GCRs are high energy particles, mostly protons, that come into our atmosphere from supernovae.
The Sun's magnetosphere modulates the amount of GCR that hit earth.
If the magnetosphere is in a weak phase, more GCR get through.
The weak phase of the magnetosphere occurs when the sunspot cycle is at its weakest, because sunspots are intensely magnetic.
Svensmark posits that GCRs cause ionisation in the atmosphere, which can seed clouds.
This was confirmed experimentally in the CERN CLOUD experiment.
The hypothesis is therefore that cosmic rays will increase the amount of cloud - to be specific, low cloud, which has a more cooling effect than high cirrus cloud, which has a warming effect.
Sunspots therefore should have a warming effect on global climate, partly through a direct effect: (solar intensity is greater during sunspots)
and also through an indirect effect :
(Sunspots high, > magnetic activity high, > GCR low, > clouds low, > temperature high)
One rather beautiful aspect of the theory is that as the solar system orbits the Milky Way Galaxy every 250,000 years, it gets a dose of cooling every time it passes through one of the four arms of the galaxy.
Does this GCR-Temperature effect show through? It looks like it:
There is a fair correlation between the sunspot activity and temperature. Here is another graph, which shows the way the temperature diverges from solar activity:
It shows too on this crude graph I made by simply superimposing a sunspot graph and a temperature graph. It involved stretching and cropping. I am not proud of it, and it comes with a health warning:
I include it because the red line is a smoothed global temperature which has a definite sinuosity about it. John Cook emphasises this in his excellent Escalator, designed to answer the claim that global warming has stopped. (It keeps stopping, repeatedly, and rather regularly). The periodicity of the wiggle shows that a downstroke was due from about 2000. It is a bit blurred, but I conjectured that if the sunspot cycle were to be given more weight in the models, to account for a GCR effect, the models might have predicted the 1999-present plateau which the contrarians get so excited about.
However, when I superimposed 2 graphs, one of sunspot activity since 1900 and one of temperatures since 1900, although the post 1960 correlation is there, earlier years show no correlation. It could be that my superimposition is not accurate.
|Composite; sunspote (blue) from climate4you. temps from Rohde|
Skeptical Science has more detail on Svensmark here. He reviews the criticisms that have been made of his work, but it is noticable that people do not dismiss him out of hand. It is likely that his work will lead to be incorporated into the list of climate forcings. IN fact, it seems that work has started on this.
which are, to summarise
- Solar variation from changes in the 11 year cycle (sunspots and now ?CGRs?), orbital changes, precessional changes.
- Aerosols from volcanoes and industry
- Ocean current variations
So Svensmark is interesting. The magnitude of his effect needs to be quantified and fed into models to see if the models' accuracy is thereby improved. One thing is clear though: GCRs are a supplement to present climate science. They do not overthrow climate science and create a new paradigm, no matter how much contrarians may wish it, nor how loudly they proclaim it.