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EOS, TRANSACTIONS AMERICAN GEOPHYSICAL UNION, VOL. 84, NO. 28, doi:10.1029/2003EO280001, 2003

Are Noctilucent Clouds Truly a “Miner's Canary” for Global Change?

Ulf von Zahn

Liebniz-Institute of Atmospheric Physics, kühlungsborn, Germany

Abstract

Noctilucent clouds (NLC) occur close to 83 km altitude during summer at polar, high, and mid-latitudes. They are frequently visible to Earth-bound observers, provided the observers are on the night side of Earth and the clouds are still illuminated by the Sun. Under these conditions, NLC can become a quite impressive sight. NLC owe their existence to the extremely low temperatures (well below 150 K) which prevail during summer over a wide latitude band in the 82 to 90 km altitude region. For a major review of NLC science, the reader is referred to Gadsden and Schröder [1989]. Because of their likely sensitivity to the ambient water vapor mixing ratio and temperature, NLC could be expected to react with particular sensitivity to long-term changes in composition and thermal structure of our atmosphere as caused by human activities. Indeed, such speculations have been voiced earlier. In particular, Thomas et al. [1989] suggested that the NLC brightness has notably increased in the past century due to an increase of water vapor at mesopause altitudes. This increase is supposedly caused by oxidation of methane in the middle atmosphere, the enhanced levels of which are due to human activities. Subsequently, Thomas [1996] developed this picture by numerical modeling of the effects that increased levels of CH₄ and CO₂ will have on the latitude range of NLC. Thomas summarizes his results as follows (quote): “I have described a mesospheric climate change that has been underway for over a century (hence my term the “miner's canary” of global change).” Thomas and Olivero [2001] recently discussed their views on the use of NLCs as possible indicators of global change in the mesosphere.

Index Terms: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry.

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Citation: von Zahn, U. (2003), Are Noctilucent Clouds Truly a “Miner's Canary” for Global Change?, Eos Trans. AGU, 84(28), doi:10.1029/2003EO280001.

Copyright 2003 by the American Geophysical Union.