Reuters: 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 U.S. airspace after September 11, 2001, experts say. The climate effects of jet fuel burned 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 vapor trails. Scientists will pore over European temperature records, …