Comment on “Are noctilucent clouds truly a “Miner's Canary” for Global Change?”



A recent Eos article [von Zahn, 2003] has challenged the notion that Noctilucent Clouds may be a “miner's canary” of global change [Thomas, 1996]. We first note our terminology: we use the generic term Mesospheric Clouds (MC) to denote this phenomenon encompassing both the terms Polar Mesospheric Clouds (PMC) and Noctilucent Clouds (NLC). In this article, we address his specific criticisms on a point-by-point basis. We critically address his assertion that available data sets derived from satellite measurements of cloud radiance are too short for assessing long-term trends. We argue that his inference of large and irregular natural variability of MC is based on statistically-unreliable data from the older literature. We show from published space-based data that substantial interdecadal trends are present in MC brightness. We point out that MC heights (which have apparently remained constant) are not necessarily sensitive indicators of changes of MC properties. Finally we address the question of attribution, raised by von Zahn. We argue that the space observations are readily explained by well-documented water vapor variability.