The Effect of UV Irradiance Variations on the Earth's Atmosphere and Climate

21 July 1999

Joanna Haigh and Alice Larkin of Imperial College, London, will today (See Notes to Editors 1) announce at the 1999 IUGG(2) meeting in Birmingham, the results of modelling experiments which show how solar-induced changes in the stratosphere can affect a number of meteorological phenomena.

At their talk entitled "The Effect of UV Irradiance Variations on the Earth's Atmosphere and Climate" Haigh and Larkin will describe research showing that variations in solar irradiance and ozone levels in the stratosphere may lead to small but significant changes in global weather patterns. These include, at the height of solar activity, bands of warming and cooling in mid-latitudes and poleward movements of mid-latitude storm tracks.

For centuries measurements of solar activity (e.g. variations in sunspot numbers) have been observed to correlate with weather and climate anomalies. The level of water in lakes, local temperatures and precipitation rates are some of the phenomena which have been linked to the 11-year solar cycle(3) of sunspot activity. Previously Dr Haigh has suggested that links between the stratosphere(4) and the lower atmosphere might provide a plausible mechanism to explain some of these correlations. Now work by PhD student Alice Larkin, using a computer model(5) of the atmosphere up to 65km, has confirmed these ideas.

Dr Haigh will describe the mechanism which suggests that a 0.1 per cent variation in the solar "constant" - the rate at which solar radiation is received outside the Earth's atmosphere - can cause a two per cent change in ozone concentration. It is the level of ozone in the stratosphere which modifies the radiative flux entering the lower atmosphere (known as "radiative forcing") and hence causes changes in the patterns of the weather.

Because the results depend critically on stratospheric ozone, as well as on changes in solar ultraviolet radiation, Dr Haigh adds that there is a need for accurate long-term measurements of both ozone levels in the atmosphere and the spectrum of solar radiation.

For further information please contact:

Dr Joanna Haigh, Department of Physics
Blackett Laboratory, Imperial College
London SW7 2BZ, UK
Telephone: +44 171 594 7671
Fax: +44 171 594 7900
Email: j.haigh@imperial.ac.uk

Notes to editors:

1. The talk will take place on Wednesday 21 July at 2.30pm at the University of Birmingham in the School of Education G33 LT, during the symposium on Solar Variability and Climate (code on the programme is JSA16/W/03-A3).

2. The International Union of Geodesy and Geophysics (IUGG) exists to promote and coordinate studies of the Earth and its environment in space. IUGG General Assemblies take place every four years and are attended by thousands of geo-scientists from around the world.

3. The solar cycle is a well-observed variation in solar activity (seen in sunspots, magnetic fields, solar flares, solar particle emissions, as well as in the emission of electromagnetic radiation) which occurs with periods between nine and 13 years.

4. The stratosphere exists between 15 and 50km above the Earth and contains the ozone layer.

5. The UK Universities' Global Atmospheric Modelling Programme (UGAMP), funded by the UK Natural Environment Research Council, is a community research programme bringing together university centres of atmospheric science with the aim of addressing high priority issues in climate research.

6. Imperial College of Science, Technology and Medicine has the largest income (£309 (£309 (£309m) and the largest research income (£117m) of any university institution in the UK. It has 8823 full time students, 30 per cent of whom are postgraduate and 35 per cent non-British nationalities. Imperial College was rated second overall in both The Times Good University Guide (23 April 1999) and the Financial Times Guide to Britain's top 100 universities (1 April 1999).