An ultraviolet image of the Sun shows a solar flare leaping out from an active sunspot. Photograph: Nasa/EPA
It has been one of the central claims of those who challenge the idea that human activities are to blame for global warming. The planet's climate has long fluctuated, say the climate sceptics, and current warming is just part of that natural cycle - the result of variation in the sun's output and not carbon dioxide emissions.
But a new analysis of data on the sun's output in the last 25 years of the 20th century has firmly put the notion to rest. The data shows that even though the sun's activity has been decreasing since 1985, global temperatures have continued to rise at an accelerating rate.
The solar hypothesis was championed publicly in March by the controversial Channel 4 documentary The Great Global Warming Swindle.
The programme has been heavily criticised for distorting scientific data to fit the sceptic argument and Carl Wunsch, a professor of physical oceanography at MIT who featured in the programme, later said that he was "totally misled" by the film makers and that his comments were "completely misrepresented".
The new analysis is designed to counter the main alternative scientific argument put forward by the programme - that solar activity may be to blame for global warming.
"The temperature record is simply not consistent with any of the solar forcings that people are talking about," said lead author Mike Lockwood at the Rutherford Appleton Laboratory in Oxfordshire.
"They changed direction in 1985, the climate did not ... [the temperature] increase should be slowing down but in fact it is speeding up."
Global temperatures are going up by 0.2 degrees per decade and the top 10 warmest years on record have happened in the past 12 years.
One way that the sun affects the climate is through clouds. The sun's magnetic field shields the Earth from its high energy particles called cosmic rays. The rays help form clouds that reflect the sun's energy back into space and cool the planet.
So if the sun's magnetic field is high, there should be a fall-off in cosmic rays, fewer clouds and more warming. But Prof Lockwood's data, published today in the Proceedings of the Royal Society, shows the sun's magnetic field has declined since 1985, even as the world heats up.
James Hansen, a Nasa climate scientist who was once gagged by the Bush administration for speaking out on global warming, said the issue of whether the sun's activity is causing global warming had been dispensed with by most scientists long ago. "The reason [this paper] has value is that the proponents of the notion that the sun determines everything come up with various half-baked suggestions that the sun can somehow cause an indirect forcing that is not included in the measurements of radiation coming from the sun," he said. "These half-baked notions are usually supported by empirical correlations of climate with some solar index in the past. Thus, by showing that these correlations are not consistent with recent climate change, the half-baked notions can be dispensed with."
Prof Lockwood said the study was "another nail" in the coffin of the notion that solar activity is responsible for global warming.
Nir Shaviv, an astrophysicist at the Hebrew University of Jerusalem and a proponent of the solar hypothesis, has tried to rescue the idea by invoking a time lag between changes in the sun and its effect on the Earth's climate. But Prof Lockwood dismissed this as "disingenuous".
"Nobody has invoked that kind of lag before. It's only been invoked now as a way out," he said. Even if the lag were 50 years then he believes we would begin to see the rise in global temperatures slowing down.
Even though there is almost no argument among scientific circles about the role of human activities as the main driver of climate change, a recent poll suggested that the public still believes there is significant scientific uncertainty. Despite the efforts of government and campaigns such as Live Earth to educate the public, the Ipsos Mori poll of over 2,031 people, released this month, found 56% of people thought there was an active scientific debate into the causes of global warming.
A spokesman for the Royal Society, the UK's leading scientific academy, said: "This is an important contribution to the scientific debate on climate change. At present there is a small minority which is seeking to deliberately 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. We have reached a point where a failure to take action to reduce carbon dioxide and other greenhouse gas emissions would be irresponsible and dangerous."