Do Cosmic Rays Cause Global Warming?

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How do you make a cloud? Well, first you start with an aerosol particle, a small particle around which the much larger cloud condensation nuclei (CCNs) can condense. It takes a large CCN – at least 100 nanometres in size – for water vapour to be able to condense from water vapour. Clouds are made up of many CCNs with water condensed on them. Clouds can reflect sunlight back into space, cooling the Earth – but they can also reflect heat back to Earth, warming the Earth instead. The total cooling effect of clouds is about 44W/m2, contrasted with about 31W/M2 of warming from them reflecting heat back to Earth – a net effect of about -13W/m2 (Ramanathan et. al, 1989).

A new paper, Kirkby et. al. (2011), published by scientists at CERN claims to shed light on the role of cosmic rays in the formation of these aerosol particles. Cosmic rays are mostly the remnants of atoms which have been accelerated to near the speed of light, along with some more exotic particles such as stable antimatter. Most cosmic rays reach such speeds while bouncing around in the magnetic fields and remnants of supernova, though some reach even higher energies through not-yet-fully-understood processes. The sun’s magnetic field diverts most cosmic rays away from the Earth, so the solar maximum is the low for cosmic rays, and during the solar minimum is when we get the most cosmic rays here on Earth. As they pass through the atmosphere, they collide with gasses, donating their energy and ionizing the molecules.

Kirkby et. al. used a particle accelerator to create analogs to cosmic rays, ionizing the gasses. They found that this increased the formation – the “nucleation” – of small aerosols – nanometre sized particles – by a factor of 2-10. They also showed a much larger – 100 to 1000 times – increase in nucleation from the presence of ammonia in addition to sulphate. It remains the case that they observed far more nucleation than in previous lab studies, it was still several times lower than is observed in the atmosphere.

Their setup only considered small, nanometre-sized particles – it was in principle incapable of producing larger ones. No doubt, future experiments, including future work by these same scientists, will investigate whether similar increases in the formation of larger, potentially cloud-forming, particles are found. However, other studies have found results that cast doubt over whether this will be the case. For example, Snow-Kropla et. al. (2011) found that the observed variation in cosmic rays made less than a 0.2% difference in the concentration of 80 nm particles. They saw a larger increase (1%) in 10 nm particles, suggesting that the impact of cosmic rays falls off as you look at larger particles. Pierce & Adams (2009) find similar results, and suggest that this could be because of a lack of condensable gasses that would allow the particles to grow larger – that it’s not the lack aerosol particles limiting the growth of potential cloud condensation nuclei. If so, it doesn’t matter how many aerosol particles there are about, so far as CCNs are concerned – they are limited by a different factor. It would certainly be possible for the results of a future experiment to contradict these results – and it would be interesting to see how the question was resolved.

We can see that there’s rather a long way to get from this finding – that cosmic rays increase the production of nanometre-sized particles – to have an effect on cloud formation. And to get to the idea that cosmic ray variation can explain global warming, we must assume that:

  1. An increase in aerosol nucleation increases the concentration of large cloud condensation nuclei (though it’s not out of the question, we’ve already seen that it has been contradicted by other studies).
  2. That an increase in CCNs increases cloud cover (this, at least, seems plausible).
  3. That there is a downwards trend in cosmic rays (which would decrease nucleation, which may reduce CCN formation, which may decrease cloud formation).

So what are cosmic rays doing? Well… not much. The data just doesn’t contain the sort of decline that would be necessary for any possibility that cosmic rays could be the cause for global warming.

Cosmic rays seen by monitoring stations in Climax, New Mexico and Oulu, Finland. Shown as the variation from a 1970-1999 baseline.

Many in the media jumped to the conclusion that cosmic rays were affecting the climate. Lawrence Solomon wrote a hyperbolic article claiming that the paper proved absolutely, beyond a shadow of doubt, that cosmic rays were responsible for all the observed global warming. The International Business Times claimed that anthropogenic global warming had been disproved. James Delingpole has got a wonderful conspiracy theory going. One must wonder why people so “sceptical” about the well-established link between greenhouse gasses and global temperatures would be so quick to jump to conclusions without even a correlation. No such conclusions are supported, or even suggested, in the paper they each claim to be writing about.

Cosmic rays as above, plotted alongside NOAA’s monthly global index (which uses 1901-2000 as a baseline).


Kirkby, J., Curtius, J., Almeida, J., Dunne, E., Duplissy, J., Ehrhart, S., Franchin, A., Gagné, S., Ickes, L., Kürten, A., Kupc, A., Metzger, A., Riccobono, F., Rondo, L., Schobesberger, S., Tsagkogeorgas, G., Wimmer, D., Amorim, A., Bianchi, F., Breitenlechner, M., David, A., Dommen, J., Downard, A., Ehn, M., Flagan, R., Haider, S., Hansel, A., Hauser, D., Jud, W., Junninen, H., Kreissl, F., Kvashin, A., Laaksonen, A., Lehtipalo, K., Lima, J., Lovejoy, E., Makhmutov, V., Mathot, S., Mikkilä, J., Minginette, P., Mogo, S., Nieminen, T., Onnela, A., Pereira, P., Petäjä, T., Schnitzhofer, R., Seinfeld, J., Sipilä, M., Stozhkov, Y., Stratmann, F., Tomé, A., Vanhanen, J., Viisanen, Y., Vrtala, A., Wagner, P., Walther, H., Weingartner, E., Wex, H., Winkler, P., Carslaw, K., Worsnop, D., Baltensperger, U., & Kulmala, M. (2011). Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation Nature, 476 (7361), 429-433 DOI: 10.1038/nature10343

Pierce, J., & Adams, P. (2009). Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? Geophysical Research Letters, 36 (9) DOI: 10.1029/2009GL037946 [PDF]

Ramanathan, V., Cess, R., Harrison, E., Minnis, P., Barkstrom, B., Ahmad, E., & Hartmann, D. (1989). Cloud-Radiative Forcing and Climate: Results from the Earth Radiation Budget Experiment Science, 243 (4887), 57-63 DOI: 10.1126/science.243.4887.57 [PDF]

Snow-Kropla, E., Pierce, J., Westervelt, D., & Trivitayanurak, W. (2011). Cosmic rays, aerosol formation and cloud-condensation nuclei: sensitivities to model uncertainties Atmospheric Chemistry and Physics, 11 (8), 4001-4013 DOI: 10.5194/acp-11-4001-2011


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