The wispy, weaving trails of nigh-shining clouds – those high enough in the atmosphere that they can still reflect the sun’s rays even after sunset – are shining brighter in recent decades. Usually a polar event, these clouds are also appearing closer to the equator – dumbfounding atmospheric scientists.
The clouds first caused a scene back in 1885, when both Thomas William Blackhouse and Robert Leslie independently identified the noctilucent (night-shining) clouds as a special atmospheric phenomenon. Appearing 50 miles above the ground in the mesosphere – the clouds are high enough for the sun’s rays to illuminate them from beyond the visible horizon.
Scientists don’t know for sure why they form; what their link is to the solar cycle, a period of 11 years in which solar activity rises and falls; or, most importantly, why they seem to have been getting brighter and more frequent in recent decades, especially in lower latitudes.
Where historically the glowing dusk vapors were only seen from the ground between 50 and 65 degrees latitude, giving them the name Polar Mesospheric Clouds (PMCs), in the last 10 to 15 years ground observations have spotted the clouds from as far as 40 degrees latitude.
Atmosphere scientist Matthew DeLand, of Science Systems and Applications, Inc., and NASA’s Goddard Space Flight Center, says that “seeing the clouds at lower latitudes leads to the question of whether the atmosphere is getting colder at those latitudes.”
The noctilucent clouds start forming around minus 200 degrees Fahrenheit when water vapor condenses onto particles of cosmic dust floating in the upper atmosphere. Covered in ice crystals, the dust, remnants of burned up meteorites, catch the sun and glow bright again. The mesosphere is cooler when the lower atmosphere is at its warmest. The change in temperature causes a pressure differentiation that sends winds rapidly into the upper atmosphere.
“The interesting part of our work has been the question of whether the changes in the clouds that we see are an example of what global warming would be doing,” DeLand
The night clouds only form in the summer, when the mesosphere is at its coldest. But global warming causes changes in atmospheric weather that block the necessary upward motion of air that cools the mesosphere and can delay the clouds’ seasonal formation.
Based on data collected by NASA’s Aeronomy of Ice in the Mesophere, or AIM, satellite mission launched in 2007, it seems that the direct effect of global warming on the clouds ends there.
“We’ve been thinking for years that the lower mesosphere has cooled because of carbon dioxide, which radiated energy into space, and that the clouds themselves would respond to this, but it appears now that the cloud region doesn’t seem to respond much to the carbon dioxide itself,” said Gary Thomas, a co-investigator for AIM.
With global warming-caused carbon dioxide looking less likely as the culprit behind the clouds’ long-term behavior changes, Thomas is turning his attention to increased water vapor in the upper atmosphere, linked to a rise in methane in the lower atmosphere. Solar radiation in the upper atmosphere oxidizes methane and breaks it down into two water molecules.
But he is not yet declaring a guilty verdict. Some satellite data from AIMS ties the phenomena to fluctuations in the ozone hole. “What’s causing the long term changes,” asks Thomas. “Is it temperature, is it water, is it a combination, or is it none of the above?”
In a surprise development, it seems that the Earth’s atmospheric wind cells, which move in opposite directions on either side of the equator are so strongly linked that warming events in the north are reflected in day-to-day cloud variability in the south. Whether the solar cycle has a role in this is under investigation but the amount of data needed to help resolve the answer is trickling in slowly.
“Over the last 27 years we’ve seen an increase in frequency and brightness,” says James Russell III, the principal investigator for the AIM mission. But “there’s no known natural phenomenon that we can conceive of and no one phenomenon has ever been proposed that says a natural thing will cause this to happen” he says. Leaving night-shining clouds as much a mystery now as they were 126 years ago.
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