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Are Noctilucent Clouds connected with Climate Change?


NLCs in Finland.  (c) Pekka Parviainen
NASA reported yesterday that their AIM Mission continues to add to our understanding of Noctilucent Clouds (NLC).

"The 2013 season is remarkable because it started in the northern hemisphere a week earlier than any other season that AIM has observed," reports Cora Randall of the Laboratory for Atmospheric and Space Physics at the University of Colorado. "This is quite possibly earlier than ever before."

The ScienceCast video below was posted in the STEM on Google+ Community, where the focus was aimed toward work that connects NLCs to climate change.


NLCs are cloud-like phenomena that are located near the edge of polar mesospheric clouds in the upper
atmosphere. They are made of ice crystals and are typically observed in the summer months at latitudes between 50° and 70° north and south of the equator when the Sun is below the horizon.

Their occurrence can be used as a sensitive guide to changes in the upper atmosphere.

Wikipedia pulls some of the connections together:
There is evidence that the relatively recent appearance of noctilucent clouds, and their gradual increase, may be linked to climate change.[1] The author of this study, atmospheric scientist Gary Thomas of the Laboratory for Atmospheric and Space Physics at the University of Colorado has pointed out[2] that the first sightings coincide with both Krakatoa and the nascent Industrial Revolution, and they have become more widespread and frequent throughout the twentieth century, including an uptick between 1964 and 1986. The connecting of global warming and noctilucent clouds however, remains controversial.[2]

Climate models predict that increased greenhouse gas emissions cause a cooling of the mesosphere, which would lead to more frequent and widespread occurrences of noctilucent clouds.[3] A competing theory is that larger methane emissions from intensive farming activities produce more water vapour in the upper atmosphere.[4] Methane concentrations have more than doubled in the past 100 years.[5]

One possible weakness is the timing coincidence of the first sightings.  Given the limited time of day, typical geographic location, and overall appearance of NLCs being so similar to aurora, they may have been lumped together with that phenomenon.  See photos [A, B] of recent low latitude aurora, which is also a fairly rare event.  Oddly, NLCs are anti-correlated with the peak of the solar cycle which is going on right now, and is the direct cause of the conditions that spark the aurora.

It's also curious to me that the word "competing" is used between cooling of the mesosphere and more water vapor in the upper atmosphere, as both are necessary characteristics of NLCs.
 
Relating to the last post here is the question: Is it better to push the envelope when it comes to possible connections to climate change in order to increase awareness, or is it too risky given that many people won't have adequate knowledge of varied and lessor known phenomena, and how they would relate?  The risk being that either people will dismiss these connections out of hand, or if ultimately disproved, would become fodder for denialism.


[1] Thomas, GE; Olivero, J (2001). "Noctilucent clouds as possible indicators of global change in the mesosphere". Advances in Space Research 28 (7): 939–946. 
[2] Phillips, Tony (August 25, 2008). "Strange Clouds at the Edge of Space". NASA.
[3] A. Klekociuk, R. Morris, J. French (2008). "First Antarctic ground-satellite view of ice aerosol clouds at the edge of space". Australian Antarctic Division. Retrieved 2008-10-19.
[4] "Noctilucent clouds". Australian Antarctic Division.
[5] Hsu, Jeremy (2008-09-03). "Strange clouds spotted at the edge of Earth's atmosphere". USAtoday.

 



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