Friday, October 06, 2006

Cosmic mind rays

Maybe it's time to put on the tin underpants because, wouldn't you know it, cosmic rays are going to do us all in. Well, in a roundabout way. You see, some denialists have now decided that cosmic rays cause global warming. Andrew Bolt and JF Beck are but two examples.

Here's what's got them all excited:

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. The original theory rested on data showing a strong correlation between variation in the intensity of cosmic radiation penetrating the atmosphere and the amount of low-altitude clouds. Cloud cover increases when the intensity of cosmic rays grows and decreases when the intensity declines.

It is known that low-altitude clouds have an overall cooling effect on the Earth's surface. Hence, variations in cloud cover caused by cosmic rays can change the surface temperature. The existence of such a cosmic connection to Earth's climate might thus help to explain past and present variations in Earth's climate.

Interestingly, during the 20th Century, the Sun's magnetic field which shields Earth from cosmic rays more than doubled, thereby reducing the average influx of cosmic rays. The resulting reduction in cloudiness, especially of low-altitude clouds, may be a significant factor in the global warming Earth has undergone during the last century. However, until now, there has been no experimental evidence of how the causal mechanism linking cosmic rays and cloud formation may work.

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 SKY experiment now shows how they do so, and should help to put the cosmic-ray connection firmly onto the agenda of international climate research.

Now, you'd think so-called skeptics would look at a bit of the background behind this. Not so, of course. Here's what Nature had to say:

This cosmic-ray connection drew a lot of media attention for several years, but never found favour with the mainstream of climate science, which holds that the twentieth century's global warming was caused by people, not particles. To many in the community, the attention paid to Svensmark and Friis-Christensen seemed to be at best a diversion, at worst a counter-attack. The connection with the Sun was played on by organizations with connections to oil companies, such as the right-wing George C. Marshall Institute in Washington DC.

There were also questions about Svensmark's use of data. In a 2004 article published in Eos, Paul Damon of the University of Arizona in Tucson and Peter Laut of the Technical University of Denmark discussed several examples of what they called "unacceptable handling of observational data by Svensmark and Friis-Christensen which exaggerated the correlation."

Among several flaws, including arithmetical errors, they noted that the cloud data that had been used originally did not represent total global cloud cover, and that when the correct data were used the correlation broke down. Svensmark began to use a different measure of cloudiness, justifying this by arguing that the new measure made more sense than the original one as something that the cosmic rays might be influencing.

Maybe the new experiments are great science. Maybe there's no unacceptable handling of observational data. Maybe there's no arithmetical errors. Maybe they won't change the way the data is analysed to prevent their hypothesis breaking down. Maybe their experiments will even be reproducible using different methodologies. Maybe. Or maybe not.

UPDATE: As expected, Gavin Schmidt at RealClimate destroys the "cosmic rays cause global warming" hypothesis.