Cosmic rays and clouds: Potential mechanisms

Guest Commentary by Jeffrey Pierce (Dalhousie U.)

I’ve written this post to help readers understand potential physical mechanisms behind cosmic-ray/cloud connections. But first I briefly want to explain my motivation.

Prior to the publication of the aerosol nucleation results from the CLOUD experiment at CERN in Nature several weeks ago Kirkby et al, 2011, I was asked by Nature Geoscience to write a “News and Views” on the CLOUD results for a general science audience. As an aerosol scientist, I found the results showing the detailed measurements of the influences of ammonia, organics and ions from galactic cosmic rays on aerosol formation exciting. While none of the results were entirely unexpected, the paper still represents a major step forward in our understanding of particle formation. This excitement is what I tried to convey to the general scientific audience in the News and Views piece. However, I only used a small portion of the editorial to discuss the implications to cosmic rays and clouds because (1) I felt that these implications represented only a small portion of the CLOUD findings, and (2) the CLOUD results address only one of several necessary conditions for cosmic rays to affect clouds, and have not yet tested the others.

Many of the news articles and blog posts covering the CLOUD article understandably focused much more on the cosmic-ray/cloud connection as it is easy to tie this connection into the climate debate. While many of the articles did a good job at reporting the CLOUD results within the big picture of cosmic-ray/cloud connections, some articles erroneously claimed that the CLOUD results proved the physics behind a strong cosmic-ray/cloud/climate connection, and others still just got it very muddled. A person hoping to learn more about cosmic rays and clouds likely ended up confused after reading the range of articles published. This potential confusion (along with many great questions and comments in Gavin’s CLOUD post) motivated me to write a general overview of the potential physical mechanisms for cosmic rays affecting clouds. In this post, I will focus on what we know and don’t know regarding the two major proposed physical mechanisms connecting cosmic rays to clouds and climate.

What we know and don’t know about the connection between cosmic rays and clouds and climate

These two proposed mechanisms are the ion-aerosol clear-sky hypothesis and the ion-aerosol near-cloud hypothesis (using the terminology from Carslaw et al., 2002). The ion-aerosol clear-sky hypothesis has gotten most of the mainstream attention, and the recent CLOUD results test a portion of this hypothesis. The near-cloud hypothesis has received less attention. I believe this is because little is known about many of the processes involved. Regardless, it is a fascinating and plausible hypothesis, so I will also address it here. The central question we need to answer in either of these hypotheses is “How much do clouds change due to a change in cosmic rays?”.

The ion-aerosol clear-sky hypothesis

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.

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  1. J. Kirkby, J. Curtius, J. Almeida, E. Dunne, J. Duplissy, S. Ehrhart, A. Franchin, S. Gagné, L. Ickes, A. Kürten, A. Kupc, A. Metzger, F. Riccobono, L. Rondo, S. Schobesberger, G. Tsagkogeorgas, D. Wimmer, A. Amorim, F. Bianchi, M. Breitenlechner, A. David, J. Dommen, A. Downard, M. Ehn, R.C. Flagan, S. Haider, A. Hansel, D. Hauser, W. Jud, H. Junninen, F. Kreissl, A. Kvashin, A. Laaksonen, K. Lehtipalo, J. Lima, E.R. Lovejoy, V. Makhmutov, S. Mathot, J. Mikkilä, P. Minginette, S. Mogo, T. Nieminen, A. Onnela, P. Pereira, T. Petäjä, R. Schnitzhofer, J.H. Seinfeld, M. Sipilä, Y. Stozhkov, F. Stratmann, A. Tomé, J. Vanhanen, Y. Viisanen, A. Vrtala, P.E. Wagner, H. Walther, E. Weingartner, H. Wex, P.M. Winkler, K.S. Carslaw, D.R. Worsnop, U. Baltensperger, and M. Kulmala, "Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation", Nature, vol. 476, pp. 429-433, 2011.
  2. K.S. Carslaw, "Cosmic Rays, Clouds, and Climate", Science, vol. 298, pp. 1732-1737, 2002.