By Richard Black
Environment correspondent, BBC News website
Cloud cover affects temperature - but what determines cloud cover?
Scientists have produced further compelling evidence showing that modern-day climate change is not caused by changes in the Sun's activity.
The research contradicts a favoured theory of climate "sceptics", that changes in cosmic rays coming to Earth determine cloudiness and temperature.
The idea is that variations in solar activity affect cosmic ray intensity.
But UK scientists found there has been no significant link between cosmic rays and cloudiness in the last 20 years.
Presenting their findings in the Institute of Physics journal, Environmental Research Letters, the University of Lancaster team explain that they used three different ways to search for a correlation, and found virtually none.
This is the latest piece of evidence which at the very least puts the cosmic ray theory, developed by Danish scientist Henrik Svensmark at the Danish National Space Center (DNSC), under very heavy pressure.
Dr Svensmark's idea formed a centrepiece of the controversial documentary The Great Global Warming Swindle.
"We started on this game because of Svensmark's work," said Terry Sloan from Lancaster University.
"If he is right, then we are going down the wrong path of taking all these expensive measures to cut carbon emissions; if he is right, we could carry on with carbon emissions as normal."
Cosmic rays are deflected away from Earth by our planet's magnetic field, and by the solar wind - streams of electrically charged particles coming from the Sun.
The Svensmark hypothesis is that when the solar wind is weak, more cosmic rays penetrate to Earth.
That creates more charged particles in the atmosphere, which in turn induces more clouds to form, cooling the climate.
The planet warms up when the Sun's output is strong.
Professor Sloan's team investigated the link by looking for periods in time and for places on the Earth which had documented weak or strong cosmic ray arrivals, and seeing if that affected the cloudiness observed in those locations or at those times.
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"For example; sometimes the Sun 'burps' - it throws out a huge burst of charged particles," he explained to BBC News.
"So we looked to see whether cloud cover increased after one of these bursts of rays from the Sun; we saw nothing."
Over the course of one of the Sun's natural 11-year cycles, there was a weak correlation between cosmic ray intensity and cloud cover - but cosmic ray variability could at the very most explain only a quarter of the changes in cloudiness.
And for the following cycle, no correlation was found.
Dr Svensmark himself was unimpressed by the findings.
"Terry Sloan has simply failed to understand how cosmic rays work on clouds," he told BBC News.
"He predicts much bigger effects than we would do, as between the equator and the poles, and after solar eruptions; then, because he doesn't see those big effects, he says our story is wrong, when in fact we have plenty of evidence to support it."
But another researcher who has worked on the issue, Giles Harrison from Reading University, said the work was important "as it provides an upper limit on the cosmic ray-cloud effect in global satellite cloud data".
Dr Harrison's own research, looking at the UK only, has also suggested that cosmic rays make only a very weak contribution to cloud formation.
The Svensmark hypothesis has also been attacked in recent months by Mike Lockwood from the UK's Rutherford-Appleton Laboratory.
He showed that over the last 20 years, solar activity has been slowly declining, which should have led to a drop in global temperatures if the theory was correct.
The Intergovernmental Panel on Climate Change (IPCC), in its vast assessment of climate science last year, concluded that since temperatures began rising rapidly in the 1970s, the contribution of humankind's greenhouse gas emissions has outweighed that of solar variability by a factor of about 13 to one.
According to Terry Sloan, the message coming from his research is simple.
"We tried to corroborate Svensmark's hypothesis, but we could not; as far as we can see, he has no reason to challenge the IPCC - the IPCC has got it right.
"So we had better carry on trying to cut carbon emissions."