By Alister Doyle, Environment Correspondent
OSLO (Reuters) - Aircraft condensation trails criss-crossing the sky may be warming the planet on a normal day more than the carbon dioxide emitted by all planes since the Wright Brothers' first flight in 1903, a study said on Tuesday.
It indicated that contrails -- white lines of Vapor left by jet engines -- also have big knock-on effects by adding to the formation of high-altitude, heat-trapping cirrus clouds as the lines break up.
The findings may help governments fix penalties on planes' greenhouse gas emissions in a U.N.-led assault on climate change. Or new engines might be designed to limit Vapor and instead spit out water drops or ice that fall from the sky.
"Aircraft condensation trails and the clouds that form from them may be causing more warming today than all the aircraft-emitted carbon dioxide (CO2) that has accumulated in the atmosphere since the start of aviation," the journal Nature Climate Change said in a statement of the findings.
The study, by experts at the DLR German Aerospace Center, estimated that the net warming effect for the Earth of contrails and related cirrus clouds at any one time was 31 milliwatts per square meter, more than the warming effect of accumulated CO2 from aviation of 28 milliwatts.
A milliwatt is a thousandth of a watt. Aviation emissions now account for about three percent of annual CO2 emissions from fossil fuels, more than a century since Orville and Wilbur Wright made the first powered airplane flight.
But a key difference is that CO2 lingers for decades while warming from contrails quickly ends if flights are grounded, such as after the September 11, 2001 attacks in the United States, or in Europe after last year's Icelandic volcano eruption.
"You can get rid of contrails very quickly. You can't get rid of CO2 quickly," lead author Ulrike Burkhardt at DLR told Reuters.
The main climate effect of white lines and related cirrus clouds is to trap heat radiating back from the Earth's surface. They also have a smaller, counter-effect by slightly dimming sunlight and so slowing warming. Contrails are especially dense over parts of Europe and eastern United States.
"This is a breakthrough in modeling and understanding of contrails," Olivier Boucher, of the Met Office Hadley Center in England who wrote a related article in Nature, told Reuters.
He said the findings might bring changes in air traffic control, for instance diverting planes from regions or altitudes where air moisture was high and favored cirrus formation.
But a problem was that any benefits of fewer contrails might be canceled out by higher fuel use on longer routes.
He also said that it could spur a novel engine concept that would seek to condense some of the water Vapor "before it leaves the engine. The condensed water could be vented in the form of large ice crystals or droplets that would fall quickly through the atmosphere."
The U.N. panel of climate scientists has estimated that fuel burned at altitude is roughly twice as damaging for the climate as when used at ground level. Boucher said that the study might slightly raise that estimate, adding to potential costs.
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(Editing by Mark Heinrich)