Cosmic rays and clouds: Potential mechanisms

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?

The impact of changing aerosol nucleation rates on CCN concentrations has recently been studied using several different models Spracklen et al, 2008Makkonen et al, 2009Wang and Penner, 2009Yu and Luo, 2009. In all cases, the change in CCN is smaller than the change in nucleation rates. Two other papers Pierce and Adams, 2009, Snow-Kropla et al., 2011 have specifically looked at this question in the context of cosmic-ray changes, and found that even though nucleation rates are changing by 1-5% throughout much of the troposphere, the changes in CCN are generally around 0.1-0.2% throughout much of the globe. The reason for this strong dampening is shown in the figure below.

Figure 4. Schematic showing the reasons for the small changes in CCN to changes in 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
  3. D.V. Spracklen, K.S. Carslaw, M. Kulmala, V. Kerminen, S. Sihto, I. Riipinen, J. Merikanto, G.W. Mann, M.P. Chipperfield, A. Wiedensohler, W. Birmili, and H. Lihavainen, "Contribution of particle formation to global cloud condensation nuclei concentrations", Geophys. Res. Lett., vol. 35, 2008. http://dx.doi.org/10.1029/2007GL033038
  4. R. Makkonen, A. Asmi, H. Korhonen, H. Kokkola, S. Järvenoja, P. Räisänen, K.E.J. Lehtinen, A. Laaksonen, V. Kerminen, H. Järvinen, U. Lohmann, R. Bennartz, J. Feichter, and M. Kulmala, "Sensitivity of aerosol concentrations and cloud properties to nucleation and secondary organic distribution in ECHAM5-HAM global circulation model", Atmospheric Chemistry and Physics, vol. 9, pp. 1747-1766, 2009. http://dx.doi.org/10.5194/acp-9-1747-2009
  5. M. Wang, and J.E. Penner, "Aerosol indirect forcing in a global model with particle nucleation", Atmospheric Chemistry and Physics, vol. 9, pp. 239-260, 2009. http://dx.doi.org/10.5194/acp-9-239-2009
  6. F. Yu, and G. Luo, "Simulation of particle size distribution with a global aerosol model: contribution of nucleation to aerosol and CCN number concentrations", Atmospheric Chemistry and Physics, vol. 9, pp. 7691-7710, 2009. http://dx.doi.org/10.5194/acp-9-7691-2009
  7. J.R. Pierce, and P.J. Adams, "Efficiency of cloud condensation nuclei formation from ultrafine particles", Atmospheric Chemistry and Physics, vol. 7, pp. 1367-1379, 2007. http://dx.doi.org/10.5194/acp-7-1367-2007
  8. E.J. Snow-Kropla, J.R. Pierce, D.M. Westervelt, and W. Trivitayanurak, "Cosmic rays, aerosol formation and cloud-condensation nuclei: sensitivities to model uncertainties", Atmospheric Chemistry and Physics, vol. 11, pp. 4001-4013, 2011. http://dx.doi.org/10.5194/acp-11-4001-2011