Once-rare cloud is now more common, and new study says climate change is to blame

Noctilucent clouds in Wismar, Germany. They appear blue at higher elevations and become white at lower elevations. The black spots in the center and lower right corner are normal tropospheric clouds that are no longer catching light from the sun. Institute of Atmospheric Physics

A new study bolsters the theory that human-caused climate change is making it easier to see a rare kind of cloud that is known for brightening up the sky just after sunset or just before sunrise, researchers said.

The glimmering noctilucent, or “night-shining,” clouds are the highest clouds in earth’s atmosphere, forming in the middle atmosphere (mesosphere), around 50 miles above earth’s surface.

They form when water vapor freezes and forms ice crystals, researchers said.

Researchers used computer simulations to model the Northern Hemisphere’s atmosphere and noctilucent clouds from 1871 to 2008, according to the study, which was published last week in the journal Geophysical Research Letters.

The study suggested that increased methane emissions — added to the atmosphere by humans extracting and burning fossil fuels — have increased water vapor concentrations in the mesosphere by about 40 percent since the late 1800s.

Franz-Josef Lübken, lead author of the study and an atmospheric scientist at the Leibniz Institute of Atmospheric Physics in Kühlungsborn, Germany, said you can only see noctilucent clouds under certain conditions:

■  It has to be summertime.

■  You have to be at mid- to high latitudes. (Boston, at 42 degrees, is too far to the south except for extremely rare instances, he said. But northern Germany, which is in the upper 50s and where Lübken’s institute is located, is better.)

■  The sun must be several degrees below the horizon (one to two hours after sunset or before sunrise).

Lübken said noctilucent clouds are “somewhat blue-ish,” and they’re not anything that would startle anyone.

“They are probably not so spectacular that everybody would identify them immediately,” Lübken said.

Humans first reported observing the clouds in 1885, two years after Indonesia’s Krakatoa volcano spewed large amounts of water vapor in the air, the study said.

Sightings of the clouds became more common during the 20th century, which made scientists wonder if humans were the culprit.

Gary Thomas, an atmospheric scientist who is a professor emeritus at the University of Colorado, Boulder, was one of the authors of the first study to suggest that noctilucent clouds were on the rise because of human effects on the environment.

“I concur almost totally in the results of the Lübken paper,” Thomas said. “It is inescapable that we are changing the atmosphere. This is just another manifestation of global change, and actually something that non-scientists can appreciate because these clouds are a brilliant and obvious reminder of these changes.”

“I have to be grateful that our ideas have held up,” he said.

Lübken said people can expect noctilucent clouds to become more visible with time, and it’s not clear whether they will ultimately have some effect on the world below.

“Whether thicker, more visible noctilucent clouds could influence earth’s climate themselves is the subject of future research,” Lübken said.