Just after summer sunsets in northern latitudes, shimmering, wispy clouds appear in the twilight sky. This year, these noctilucent clouds have appeared earlier and farther south than ever before.

Noctilucent clouds exist higher in Earth’s atmosphere than any other cloud type. First observed in 1885 following the eruption of Krakatoa, they were a sight reserved for Earth’s northernmost residents. In recent years, however, their intensity and frequency have increased, often at latitudes previously thought to be too far south for noctilucent clouds to form.

Noctilucent clouds over the North Pole, June 10, 2013 (LASP/Univ. of Colorado)

In 2009, scientists from the National Center for Atmospheric Research pointed to the southern creep of noctilucent clouds as an early warning signal for climate change high in the atmosphere. Now, new data from NASA's cloud-observing AIM satellite supports this possibility.

James Russell, principal investigator for AIM, says increasing methane emissions could be amping up the cloud show. “When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor,” Russell said. “This extra water vapor is then available to grow ice crystals for [noctilucent clouds].”

As polar regions warm in the summer months, water vapor is driven to higher levels of Earth’s atmosphere. There, small dust particles left over from burning meteors, volcanic eruptions, or even rocket launches act as seeds for ice formation. Transparent in broad daylight, these crystalline clouds become visible in twilight hours, reflecting the sun’s rays from below the horizon. They float so high in the atmosphere that they can even glow in the dead of night. Normally such nocturnal cloud sightings peak when the sun is at its 11-year minimum. This year marks exactly the opposite, a predicted solar maximum.

These changing clouds serve as a reminder of the interconnected nature of Earth’s atmosphere, what the scientists refer to as “atmospheric teleconnections.” Cooling in the stratosphere, a consequence of elevated greenhouse gas levels, can alter air circulation patterns across the globe, although exactly how remains a mystery.

“Models will be catching up with AIM for a long time,” said space physicist Thomas Immel of Berkeley’s Space Sciences Laboratory, chief scientist for NASA’s next mission to study fluctuations in the upper atmosphere, the ICON satellite, expected to launch in 2017.