Restrictions on night flights could ease the aviation industry's fast-growing contribution to global warming, say UK scientists.
Dr Nicola Stuber and fellow meteorologists at the University of Reading report
their findings in today's issue of the journal Nature.
certain altitudes, aircraft produce contrails - condensation trails caused
when the plane's hot exhaust hits the chilly atmosphere.
contrails have a surprisingly big but also complex effect on the climate.
Because they are clouds, they trap heat that is emitted by the Earth's
surface, creating a "greenhouse effect" that adds to warming.
during daytime, these clouds have a cooling effect because they are white and
thus reflect some of the Sun's energy back into space. In certain conditions,
contrails can exist for several hours.
Stuber and team estimate the
radiation caused by contrails at a busy flight corridor in southeast
Using high-resolution aircraft flight data and routine weather
balloon data, they looked at "persistent" contrails: wakes that remained for
an hour or more after the aircraft had flown over.
account for only 22% of Britain's annual air traffic but contribute between 60
to 80% of the greenhouse effect from contrails, the scientists
Winter flights warm more
Stuber and team also
found that flights during the winter months could contribute more to global
"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," says Stuber.
there are fewer flights during the winter months, the conditions needed to
form contrails - the right temperature, amount of moisture in the air and
aircraft altitude - are found more often then.
Global emissions of man-made CO2 are between 6.2 billion
and 6.9 billion tonnes per year. Added to this are around 1.5 billion tonnes
from land use.
Commercial aircraft account for only a small
contribution compared with power stations, industry and road traffic.
However, passenger travel is growing at the rate of around 5% a year,
which means that this share will grow fast.
A 1999 estimate by the UN's
Intergovernmental Panel on Climate Change (IPCC) found that the airline
industry accounted for 2% of man-made CO2 emissions in 1992. But it would rise
to as much as 15% by 2050.
Environmentalists are angry, complaining
that airlines get a free ride when it comes to environmental taxes.
Changing altitude could also help
In addition to
rescheduling night flights for the daytime, planes could diminish their
contribution to global warming by changing their altitude.
published last year in the journal Transportation Research suggests
that the regions of "ice-supersaturated" air where contrails form is only
about 500 metres thick.
The goal would be to fit sensors on aircraft
that could inform pilots where this layer lies, thus enabling them to shift