By Richard Black
Environment correspondent, BBC News website
In February 2007, depending on what newspaper you read, you might have seen an article detailing a "controversial new theory" of global warming.
A web of theory has been spun around the Sun's climate influence
The idea was that variations in cosmic rays penetrating the Earth's atmosphere would change the amount of cloud cover, in turn changing our planet's reflectivity, and so the temperature at its surface.
This, it was said, could be the reason why temperatures have been seen to be varying so much over the Earth's history, and why they are rising now.
The theory was detailed in a book, The Chilling Stars, written by Danish scientist Henrik Svensmark and British science writer Nigel Calder, which appeared on the shelves a week after the Intergovernmental Panel on Climate Change (IPCC) had published its landmark report concluding it was more than 90% likely that humankind's emissions of greenhouse gases were warming the planet.
In truth, the theory was not new; Dr Svensmark's team had proposed it a decade earlier, while the idea of a cosmic ray influence on weather dates back to 1959 and US researcher Edward Ney.
The bigger question is whether it amounts to a theory of global warming at all.
Over the course of the Earth's history, the main factor driving changes in its climate has been that the amount of energy from the Sun varies, either because of wobbles in the Earth's orbit or because the Sun's power output changes.
Most noticeably, it changes with the 11-year solar cycle, first identified in the mid-1800s by astronomers who noticed periodic variations in the number of sunspots.
If it varied enough, it could change the Earth's surface temperature markedly. So is it?
"Across the solar cycle, the Sun's energy output varies only by about 0.1%," says Sami Solanki from the Max-Planck Institute for Solar System Research in Germany.
Cosmic rays appear to influence the formation of clouds
"When you look across much longer timescales, you also see changes only of about 0.1%. So just considering directly variations in energy coming from the Sun, this is not enough to explain the climatic changes we have seen and are seeing now."
This is why scientists have been investigating mechanisms which could amplify the changes in solar output, scaling up the 0.1% variation into an effect that could explain the temperature rise of almost half a degree Celsius that we have seen at the Earth's surface in just the last few decades.
One is Joanna Haigh from Imperial College, London, UK. She realised that although the Sun's overall energy output changes by 0.1%, it changes much more in the ultraviolet part of the spectrum.
"The changes in the UV are much larger, between 1% and 10%," she says.
"And that primarily has an impact in the stratosphere (the upper atmosphere) - UV is absorbed by ozone in the stratosphere and also produces ozone, and this warms the air."
Using computer models of climate, Dr Haigh's team showed that warming in the stratosphere could change the way energy is distributed across the troposphere, the lower atmosphere, changing wind and weather patterns. But not by much.
FEELING THE HEAT
Three theories on how the Sun could be causing climate change
"We found it might raise temperatures by a maximum of half to one Celsius in certain regions," she says. "But in terms of an impact on the global average temperature, it's small, maybe about 0.2C."
Which is not enough to explain the warming that has occurred since the late 1970s.
Henrik Svensmark and his collaborators at the Danish National Space Center (DNSC) believe the missing link between small solar variations and large temperature changes on Earth are cosmic rays.
"I think the Sun is the major driver of climate change," he says, "and the reason I'm saying that is that if you look at historical temperature data and then solar activity and cosmic ray activity, it actually fits very beautifully.
"If CO2 is a very important climate driver then you would expect to see its effect on all timescales; and for example when you look at the last 500 million years, or the last 10,000 years, the correlation between changes in CO2 and climate are very poor."
When hugely energetic galactic cosmic rays - actually particles - crash into the top of the atmosphere, they set in train a sequence of events which leads to the production of ions in the lower atmosphere.
The theory is that this encourages the growth of tiny aerosol particles around which water vapour can condense, eventually aiding the formation of clouds.
And the link to the Sun? It is because cosmic rays are partially deflected by the solar wind, the stream of charged particles rushing away from the Sun, and the magnetic field it carries. A weaker solar wind means more cosmic rays penetrating the atmosphere, hence more clouds and a cooler Earth.
The theory makes some intuitive sense because over the last century the Sun has been unusually active - which means fewer cosmic rays, and a warmer climate on Earth.
"We reconstructed solar activity going back 11,000 years," relates Sami Solanki.
"And across this period, the level of activity we are seeing now is very high - we coined the term 'grand maximum' to describe it. We still have the 11-year modulation on top of the long-term trend, but on average the Sun has been brighter and the cosmic ray flux lower."
The Sky experiment showed ions could influence aerosol formation
There is evidence too that cosmic rays and climate have been intertwined over timescales of millennia in the Earth's past.
And the theory received some experimental backing when in October 2006, Henrik Svensmark's team published laboratory research showing that as the concentration of negative ions rose in air, so did the concentration of particles which could eventually become condensation nuclei.
Other scientists, meanwhile, had started putting the idea to the test in the real world.
Seeing the light
In 1947, British meteorologists began deploying instruments in various sites across the country to measure sunlight.
Whether through foresight or luck, they included one feature which was to prove very useful; the capacity to measure the relative amounts of direct and diffuse light.
It is the difference between a sunny day, when light streams directly from above, and a cloudy day, when it seems to struggle in from everywhere, and photographers give up and go home.
Giles Harrison from Reading University realised that the UK Met Office's record of hourly readings from its sunlight stations could be used to plot the extent of cloud cover over a period going back more than 50 years; the larger the ratio of diffuse to direct light, the cloudier the skies.
By chance, cosmic rays have been recorded continuously over almost exactly the same period. So Dr Harrison's team compared the two records, looking for a correlation between more intense cosmic rays and more clouds.
"We concluded that there is an effect, but that it is small - 'small but significant' was how we described it," he recalls.
"It varied UK cloud cover only by about 2%, although we suggested it would have a larger effect on centennial timescales; and it's difficult to assess what effect this would have on global surface temperature."
He concludes it would be premature to lay global warming at the door of cosmic rays. Perhaps surprisingly, you will find no references to his work in The Chilling Stars.
In July, Mike Lockwood from the UK's Rutherford-Appleton Laboratory attempted a definitive answer to the question with what appeared to be a simple method. He simply looked at the changing cosmic ray activity over the last 30 years, and asked whether it could explain the rising temperatures.
His conclusion was that it could not. Since about 1985, he found, the cosmic ray count had been increasing, which should have led to a temperature fall if the theory is correct - instead, the Earth has been warming.
"This should settle the debate," he told me at the time.
It has not. Last month Dr Svensmark posted a paper on the DNSC website that claimed to be a comprehensive rebuttal.
"The argument that Mike Lockwood put forward was that they didn't see any solar signal in the surface temperature data," he says.
"And when you look at [temperatures in] the troposphere or the oceans, then you do see a solar signal, it's very clear."
Dr Lockwood disagrees; he says he has re-analysed the issue using atmospheric temperatures, and his previous conclusion stands. And he thinks the Svensmark team has been guilty of poor practice by not publishing their argument in a peer-reviewed scientific journal.
"Lots of people have been asking me how I respond to it; but how should I respond to something which is just posted on a research institute's website?" he asks.
"This isn't on, because the report title says it is a 'comprehensive rebuttal'; if it were that, then it would be his duty to publish it in a scientific journal and clean up the literature - that's how science filters out what is incorrect, and how it comes to a consensus view as to what is correct."
Droplets of doubt
This dispute presumably has some distance to run.
But Mike Lockwood's larger conclusion that current warming has nothing to do with solar changes is backed up by others - notably the IPCC, which concluded earlier this year that since temperatures began rising rapidly in the 1970s, the contribution of humankind's greenhouse gas emissions has outweighed that of the Sun by a factor of about 13 to one.
Even though misguided journalists have sometimes mistaken his work as implying a solar cause to modern-day warming, Sami Solanki agrees with the IPCC verdict.
"Since 1970, the cosmic ray flux has not changed markedly while the global temperature has shown a rapid rise," he says. "And that lack of correlation is proof that the Sun doesn't cause the warming we are seeing now."
Even to prove that the link between cosmic rays and cloud cover matters in the real world needs a lot more work, observes Joanna Haigh.
"You need to demonstrate a whole long chain of events - that the atmosphere is ionised, then that the ionised particles act to nucleate the condensation of water vapour, then that you form droplets, and then that you get clouds; and you have to show it's important in comparison to other sources of nucleation.
A weakening Sun could soon see the issue die down
"And that hasn't been demonstrated. Proponents of this mechanism have tended to extrapolate their results beyond what is reasonable from the evidence."
And Giles Harrison believes climate sceptics need to apply the same scepticism to the cosmic ray theory as they do to greenhouse warming - particularly those who say there are too many holes in our understanding of how clouds behave in the man-made greenhouse.
"There is some double-speak going on, as uncertainties apply to many aspects of clouds," he says.
"If clouds have to be understood better to understand greenhouse warming, then, as we have only an emerging understanding of the electrical aspects of aerosols and non-thunderstorm clouds, that is probably also true of any effect of cosmic rays on clouds."
Dr Svensmark agrees it would be wrong for anyone to claim the case has been proved.
"If anyone said that there is proof that the Sun or greenhouse gases alone are responsible for the present-day warming, then that would be a wrong statement because we don't really have proofs as such in the natural sciences," he says.
Two events loom on the horizon that might settle the issue once and for all; one shaped by human hands, one entirely natural.
At Cern, the giant European physics facility, an experiment called Cloud is being constructed which will research the notion that cosmic rays can stimulate the formation of droplets and clouds. There may be some results within three or four years.
By then, observations suggest that the Sun's output may have started to wane from its "grand maximum".
If it does, and if Henrik Svensmark is right, we should then see cosmic rays increase and global temperatures start to fall; if that happens, he can expect to see a Nobel Prize and thousands of red-faced former IPCC members queuing up to hand back the one they have just received.
But if the Sun wanes and temperatures on our planet continue to rise, as the vast majority of scientists in the field believe, the solar-cosmic ray concept of global warming can be laid to eternal rest.
And if humankind has done nothing to stem the rise in greenhouse gas emissions by then, it will be even harder to begin the task.