SAN FRANCISCO -- Shining clouds at the edge of space are growing in number and brightness. For years, scientists have puzzled over the observed increase in these noctilucent clouds.
Now two groups modeling the behavior of the atmosphere have found new support for the idea that human-induced climate change is the cause.
Their models, presented here at the American Geophysical Union meeting, accurately reproduced both the variability induced by the solar cycle and the intensification trend. The intensification is driven by changes in the atmosphere below the clouds, triggered by increasing amounts of greenhouse gases.
"They are showing that it's largely due to change farther down in the atmosphere," said John Plane, an atmospheric chemist at University of Leeds who was not involved with the work. "That's a really good thing because a lot of the rationale for studying these clouds is that you can use them as an early warning of climate change elsewhere in the atmosphere."
Noctilucent clouds are one of the atmosphere's strangest visible phenomena. As the sun goes down, these wispy clouds in the northern latitudes appear to glow as the ice particles in the clouds reflect the sun's rays. They aren't as dramatic as aurorae, but they are a distinct and mysterious phenomenon that's much more common now than it was when our great grandparents bought their first cars and switched on the lights for the first time.
The new work comes from two separate groups of scientists using different models of the atmosphere. One group, led by Daniel Marsh of the National Center for Atmospheric Research used the Whole Atmosphere Community Climate Model, which simulates how the various layers of the Earth's atmosphere respond to changes in chemistry and solar radiation. They were able to model the increasing amounts of ice mass at the level in the atmosphere where noctilucent clouds and a close cousin, polar mesospheric clouds, form.
"My model does reproduce the trend, and its drivers are the increasing methane and increasing CO2," Marsh said.
Marsh's work broadly agrees with that of Franz-Josef Lübken, an atmospheric physicist at the Leibniz Institute for Atmospheric Physics in Kühlungsborn, Germany. Lübken used the Leibniz Institute Middle Atmosphere Model, which looks just at the specific layer of the atmosphere where the glowing clouds appear. Lübken's also reproduced the variability and long-term increase in noctilucent clouds. His model implicitly takes increasing carbon dioxide into account because it's "nudged" by activity in lower layers where carbon dioxide is causing changes in atmospheric chemistry.
In 2003, a scientist at Lübken's own institution questioned whether noctilucent clouds [pdf] were a good indicator of global change. Physicist Gary Thomas, who had been one of the first to float the idea, concluded in a review of noctilucent cloud data that "longer time series and more comprehensive models are needed before the link with global change can be established."
And that's exactly what Marsh and Lübken presented here this week.
"The new thing is that we have this consistency between the models and observations as far as the [upward] trend is concerned, which had not been the case," Lübken said. "The past models hadn't been able to reproduce the trends because the microphysics that goes into them was not good enough."
The increase in noctilucent clouds appears to result from a decrease in temperature at the altitude where the clouds form along with an increase in water vapor. Warming in the in the layers of the atmosphere below the clouds actually causes the cloud layer to cool. The clouds only form under very specific circumstances, so small changes in the atmosphere can lead to large changes in the observed occurrence of the phenomenon.
"A very small temperature decrease, just 0.08 Kelvin per year, together with an increase in water vapor, causes a substantial increase in occurrence rates and brightness," Lübken told scientists here Wednesday.
There are still differences between the two models and details to be worked out. In Lübken's model, the temperature decrease is driven by the cooling of the stratosphere, which has long been a predicted impact of climate change induced by increasing carbon dioxide emissions. The increase in water vapor is a dynamic process caused by increased freeze drying of water particles a couple of miles up from where noctilucent clouds form. The atmospheric dynamics push more humidity to exactly the altitude at which the clouds form.
Marsh's model includes the dynamics of methane, which, as it breaks down in the upper atmosphere, creates more water vapor at higher altitudes. Lübken's does not, but the trend is still apparent, so it's unclear how important methane's role in the increase in noctilucent clouds is.
The clarity and similarity of the pattern coming from these two different models gives the greenhouse-gas hypothesis an edge over other explanations for the increase in noctilucent clouds, like an increase in water vapor driven by Space Shuttle launches.
Up next, Marsh will try to explain one of the most persistent and strange mysteries associated with noctilucent clouds. While historical astronomers and sky watchers reported seeing many of the phenomena we now know and understand, there are isn't a single account of a noctilucent cloud before 1885. It's unclear whether they just weren't there or people just weren't looking hard enough. The first descriptions of the clouds come two years after the eruption of Krakatoa, which caused spectacular sunsets and may have encouraged people to look up more often.
Marsh plans to run his model back in time to see if it can reproduce the appearance of the clouds in the 1880s solely in response to the increase of greenhouse gases in the atmosphere caused by coal-powered industrialization.
*Images: 1) *The sky over Omaha on July 14th./ Mike Hollingshead, Extreme Instability. 2) NASA.
See Also:
- Mysterious, Glowing Clouds Appear Across America's Night Skies
- Weird, Rare Clouds and the Physics Behind Them
- Baffling Patterns Form in Scientific Sandbox
- Dry-Ice Clouds Shade Martian Landscape
- Lie on Mars, Look Up and Watch the Clouds Go By
- Brown Clouds are Increasing High Altitude Melting
- UAVs Search For Scientific Silver Lining in Beijing Pollution
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