Mysterious, glowing clouds previously seen almost exclusively in Earth’s polar regions have appeared in the skies over the United States and Europe over the past several days.

Photographers and other sky watchers in Omaha, Paris, Seattle, and other locations have run outside to capture images of what scientists call noctilucent (“night shining”) clouds. Formed by ice literally at the boundary where the earth’s atmosphere meets space 50 miles up, they shine because they are so high that they remain lit by the sun even after our star is below the horizon.

The clouds might be beautiful, but they could portend global changes caused by global warming. Noctilucent clouds are a fundamentally new phenomenon in the temperate mid-latitude sky, and it’s not clear why they’ve migrated down from the poles. Or why, over the last 25 years, more of them are appearing in the polar regions, too, and shining more brightly.

“That’s a real concern and question,” said James Russell, an atmospheric scientist at Hampton University and the principal investigator of an ongoing NASA satellite mission to study the clouds. “Why are they getting more numerous? Why are they getting brighter? Why are they appearing at lower latitudes?”

Nobody knows for sure, but most of the answers seem to point to human-caused global atmospheric change.

Noctilucent clouds were first observed in 1885 by an amateur astronomer. No observations of anything resembling noctilucent clouds before that time has ever been found. There is no lack of observations of other phenomena in the sky, so atmospheric scientists are fairly sure that the phenomenon is recent, although they are not sure why.

Over the last 125 years, scientists have learned how the clouds form. At temperatures around minus 230 degrees Fahrenheit, dust blowing up from below or falling into the atmosphere from space provides a resting spot for water vapor to condense and freeze. Right now, during the northern hemisphere’s summer, the atmosphere is heating up and expanding. At the outside edge of the atmosphere, that actually means that it’s getting colder because it’s pushed farther out into space.

It’s not hard to see how a warming Earth could change those dynamics: as the globe heats up, the top of the atmosphere should get colder.

“The prevailing theory and most plausible explanation is that CO2 buildup, at 50 miles above the surface, would cause the temperature decrease,” Russell said. He cautioned, however, that temperature observations remain inconclusive.

The global changes that appear to be reshaping noctilucent cloud distribution could be much more complex, said Vincent Wickwar, an atmospheric scientist at Utah State University whose team was first to report a mid-latitude noctilucent cloud in 2002. Temperature does not explain their observations from around 42 degrees latitude.

“To get the noctilucent clouds you need temperatures that are about 20 degrees Kelvin colder than what we see on average up there,” Wickwar said. “We may have effects from CO2 or methane but it would only be a degree or a fraction of a degree.”

Instead, Wickwar’s explanation is that a vertical atmospheric wave discovered in their LIDAR data lowered the temperature in the region above their radar installation near Logan, Utah. But then you have to ask, he noted, “Where’d the wave come from?”

They don’t really have an answer yet. Other facilities around the world with similar LIDAR capacity haven’t reported similar waves. And the Rocky Mountains, near Wickwar’s lab, can cause atmospheric waves, which could be a special feature of his location.

Other hypotheses abound to explain the observed changes in the clouds. Human-caused increases in atmospheric methane, which oxidizes into carbon dioxide and water vapor, could be providing more water for ice in the stratosphere. Increases in the amount of cosmic or terrestrial dust in the stratosphere could also increase the number of brightly shining clouds.

Two years into Russell’s NASA project, more questions exist than firm answers. They will have at least three and a half more years, though, to gather good data on upper atmospheric dynamics.

The recent observations of noctilucent clouds at all kinds of latitudes provide an extra impetus to understand what is going on up there. Changes are occurring faster than scientists can understand their causes.

“I suspect, as many of us feel, that it is global change, but I fear we don’t understand it,” Wickwar said. “It’s not as simple as a temperature change.”

Image: 1. The sky over Omaha on July 14th, snapped by Mike Hollingshead at Extreme Instability 2. A rendering of the noctilucent clouds created from data obtained by Russel’s NASA project, AIM. Video: NASA.

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