Of the many unprecedented events of Sept. 11, one of them — the near-absence of airplanes in the skies that day and the next — has given scientists important clues for studying the impact of aviation on climate change. Normally, the sky above 25,000 feet is full of contrails, cloud-like phenomena that form when ice crystals bond to jet fumes. Climatologists have long suspected that contrails contribute to global warming, but with constant air traffic, there was no way to compare the sky with and without airplanes. Now scientists are optimistic that data from Sept. 11 and 12 will help refine climate change computer modeling. Meanwhile for an update on the climate negotiation scene, as the latest round of international talks begins this week in Morocco, visit the Grist Magazine website.