A study by the Meteorology Department at the University of Reading (UK) suggests that a reducing night flights could help minimize the impact of aviation on climate change.
The study, published in Nature this week, shows that even though 25% of flights over the UK occur during the night, these flights are responsible for at least 60% to 80% of the radiative forcing associated with aircraft condensation trails (contrails).
For countries without night-flying restrictions the contribution of these flights to the contrail warming can be even larger.
Although the warming effective of contrails is currently small compared to that induced by other sources of human emissions, the comparably large growth rate of air traffic requires an improved understanding of the resulting impact of aircraft radiative forcing on climate, according to the research team.
Aircraft currently only have a small effect on climate. However, the fact that the volume of air traffic is set to rapidly grow in coming years makes it important to investigate the effects of contrails on our climate.—Piers Forster, Project leader (now at University of Leeds)
|The contribution of flights at different times of the day. Click to enlarge.|
Contrails have an effect on the Earth’s energy balance similar to that of high thin ice clouds. They enhance the natural greenhouse effect by trapping outgoing longwave radiation emitted by the Earth and atmosphere (positive radiative forcing), leading to warming. At the same time, they reflect some of the incoming solar radiation back to space, leading to a cooling effect (negative radiative forcing).
On average, the warming longwave effect dominates and the net contrail radiative forcing is believed to be positive (i.e., warming). Over daily and annual timescales, varying levels of air traffic, meteorological conditions, and solar insolation influence the net forcing effect of contrails, according to the researchers.
The contribution that night-time flying makes to climate warming is so high because the cooling effect only happens when the sun is up, whereas the warming effect occurs both day and night.
Researchers combined high resolution aircraft flight data and routine weather balloon data in a computer program that models the interaction of solar and infrared radiation with the atmosphere.
We conducted our study for a site in southeast England, located in the entrance region to the North Atlantic flight corridor. For this investigation, we concentrated on “persistent contrails”—contrails which remain for an hour or so after the aircraft have gone. As well as discovering that this small proportion of night-time flights contributes in such a significant way to climate warming, we also found that flights between December and February contribute half of the annual mean climate warming even though they account for less than a quarter of annual air traffic.
The findings have implications beyond their pure scientific value; they could be used if policy makers decided to modify flight management systems in order to reduce the climate impact of aviation.—Nicola Stuber, University of Reading
The study was supported by the UK Department of Transport, the UK Department of Trade and Industry, Airbus and Chris Eyers at QinetiQ.
“The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing”; Nicola Stuber, Piers Forster, Gaby Rädel and Keith Shine; Nature 441, 864-867 (15 June 2006) | doi:10.1038/nature04877