Very high altitude ice clouds are increasing in polar regions and space traffic may be the cause.
It’s all happening in the mesosphere, a part of the atmosphere about 50 to 100 km (31 to 62 miles) up, which is too high for weather balloons to reach and still not quite in space -- making it a difficult place to study.
A team of researchers looking for an expected decrease in the number of clouds in this layer, as solar activity and heating have ramped up, were instead surprised to find an increase in the number and brightness of clouds in this near-outer-space region over the last two years.
“Polar mesospheric clouds now seem more pervasive on a broader scale than we expected,” said David Siskind of the Space Science Division of Naval Research Laboratory. He is the lead author on a paper reporting the matter in the journal Geophysical Research Letters.
Siskind and his colleagues used data from NASA’s Aeronomy of Ice in the Mesosphere satellite to study the brightness and number of polar mesospheric clouds.
The source of the water to make the clouds is a puzzle, Siskind explained, because there is not much sign of it coming up into the mesosphere. On the other hand, rockets and, until recently, shuttles roaming in space could rain water exhaust down into the mesosphere.
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“We’re still working on how that all works,” Siskind said. He’s hoping that the end of the Space Shuttle program and shifts in rocket uses will continue to leave a signature on the changes in polar mesospheric clouds that could make the case one way or the other. And once the space traffic signal is sorted out, he expects it will be that much easier to see what sorts of changes are happening at that height that are being caused by climate change.
“That’s the holy grail: climate change,” Siskind said of the research on the mesosphere. Otherwise, the clouds themselves are not particularly important, since they do not necessarily create any special hazards. They could, however, be indicators of changes in the mesosphere related to rising carbon dioxide levels. More CO2 is expected to cool the mesosphere -- just the opposite of the effect in the lower troposphere, where weather happens and we all live.
Other researchers working on the mesosphere were equally surprised by the discovery of the clouds bucking the expected trend, and agree that a top suspect is rocket exhaust.
The increase in clouds "was very surprising," said Hanli Liu of the National Center for Atmospheric Research. "You do expect a decrease in the clouds." But this is what makes it such an interesting place to study, he added. “It’s special because the mesosphere is a crossover region between the atmosphere and space.” And so it is sensitive to changes in both.