Methane clouding up the twilight.

Methane clouding up the twilight

Rising concentrations of atmospheric methane may be creating a phenomenon called noctilescent clouds that light up the twilight in certain areas of the world. Forming at the top of the mesosphere, about 85 kilometers above the surface, these clouds are the highest on Earth. They appear in summertime at latitudes between Paris and the Arctic Circle and at comparable latitudes in the Southern Hemisphere.

"What we're seeing here is a very visible, very dramatic sign that the atmosphere is changing," says Gary E. Thomas of the University of Colorado in Boulder. In the April 6 NATURE, Thomas and his colleagues report that methane--accumulating at a rate of more than 1 percent per year -- has stimulated the formation of noctilescent clouds because it breaks down to form water vapor upon reaching the stratosphere. This water vapor rises into the extremely dry mesosphere and condenses onto particles to form clouds.

Before the late 1800s the mososphere was too dry to create visible clouds, but since then methane levels have slowly added enough water vapor to form noctilescent clouds, Thomas says. Indeed, no sightings of the clouds appear in historical records before 1885, a year when the clouds developed in great numbers. The researchers attribute the many clouds observed in that year to a large volcanic eruption on the Indonesian island of Krakatoa two years earlier, which temporarily injected both water vapor and particles into the mesosphere. Without the eruption, the clouds would have appeared gradually during the early 1900s.