June 14, 2006

Fewer Night Flights Could Cut Climate Change Impact

By Patricia Reaney

LONDON -- Cutting the number of flights that take off at night could help to reduce the contribution of aviation to global warming, researchers said on Wednesday.

Night flights contribute to climate change because the white streaks of condensation, or contrails, left behind by jets trap energy emitted from the Earth's surface.

Daytime flights have less impact because contrails also reflect some of the sun's energy back into space which has a cooling effect.

"It you wanted to minimize the contrail climate effect you might want to think about rescheduling flights," Dr Nicola Stuber of Reading University said in an interview.

The researchers discovered that although only about 25 percent of flights in Britain take off between 6 p.m. and 6 a.m., they account for 60-80 percent of global warming linked with contrails.

On the east coast of the United States the 36 percent of flights that take off at night account for 53 percent of the annual warming due to contrails. In southeast Asia, which had slightly fewer late flights, the percentage is about 70.

"Night flights contribute disproportionately to the daily mean effect of contrails," said Stuber.

Although the overall effect of contrails on climate change is small, about 3.5 percent of the potential from all human causes, the scientists warned it could gain importance as the volume of air traffic increases.

Stuber and her team used a computer program to analyze flight data and atmospheric conditions to determine when contrails are most likely to form and what impact they would have on the Earth's temperature.

They found that, in addition to night travel, flights during the winter months account for half of the annual warming from contrails.

"So you get half of the climate warming effects from flights during one quarter of the year," said Stuber, who reported the findings in the journal Nature.

Although 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.

"If you have all these conditions, you will have formation of persistent contrails. The effect of these conditions is larger than the effects of the air traffic," Stuber said.