You will recall that we recently had a look at work done by Dr Jasper Kirkby and others in the CERN/CLOUD project.
The possible story in my words from cosmic rays to temperature change is this.
The earth is constantly being bombarded with galactic cosmic rays (GCR), high-energy particles from exploding stars. These act on tiny particles (aerosols) of sulphuric acid and ammonia molecules which cluster together to forms “seeds” called cloud condensation nuclei (CCN) from which clouds grow. Clouds are assumed to have a net cooling effect on surface temperature.
An active sun protects the Earth from GCRs to some extent. Conversely with a less active sun more cosmic rays get through. More cosmic rays mean more clouds and the cooler global temperatures.
It’s important to understand that CCNs can and do form without the assistance of GCRs, and that it is the change in GCRs operating at the margin which may or may not cause a significant change in temperature.
Every step of the way must be scientifically examined and proven. The mechanism must work, and in quantities large enough to make a difference.
A BBC article which I hadn’t seen when I wrote the earlier post, explains what the CERN/CLOUD project did and didn’t do. The CLOUD project did show that cosmic rays cause a ten-fold increase in the formation rate of nanometre-sized aerosol particles. But these particles are far too small to form droplets. Professor Mike Lockwood of Reading University said that the particles only grew to 2 nanometres whereas to influence incoming or outgoing radiation:
droplets must be of the order of 100 nanometres (nm). The growth rates would be really slow from 2 to 100nm because there simply is not enough sulphuric acid in the atmosphere.
Kirkby told BBC News:
“We’ve shown sulphuric acid and ammonia can’t account for nucleation (the very early stages of cloud seed formation) observed in the lower atmosphere.
“We’ve found that this can only account for a tenth to a thousandth of the rate that’s observed.”
On another thread Jess linked to an an ars technica article reporting on other studies. These studies are not helpful to the Svensmark hypothesis that cosmic rays “have more effect on the climate than manmade CO2”.
These studies have indicated that the number of cloud condensation nuclei is not very sensitive to the nucleation of aerosols by cosmic rays. There are a few reasons for this. First, there are many other sources of aerosols (including particles in sea spray and anthropogenic emissions), so the total change in aerosols due to fluctuations in cosmic-ray-induced nucleation is not as significant as it might otherwise be. Second, the aerosols are competing with each other to condense a limited supply of vapor, meaning that any increase in the total cloud condensation nuclei is limited.
In addition, most aerosols collide and combine with other particles long before reaching the size of a cloud condensation nucleus—a process called “coagulation.” Increasing the number of aerosols increases the frequency of these collisions, again dampening the effect on the number of condensation nuclei.
In model simulations, a 15 percent increase in cosmic rays (which is about the variation in one 11-year solar cycle) leads to an increase in condensation nuclei of less than 0.2 percent. Even assuming that cosmic rays could have a significant effect on cloud condensation nuclei, it remains to be shown that this would, in fact, account for the observed fluctuations in global low cloud cover. (Emphasis added)
Moreover while the relationship between cosmic ray intensity and solar activty seems to work quite well, in the following graph cloud cover (grey) seems to have decisively parted company with cosmic rays (solid) from 2004:
Jeffrey Pierce at RealClimate has a guest post which goes into the mechanisms in some detail. He is left looking for one or more amplification factors which at present are simply not there. His final thoughts:
While reported observed correlations between cosmic rays and clouds are suggestive of effects of cosmic rays on clouds, cosmic rays rarely change without other inputs to the Earth system also changing (e.g. total solar irradiance or solar energetic particle events, both also driven by changes in the sun, but distinct from cosmic rays). Thus, we must understand the physical basis of how cosmic rays may affect clouds. However, it is clear that substantially more work needs to be done before we adequately understand these physical connections, and that no broad conclusions regarding the effect of cosmic rays on clouds and climate can (or should) be drawn from the first round of CLOUD results. Finally, there has been no significant trend in the cosmic ray flux over the 50 years, so while we cannot rule out cosmic-ray/cloud mechanisms being relevant for historical climate changes, they certainly have not been an important factor in recent climate change.
Pierce looks at this and can find no trend:
If the shape of the above approached that of the graph below for the last 50 years the prospects for Svensmark’s thesis would no doubt improve:
It’s perhaps worth repeating here that establishing a significant GCR/cloud/climate link would require demonstrating the following:
- … that increased nucleation gives rise to increased numbers of (much larger) cloud condensation nuclei (CCN)
- … and that even in the presence of other CCN, ionisation changes can make a noticeable difference to total CCN
- … and even if there were more CCN, you would need to show that this actually changed cloud properties significantly
- … and that given that change in cloud properties, you would need to show that it had a significant effect on radiative forcing.
Eric Steig says none of these are yet anywhere close to being shown. But if you really want to get into the topic, read the whole RealClimate thread on Pierce’s post.