If one were to reach into the grab bag of global warming skeptics' favorite theories, one might pull out any number of speculation-laden papers and editorials regarding the supposed effect of solar activity on the Earth's climate.
For example, here's an excerpt from an October 2007 presentation given by a member of the Exxon-funded Heartland Institute:
How long will the global warming alarmists be able to sustain the public hysteria without strongly rising temperatures? This will be a key factor in the short-term future of climate warming legislation.
Henrik Svensmark of the Danish Space Research Institute says cosmic rays are the link between the sun’s variability and Earth’s temperatures. More or fewer cosmic rays, depending on the strength of the “solar wind,” seed more or fewer of the low, wet clouds that cool the Earth. Further experiments to document this impact are planned in Europe.
The research to which the presentation refers is described in this paper by Svensmark, which, oddly, does not mention climate change, although the (non-peer-reviewed) press release for his research does:
The experimental results lend strong empirical support to the theory proposed a decade ago by Henrik Svensmark and Eigil Friis-Christensen that cosmic rays influence Earth’s climate through their effect on cloud formation.
'Many climate scientists have considered the linkages from cosmic rays to clouds to climate as unproven,’ comments Eigil Friis-Christensen, who is now Director of the Danish National Space Center. ‘Some said there was no conceivable way in which cosmic rays could influence cloud cover. The [current research] now shows how they do so, and should help to put the cosmic-ray connection firmly onto the agenda of international climate research.
(Click here for the Real Climate discussion of Svensmark's et al.'s claims.)
Unfortunately for the “sunspots and cosmic rays, not humans, cause global warming” crowd, British scientists have just blown their claims out of the water. The BBC News website has the story:
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 favored 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 Lancaster University scientists found there has been no significant link between them in the last 20 years.
Presenting their findings in the Institute of Physics journal, Environmental Research Letters, the UK team explain that they used three different ways to search for a correlation, and found virtually none.
The article points out the obvious:
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.
The Great Global Warming Swindle was essentially a global warming skeptic-laden response to Al Gore's fact-based documentary, An Inconvenient Truth. It came out in May 2007. Its focus on Svensmark's theory is perplexing, given that three years earlier, scientists reported:
A new scientific study concludes that changes in the Sun's output cannot be causing modern-day climate change.
It shows that for the last 20 years, the Sun's output has declined, yet temperatures on Earth have risen.
It also shows that modern temperatures are not determined by the Sun's effect on cosmic rays, as has been claimed.
Writing in the Royal Society's journal Proceedings A, the researchers say cosmic rays may have affected climate in the past, but not the present.
'This should settle the debate,' said Mike Lockwood, from the UK's Rutherford-Appleton Laboratory, who carried out the new analysis together with Claus Froehlich from the World Radiation Center in Switzerland.
In other words, there is repeated evidence from multiple researchers that global warming is caused by human activity. Not by sunspots.
Not by cosmic rays.
What will it take to convince the skeptics?