According to a research paper published in the Journal of Geophysical Research, the distance it has fallen could double in the next 100 years.

The discovery suggests that while the Earth's lower atmosphere is warming, its upper atmosphere, or thermosphere, is cooling.

This causes the thermosphere to shrink, bringing it closer the surface.

Scientist Dr Martin Jarvis said that there is no cause for alarm.

"The 8km drop in altitude is not, in itself, harmful to people.

"It is, however, another warning signal about what changes to the atmosphere can be caused by human impact," he said.

Greenhouse gases blamed

Greenhouse gases like carbon dioxide are believed to be responsible for creating the effect.

The temperature changes experienced at ground level are relatively tiny compared to the massive fluctuations - more than 100 times greater - in the thermosphere, which is found at an altitude of 300km.

Carbon dioxide is a very efficient radiator of the heat it absorbs from the sun, and at ground level this contributes to warming, because the heat remains trapped close to the Earth's surface.

However, in the thermosphere heat rapidly escapes into space, so carbon dioxide has a cooling effect. An increase in the levels of carbon dioxide at that altitude causes the thermosphere to cool and it shrinks.

Scientists have measured the fall using the ionosphere - an atmospheric layer which behaves like "the Earth's high altitude barometer".

Fall to double

The ionosphere is a relatively poorly understood layer within the thermosphere, but it is known that it reflects radio waves, allowing scientists to judge its height and therefore the height of the thermosphere.

Researchers from the BAS and the Oxfordshire-based Rutherford Appleton Laboratory have collected more than 600,000 records of these radio "echo-sounding signatures" from the ionosphere since 1958, allowing them to chart the thermosphere as it shrinks.

Scientists working on the project predict that if the levels of greenhouse gases like carbon dioxide and methane continue to increase at the expected rate, the temperature of the thermosphere could drop by more than 50ºC, causing a drop double that seen in the last 38 years.