Galactic cosmic rays

There’s a recent Nature Communications paper by Svensmark et al. called [i]ncreased ionization supports growth of aerosols into cloud condensation nuclei. The basic idea is that cosmic rays (energetic particles typically accelerated by shock waves) can influence the growth of aerosol particles that can then act as cloud nucleation sites. This may, therefore, play some kind of role in climate change.

The problem is that this idea has been around for quite some time and has essentially been pre-bunked. There’s a relevant Realclimate review called a cluttered story of little success. In fact, if you search Realclimate for Svensmark, you get a number of posts that cover this topic.

There have also been recent CERN results which suggest that the Sun-clouds connection takes a beating. The new paper has also not be all that well received.

It did, however, remind of something that I thought I would add here. I used to work with someone called Gary Zank, who was involved in research that was used to suggest that maybe dinosaurs were wiped out by genetic mutations from cosmic rays. The basic idea is that the Sun has a wind that blows a bubble of magnetised plasma – called the Heliosphere – into the interstellar medium. This acts to shield the inner Solar System from galactic cosmic rays.

However, the Solar System actually moves through the interstellar medium. The size of this bubble depends on the density and temperature of the local interstellar medium. Currently we’re in a warm, diffuse region, and so the bubble extends out to well beyond Pluto. However, when we are in a cold, dense region, it may only extend to about the orbit of Jupiter; much smaller than it is today. This would mean that the local cosmic ray flux would be much higher than when the bubble is larger. It is thought that we were in a denser region about 66 million years ago and, hence, maybe the enhance cosmic ray flux wiped out the dinosaurs.

I don’t think that is really regarded as a viable explanation. However, it does suggest that the local cosmic ray flux could vary quite substantially on these timescales. If cosmic rays can play a role in cloud formation, and hence our climate, you might expect to see some kind of signal in our past temperatures. I even found a paper that discusses exactly this, saying

We have demonstrated that the cosmic ray spectra at the Earth, particularly that of anomalous protons can vary by
orders of magnitude even for moderate changes in the interstellar medium……The assumed changes
in the state of the interstellar medium certainly occur along the galactic orbit of the Sun on time scales of a few thousand to a few million years. In conclusion, we state that, on these time scales, if the cosmic ray–climate link will be confirmed, one should consider an effect of interstellar–terrestrial relations on the climate on Earth.

As far as I’m aware, there is no indication of such a signal in our past climate. If, as suggested above, the cosmic ray flux vary by orders of magnitude, you might expect such a signal to be easy to find.

It seems highly unlikely that any cosmic ray effect on clouds (if there even is one) can be large enough to be climatically relevant. As far as I can tell, this recent paper is simply another attempt to suggest that our climate is somehow very sensitive to small changes in something that isn’t CO₂ so as to imply that maybe it isn’t sensitive to much larger changes in CO₂ itself.

Update:
Something I meant to include in the post, but forgot to do, was to point out something odd about Svensmark et als. reference list. This comes from a post by Ari Jokimaki. They do not cite a paper by Kulmala et al. called Atmospheric data over a solar cycle: no connection between galactic cosmic rays and new particle formation , but do cite earlier papers by the same author. As Ari points out, Svensmark et al. seems to ignore a number of recent papers that appear to contradict what they’re suggesting in this new paper.