 Plane-free skies over Europe during Iceland's volcanic eruption may yield rare clues about how flights stoke climate change, adding to evidence from a closure of US airspace after Sept. 11, 2001, experts say. The climate effects of jet fuel burnt at high altitude are poorly understood, partly because scientists cannot often compare plane-free skies with days when many regions are criss-crossed by white vapour trails. Scientists will pore over European temperature records, satellite images and other data from days when flights were grounded by ash -- trying to isolate any effect of a lack of planes from the sun-dimming effect of Iceland's volcanic cloud. "The presence of volcanic ash makes this event much more challenging to analyse," said David Travis, of the University of Wisconsin-Whitewater, who found that an absence of vapour trails influenced US temperatures after the Sept. 11 attacks. One possibility was to study areas of Europe where ash was minimal and flights were cancelled mainly as a precaution. "But this becomes very challenging to measure," he told Reuters. Progress in figuring out the impact of planes might make it easier to include aviation in any UN climate deal -- international flights are exempt from emissions curbs under the UN's Kyoto Protocol for combating climate change until 2012. CARBON That might in turn push up ticket prices if flights include a penalty for emissions. Flights in Europe emitted 186 million tonnes of carbon dioxide in 2007, the European Environment Agency said, more than the total emissions of Belgium. Many studies estimate that aviation, the fastest growing transport sector, accounts for 2-3 percent of global warming from human activities that could bring more heat waves, species extinctions, mudslides and rising sea levels. No one wants disasters that close airspace but scientists will seize on European data from days of clear skies, said Gunnar Myhre of the Center for International Climate and Environmental Research in Oslo. "There will be initiatives," he said, adding that it was hard to separate ash from industrial pollution. Travis's 2002 study found that an absence of condensation trails during the Sept. 11-14 closure of US airspace to commercial flights after the suicide hijacker attacks led to bigger swings in daily temperatures. That was evidence that jets affect temperatures, but did not say if contrails were boosting climate change or not. The UN panel of climate experts reckons that aviation is damaging the climate and that non-carbon factors -- such as nitrogen oxides, soot or contrails -- may have an effect 2 to 4 times as great as carbon dioxide alone. The current European Union emissions trading scheme only covers carbon dioxide, and wants more studies. "All the impacts of aviation should be addressed to the extent possible," European Commission spokeswoman Maria Kokkonen said. High clouds -- such as contrails or cirrus clouds -- tend to trap heat, preventing it escaping from the thin atmosphere. By contrast, lower clouds usually dampen climate change since their white tops are better at reflecting sunlight.