High clouds may soon light U.S. sunsets.

Since the late 1800s, residents of the far northern latitudes have witnessed a strange type of cloud appearing high in the sky during the twilight of summer days. These so-called noctilucent ice clouds may become visible as far south as Philadelphia in the next century because of greenhouse gases accumulating in Earth's atmosphere, according to calculations by Gary Thomas of the University of Colorado at Boulder and his colleagues. They reported their work at the AGU meeting.

Noctilucent clouds develop in the mesosphere at an altitude of 85 kilometers. Prior to the Industrial Revolution, the air in this region was too dry to form clouds. But rising concentrations of methane added water vapor to the mesosphere, prompting clouds to form in the coldest latitudes.

Greenhouse gases will enhance noctilucent clouds in two ways. The addition of more methane to the atmosphere will further increase water vapor concentrations in the dry mesosphere. Furthermore, rising carbon dioxide concentrations tend to cool the mesosphere, even as they warm Earth's surface. The temperature drop will prompt noctilucent clouds to extend farther from the poles, according to the team. Although beautiful, the clouds provide a reminder that humans are changing the natural state of the atmosphere, Thomas says.