Scientists have delivered the final blow to the theory that recent global warming can be explained by variations in the natural cycles of the Sun - a favourite refuge for climate sceptics who dismiss the influence of greenhouse-gas emissions.
An analysis of the records of all of the Sun's activities over the past few decades - such as sunspot cycles and magnetic fields - shows that since 1985 solar activity has decreased significantly, while global warming has continued to increase.
Mike Lockwood, of the Rutherford Appleton Laboratory in Chilton, Oxfordshire, said: "In 1985, the Sun did a U-turn in every respect. It no longer went in the right direction to contribute to global warming. We think it's almost completely conclusive proof that the Sun does not account for the recent increases in global warming."
The study, published in the journal Proceedings of the Royal Society A, shows there is no doubt that solar activity over the past 20 years has run in the opposite direction to global warming, and therefore cannot explain rises in average global temperatures.
Dr Lockwood and his colleague Claus Fröhlich, of the World Radiation Centre in Davos Dorf, Switzerland, have produced the most powerful counter argument to suggestions that current warming is part of the natural cycle of solar activities. "There is considerable evidence for solar influence on Earth's pre-industrial climate, and the Sun may well have been a factor in post-industrial change in the first half of the last century," they write.
However, since about 1940 there has been no evidence to suggest that increases in global average temperatures were caused by solar activity. "Our results show that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified," the two scientists said.
The theory that past changes in solar activity may have explained some changes in the climate before the industrial revolution is not in dispute. In previous centuries, for instance, notably between about 1420 and 1570, when the Vikings had to abandon their Greenland settlements, solar minima corresponded with unusually cool weather, such as the "little ice age" of the 17th century.
But climate sceptics have exploited this to dispute the idea that man-made emissions are responsible for global warming. In the recent Channel 4 programme The Great Global Warming Swindle, the rise in solar activity over the latter half of the 20th century was erroneously presented as perfectly matching the rise in global average temperatures.
Dr Lockwood said he was outraged when he saw the documentary, because of the way the programme-makers used graphs of temperature rises and sunspot cycles that were cut off in the 1980s, when the two trends went in the opposite direction.
"The trouble is that the theory of solar activity and climate was being misappropriated to apply to modern-day warming. The sceptics were taking perfectly good science and bringing it into disrespect," Dr Lockwood said.
The Royal Society said : "There is a small minority which is seeking to confuse the public on the causes of climate change. They are often misrepresenting the science, when the reality is that the evidence is getting stronger every day."