The researchers made the discovery when they analysed records of a 40- year-old experiment to bounce radio waves off the ionosphere, the electrically charged part of the upper atmosphere. The radio waves have been taking less time to return than they did in 1958. This could only be explained by a fall in the ionosphere's height.
The most likely explanation is global warming. This, paradoxically, causes the upper atmosphere to cool, resulting in a drop in barometric pressure and a fall in the uppermost region of the sky.
``This is not in itself harmful,'' said Martin Jarvis, a BAS scientist who helped with the study. ``It is, however, another warning signal about what changes to the atmosphere can be caused by human impact.''
The average height of the ionosphere is 185 miles, but this can vary from one part of the Earth to another, and from one time to another.
However, the BAS measurements were taken in the Antarctic, which is relatively immune to these fluctuations.
The researchers are publishing their findings in the Journal of Geophysical Letters.
Dr Jarvis said: ``The upper atmosphere is a very sensitive indicator of global change. A rise in temperature of two or three degrees at the ground would correspond to a temperature drop of about 50C higher up."
The sky will not fall indefinitely, he said. ``It will fall less and less for the same increase in greenhouses gases, so there will come a point when it reaches an equilibrium.''Reuse content