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Is the Sky Falling?
Rishbeth, H.  1999.  Chances and changes: The detection of long-term trends in the ionosphere.  EOS: Transactions, American Geophysical Union 80: 590, 593.

What was done
The author reviews what is known about a possible greenhouse-gas-induced change in the vertical location of portions of the upper atmosphere.  Based on model predictions that suggest that increased greenhouse gas concentrations will cool the middle and upper atmosphere, Rishbeth predicted ten years ago that the accompanying thermal contraction would lower the ionospheric F2 layer by 15 to 20 km for a doubling of CO2 and methane at a height of 60 km, which potential phenomenon has been called the "falling sky effect."

What was learned
In analyzing the many measurement and analytical complexities associated with the task of detecting this "falling of the sky," the author concludes that "two or three more decades must elapse before the reality of long-term secular changes is fully established."  Even then, the issue is further complicated by the problem of attribution, i.e., identifying the cause of the fall, if it is found to have occurred.  In addition to greenhouse gases, other potential candidates the author mentions in this regard are "chemical contamination due to volcanic events and man-made causes (e.g., ozone depletion, spacecraft launchings), long-term changes in solar activity, and secular or cyclical changes in the geomagnetic field."

What it means
In the words of the author, "years of interesting science lie ahead, provided the research is carefully thought out, the data are carefully examined for the unexpected as well as for the expected, and the outcomes are well presented."  In other words, there are no shortcuts to obtaining definitive answers to many important questions related to global climate change.  Hence, we must, as the author notes in his last two words: "Keep going!"

Reviewed 1 February 2000