Tom Chivers says he enjoyed his foray into climate a couple of days ago and has returned to the subject with a piece about Svensmark's cosmoclimatology theory.
...it's an interesting piece of research which adds to our understanding of atmospheric behaviour. As always, it's been leapt upon by "sceptics" who think all climate scientists are charlatans until those scientists say something they agree with, whereupon they're modern-day Galileos being placed under house arrest for heresy by the Church of AGW.
So let's look at what the research actually found. It did, as Svensmark predicted, find that cosmic rays could cause aerosol particles to form. But what it didn't find was that this leads to cloud formation; nucleation may lead to cloud formation if the particles are large enough, but it has not, yet, been shown that they are. As the Cern authors say themselves in the paper: "The fraction of these freshly nucleated particles that grow to sufficient sizes to seed cloud droplets… remain open questions experimentally.
The sceptic-bashing aside, this is pretty much a fair representation of my understanding of where Svensmark's work has got to. There is now some experimental evidence to support it, but more work is required to show that you can go from cosmic ray to cloud and also to demonstrate the size of the effect. If I recall correctly, Svensmark reckons that the effect is big enough to knock the CO2 hypothesis on the head.
We know that there is nothing in the temperature record itself to distinguish the recent warming from what has gone before - the claim that man has warmed the Earth depends on computer models whose temperature predictions are not validated. More relevantly to this posting,to the extent that the cosmic ray effect turns out to be real, the influence of CO2 will be less than previously thought. This is a potentially very big "known unknown" to go alongside the "unknown unknowns" that cannot be ruled out because of the lack of any validation of the models. It seems to me that sceptics are therefore correct to draw attention to Svensmark's work.
One other thing about Tom Chivers' posting:
...if [cosmic rays] did lead to cloud formation, that would not necessarily lead to cooling. Clouds don't only cool the planet: they reflect sunlight, but they also prevent heat from escaping from the Earth. Higher clouds and clouds further from the equator have a cooling effect; lower ones and ones near the equator tend to warm the planet.
The effect of low level clouds was touched on the Hockey Stick Illusion, where I briefly discussed a review paper by Bony et al (2006) looking at the low-level clouds (boundary layer clouds, in the jargon). I was struck by how different Bony's story on the effects of low level clouds is to Tom Chivers'. Here is what she said in her paper:
Boundary layer clouds have a strongly negative [feedback effect] . . . and cover a very large fraction of the area of the Tropics . . . Understanding how they may change in a perturbed climate therefore constitutes a vital part of the cloud feedback problem.
So my understanding of the scientific literature is that low level clouds actually cool the planet. In Tom Chivers' defence, it's easy to get confused in this area because as readers of the Hockey Stick Illusion know, when the IPCC came to discuss boundary layer clouds in the Fourth Assessment Report, they lifted Bony's text almost word for word, but making one rather important alteration:
Boundary-layer clouds have a strong impact . . . and cover a large fraction of the global ocean . . . . Understanding how they may change in a perturbed climate is thus a vital part of the cloud feedback problem.
Not all climate scientists are charlatans, and those who suggest they are are wrong. That said, I hope Tom will concede that there is a real problem with charlatanry among some scientists working on the IPCC assessments.
[Please note: I don't want this thread to turn into a Chivers-bashing session. I will snip comments that are not civil.]