Europe South Asia Asia Pacific Americas Middle East Africa BBC Homepage World Service Education



Front Page

World

UK

UK Politics

Business

Sci/Tech

Health

Education

Sport

Entertainment

Talking Point

In Depth

On Air

Archive
Feedback
Low Graphics
Help

Thursday, September 17, 1998 Published at 17:38 GMT 18:38 UK


Sci/Tech

Beware, falling sky

The Earth's upper atmosphere: getting cooler

The height of the sky has dropped by 8km in the last 38 years, according to scientists from the British Antarctic Survey.

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.

Dr Jarvis said: "Measurements taken by our European colleagues suggested there may be a drop in altitude, but these results from Antarctica confirm that it is indeed a global effect."



Advanced options | Search tips




Back to top | BBC News Home | BBC Homepage | ©


Sci/Tech Contents


Relevant Stories

01 Dec 97 | Global warming
Our changing world

23 Apr 98 | Sci/Tech
World is getting warmer

30 Nov 97 | Global warming
Life in the greenhouse

01 Aug 98 | Americas
Trading trees to save the planet





Internet Links


The British Antarctic Survey

Journal of Geophysical Research


The BBC is not responsible for the content of external internet sites.




In this section

World's smallest transistor

Scientists join forces to study Arctic ozone

Mathematicians crack big puzzle

From Business
The growing threat of internet fraud

Who watches the pilots?

From Health
Cold 'cure' comes one step closer