guide flights on more direct routes and minimise time spent in holding patterns. In both cases, the objective is to reduce fuel consumption – and hence carbon dioxide (CO2) emissions. But could there be a way to decrease the global warming impact of aviation even at the cost of a small increase in fuel consumption?

Perhaps counter-intuitively, the answer may be “yes” – by making small changes to cruise altitude that could reduce contrail formation. And the net benefit would increase with wider use of engine technology that reduces the emissions of black carbon, which seeds the formation of contrail ice particles.

Contrails near Heathrow

Source: Polly Thisdell/Flightglobal

Contrails near Heathrow airport

Under some conditions, contrails – the fine-line clouds that form in jet exhaust under certain atmospheric conditions – can reflect sunlight back to space and have a cooling effect, but otherwise contribute to warming by reflecting back downwards heat rising from the ground, which would otherwise be dissipated into space. On balance, this climate forcing of contrails and their related induced-cirrus clouds is thought to be comparable to the cumulative impacts of aviation CO2 emissions.

So, working with Japanese airspace data, researchers at the Centre for Transport Studies at Imperial College London and Germany’s DLR aerospace research institute in Oberpfaffenhofen looked at how raising or lowering cruise altitude, to avoid contrail-forming conditions, might affect the climate impact of aircraft and of the broader cirrus clouds to which they contribute.

Roger Teoh, Ulrich Schumann, Arnab Majumdar and Marc Stettler conclude that “a small-scale strategy of selectively diverting” just 1.7% of flights could reduce the net warming effect of contrails by nearly 60%, while increasing CO2 emissions by barely 0.1%. Or, they found, “a low-risk strategy of diverting flights only if there is no fuel penalty” would still cut contrail warming effects by one-fifth.

Published in February by the American Chemical Society’s Environmental Science and Technology journal, the authors note that “modern aircraft are designed with vertical altitude flexibilities of up to 3,000ft at the expense of a few per cent fuel penalty” – so, under the right conditions, directing an aircraft to fly a couple of thousand feet higher or lower could reduce a flight’s environmental impact.