One of the benefits of the flap over those leaked East Anglia e-mails is that there's been a noticeable uptick lately in good, clear writing on climate-science basics. Case in point: Steve Connor has a fascinating piece in The Independent revisiting the theory, still popular among skeptics, that solar activity is responsible for twentieth-century global warming. Connor takes us through the history of where this theory came from, how it got picked up and championed by one of Britain's most prominent science journalists, and why, ultimately, it fell out of favor. Here's the basic idea:
The theory is not that the intensity of the Sun has simply increased. Scientists are confident from 31 years of accurate, direct measurements of total solar radiation by satellites that there has been no overall increase in the amount of sunlight coming to Earth. Total solar irradiance, as it is called, has stayed remarkably constant and so cannot be held responsible for the warming of the past half century.
No, the theory of Friis-Christensen and Svensmark revolves around a far more subtle argument connected to the well-established 11-year cycle of sunspots that appear on the surface of the Sun. Sunspots are dark pools of magnetic activity that well up to the solar surface in periodic peaks of 11 years or so. When there are a lot of sunspots, the Sun is said to be more active.
From there, the theory gets even more elaborate, appealing to cosmic rays and cloud formation. But here's why it never caught on:
But other scientists working in the field have told The Independent that Laut's critique is correct and that the original papers [on the sunspot theory] published in 1991 and 1998 are seriously flawed. Six leading experts, including one Nobel laureate, agreed with Laut's analysis that the graphs of Friis-Christensen and Svensmark showing apparent correlations between global warming, sunspots and cosmic rays are deeply flawed.
Friis-Christensen now accepts that any correlation between sunspots and global warming that he may have identified in the 1991 study has since broken down. There is, he said, a clear "divergence" between the sunspots and global temperatures after 1986, which shows that the present warming period cannot be explained by solar activity alone.
Professor Jon Egill Kristjansson, a leading geoscientist at the University of Oslo, said that the divergence between global warming and solar cycles in the late 20th century "is now undisputed". He also points out that if Svensmark is right, there should have been a decrease in cosmic rays, but in fact over the past 50 years they have, if anything, slightly increased—despite statements to the contrary in the Cern proposal of 2000.
"Following Svensmark's mechanism, it seems that any cosmic ray explanation of current global warming can be ruled out," Egill Kristjansson said.
Svensmark's still at work hatching new cosmic-ray experiments, but his theory has mostly fallen out of favor—not because of any conspiracy to squelch dissenters, but because his predictions didn't pan out and the evidence wasn't on his side.