Two new pieces of research aiming to pinpoint factors which influence global warming have highlighted the role of cosmic rays and vapour trails from aircraft. The research on cosmic rays was published recently by Fangqun Yu, working at the State university of New York-Albany. It claims that the reason why the Earth's surface temperature is changing while low atmosphere temperatures appear to remain the same is that cosmic rays have 'height dependent effects' on the Earth's cloudiness. While high clouds tend to bounce sunlight back into space, low clouds tend to retain the energy at the Earth's surface. And the changes brought about in the formation of these clouds by cosmic rays (which are tiny charged particles which hit all the planets with a frequency dependent on solar winds) could explain why climate changes have taken place and are variable. Yu claims that global warming coincided with fewer cosmic rays hitting Earth during the 20th century. But his theory has not been endorsed by at least one European climate expert. Dr David Viner of the climatic research unit at the UK's University of East Anglia said that while global warming is not solely due to human activity, 'we can explain the temperature discrepancy between the surface and the low atmosphere without recourse to his theory.' A second piece of research was carried out to take advantage of the absence of air traffic following the 11 September attacks on the USA. All US flights were grounded for three days following the attacks, giving researchers the opportunity to study the effect of aeroplane vapour trails on climate. They compared temperatures at 4,000 weather stations across the USA during those three days with temperatures for the same period over the previous three decades. The results showed that there was an extra degree's difference between night and day temperatures when there were no flights. This could be explained by vapour trails reflecting sunlight and retaining heat. But the burning of large amounts of fuel is also thought to play a part.