Stockholm University noted some recent research by a team of scientists there on the brightness of high-level clouds affected by airplane exhaust. Cloud brightness is important for climate because it helps control how much sunlight (and thus energy) reaches the surface of the earth. Brighter clouds–more reflection of sunlight back to space–less energy at the surface. The exhaust from the airplanes interacts with the cloud droplets or ice crystals to make them look thicker and more reflective. This appears to be the most effect in mid-latitude clouds. You can read more about it here.
