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Night Flights Disproportionately Influence Climate Change

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.



Rafael Seidl

Not meaning to nitpick but I would have thought there are bigger global warming issues in avaiation than nighttime contrails. For example, kerosene is not taxed, allowing airlines to get away with poor utilization rates on short-hop trips to hub airports.


Discount airlines in Europe receive subsidies to attract traffic to secondary airports. Hence the dirt-cheap fares on offer, which prompt unneccessary "getaway weekend" travel. Similarly, cheap and convenient overnight courier services are often used to compensate for poor business process planning and execution.


It would be interesting to know what causes the extreme persistence of contrails such that on a clear day the whole sky gets filled with an overcast-like cloud layer, this seems like a recent occurrence. I have done timelapse photography of contrail formation from night flights (seemingly a single plane going back and forth) and it was quite interesting to see the contrails appear several minutes after the plane passed by, and then forming regular looking clouds that moved across the sky.

Video of the timelapse available here


They discoverd Global Dimming as a result of contrails that were not there
after 9/11 when the skies were clear of all airlines. It is an interesting story
that can be found on PBS.org

Roger Pham

High-altitude jet planes produce the most long lasting contrails due to the very cold and high altitude that the water vapor in the engine exhaust turns into tiny ice crystals that would float around for a long time before settling down. This acts like a radiation blanket for the earth. For this reason, short-range commercial aviation should be discouraged with perhaps taxation or other regulations. Flying itself is energy-intensive (GHG production) in comparison to trips made by trains or buses, and for short trips, the time-saving advantage of commercial aviation is minimum. High-speed electric trains similar to the French TGV (train a grand vitesse) is the most environmentally friendly and should be encouraged.

allen zheng

We should shift some R&D over surface effect vehicles. The Soviets had surface effect vehicles (one was called the Caspian Sea Monster), and Boeing has its Pelican ULTRA concept. The Pelican relies on this aerodynamic phenomenon to get an increase in range from 6,000 Nm to 10,000 Nm at mid sub-sonic speeds (Mach 0.4-0.5). It also has a 1,000+ ton (17 M1A2 SEP tanks+ammo+crew) payload rating, and may take off from Boeing 747 or A-380 capable airports. Another would to utilize fast ship designs, especially Catamaran designs that incorporate a retractable wing that generates lift (lifting the bow), and reduces drag.

allen zheng

50% fuel consumption or 41% better speed.

Roger Pham

Allen, the problem with surface effect vehicles is that the air is too thick at the earth's surface, and as such, too much drag. Another problem is that the air turbulence is too high near the earth surface that will make for an uncomfortable ride. Airliners cruise at 35,000ft where the air is less than 1/4 the density of air the surface, and you fly above the weather at that altitude.

allen zheng

Freight; sea; read info about these concepts.

John Kadlec

The point of the PBS program (Dimming the Sun) was that contrails, smoke and soot, etc are reducing the solar radiation reaching the ground. ie these particulates, ice crystals, etc are reflecting a significant percentage back into space and therefor masking the true warming effect that we are causing by release of the greenhouse gasses and carbon into our atmosphere. Global warming is significantly worsening as we clean up our smelters, coal burning and so on.

Roger Pham

John, we will still need to clean our smelters and coal burning plants and diesel vehicles for respiratory health reasons. Furthermore, a clearer sky allows for more earth cooling at night, and more intense sun light in the day time the better to power our future solar energy collectors. The best short-term solution is to encourage more jets to fly in the early morning hours instead of in late evening or at night. We should be phasing in renewable fuel economy based on solar, wind, and biomass, and phasing out fossil fuel usage. We have enough technology to start today. More technology will be developed once the momentum will gather.

John Kadlec

Roger, I totally agree, but again the point the program was making was that the global warming is considerably worse than most scientists and politicians think or speak about. The program went on to say that he believes we have about 10 years before the corner is turned and the trend will be unrecoverable.

I am concerned that we are so shortsighted and what the legacy will be for future generations?! I keep reading/hearing that it is not possible to justify the additional cost of purchasing a hybrid car. I beg to differ. We bought a Prius almost two years ago and know for a fact that already we have nearly saved the difference in fuel. But even more it's about changing the mindset that driving huge Hummers and SUV's for the ego trip is a right as Americans. We recently replaced our second car with a hybrid as well.

I know Toyota and Honda continue to develop vehicles that are even more efficient and low environmental impact. Shame on America's car makers who have not lead the way in providing the technology and educating us, but continue to stroke our egos with ridiculous guzzlers not to mention the politicians who are deep in big oil's pockets, doing little to foster public transport & alternative fuels! There is so much we could be doing today.


It's amazing to me that liberals are so frightened by global warming and it's potential disaster of Algorian proportions...while conservatives are rather ho-hum about the terror it poses. It seems that the Earth goes through cycles of heating and cooling independently of man's activities and has done so since time began.

Roger Pham

Mr. Fire, get a grip on science and learning. Don't be dumbed down by the Bush Adm. or other ignorant people who are still in denial of our real world issue. I'm a conservative when it comes to family, moral values, and social issues. But when it comes to environmental preservation, many of the so-called Conservatives do not really embrace environmental conservation.

The natural earth cycle of temperature variation took tens of thousands of years to cycle through each variation, not a few hundred years that it has taken to noticeably warm up the earth as in recent human history. The last ice age was ten thousands years ago. When a cycle takes thousands or tens of thousands of years, human and living organisms have the time to adapt. When it happens so rapidly, then we will have catastrophe.

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