Cosmic rays and clouds: Potential mechanisms

The central theme of the clear-sky hypothesis is that cosmic rays affect ion concentrations in the atmosphere. Aerosol nucleation (the formation of ~1 nm particles in the atmosphere) is generally enhanced by the presence of ions. The particles formed through nucleation may grow through condensation of sulfuric acid and organic vapors to sizes where they can act as Cloud Condensation Nuclei (CCN) (the particles on which cloud drops form). If CCN are exposed to relative humidities above 100%, cloud droplets will form on them. Thus, a change in cosmic rays could potentially affect the number of cloud drops, which in turn may affect the amount of sunlight reflected by a cloud, the formation of precipitation and the cloud lifetime.

Figure 1. Overview of freshly nucleated particles, CCN and cloud droplets.

For us to understand the clear-sky hypothesis, and answer the question, “How much do clouds change due to a change in cosmic rays?”, we must understand the following sub-questions:

  1. How much does ion formation in the atmosphere change due to changes in the cosmic-ray flux to the atmosphere (due to the solar cycle etc.)?
  2. How much do aerosol nucleation rates change due to changes in ion formation rates?
  3. How much do CCN concentrations change due to changes in aerosol nucleation rates?
  4. How much do clouds change due to changes in CCN concentrations?

Question 1: How much does ion formation in the atmosphere change due to changes in the cosmic-ray flux to the atmosphere?

Of the four questions, we understand question 1 the best. With current information about the Earth’s magnetic field and solar activity, we have fairly robust predictions of the ion formation rate from cosmic rays. The figure below shows the percent change in the ion formation rate from cosmic rays between the solar minimum (more cosmic rays) and solar maximum (fewer cosmic rays) Usoskin and Kovaltsov, 2006.

Figure 2. Percent change in the ion formation rate as a function of height and latitude in the atmosphere from cosmic rays between a typical solar minimum and solar maximum in the troposphere and lower stratosphere.

As shown in the figure above, the ion formation rate from cosmic rays varies by 5-20% throughout most of the troposphere (the region of the atmosphere where clouds form). The reported observed relative change in low cloud cover [2] is ~6% with the solar cycle (or 2% absolute change in the fraction that low clouds cover the planet). Thus, the modulation of ions is a similar order of magnitude to the amount of cloud change. In order for the clear-sky hypothesis to have a large effect on clouds, the 5-20% change in ion formation rates needs to efficiently propagate into changes in aerosol nucleation, CCN and cloud properties. So…

Question 2: How much do aerosol nucleation rates change due to changes in ion formation rates?

The recent CLOUD results in Nature directly address this question (and this question only). The results showed under the conditions of the CLOUD chamber show that ions from cosmic rays unequivocally aid aerosol nucleation. However, the CLOUD paper does not directly address how much nucleation rates will change from a 5-20% change in ion formation rates, but inspection of Figure 2 in their paper (below as our Figure 3) shows that a doubling of ion concentration leads to somewhat less than a doubling in nucleation rate. Furthermore, a doubling of ion concentration requires more than a doubling in ion formation rates (due to an increased rate of positive and negative ions re-combining with each other to form neutral molecules when ion concentrations are higher). Therefore, a 5-20% change in ion formation rates from cosmic-ray changes will lead to less than a 5-20% change in nucleation rates. (The results in Figure 3 covers a very large range in ion concentrations, much larger than would ever be modulated by relevant changes in cosmic rays.)

Figure 3. Figure 2 from Kirkby et al. (2011) showing the nucleation rate as a function of ion concentration for two different conditions (the two colored lines).

Question #3: How much do CCN concentrations change due to changes in aerosol nucleation rates?

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References

  1. I.G. Usoskin, and G.A. Kovaltsov, "Cosmic ray induced ionization in the atmosphere: Full modeling and practical applications", J. Geophys. Res., vol. 111, 2006. http://dx.doi.org/10.1029/2006JD007150
  2. H. Svensmark, and E. Friis-Christensen, "Variation of cosmic ray flux and global cloud coverage—a missing link in solar-climate relationships", Journal of Atmospheric and Solar-Terrestrial Physics, vol. 59, pp. 1225-1232, 1997. http://dx.doi.org/10.1016/S1364-6826(97)00001-1