Cosmic rays and clouds: Potential mechanisms

Firstly, primary emissions contribute to CCN as well as nucleation, and the primary emissions are not affected by cosmic rays. Secondly, the likelihood that a freshly nucleated particle will grow to become a CCN depends on whether it can grow from condensation of sulfuric acid and organic vapors onto it before the particle coagulates with a larger particle (reducing the number of particles). If the nucleation rate is increased due to cosmic rays, there will be more particles competing for a fixed amount of condensible vapors, and each new particle will grow more slowly. Additionally, the coagulation loss of the particles will increase due to the increased number of particles and the slower growth (particles are lost through coagulation more quickly at smaller sizes).

Unfortunately, as far as I know this question has only been addressed using models. While we test the model for known uncertainties in model inputs, it is always a possibility that we are missing something. Fortunately, the growth of ultrafine particles to CCN sizes should be addressed in future experiments in the CLOUD chamber, so we should soon also have controlled experimental evidence to compare with model results.

Question #4: How much do clouds change due to changes in CCN concentrations?

Increased CCN concentrations lead to increased concentrations of cloud droplets. More cloud droplets will lead to increased reflection of sunlight from the cloud to space, and may under some circumstances lead to a reduction of precipitation and an increased lifetime of the cloud. How much these cloud properties depend on CCN concentrations is a major area of research in general. CCN concentrations have more than doubled in many polluted regions due to human-generated emissions, so we are working hard to understand how this has affected clouds. Given that CCN concentrations have changed so much from human influence, a change in CCN of less than 1% due to cosmic rays seems quite minor. Indeed, cloud reflectivity, precipitation and cloud lifetime will generally change by less than the change in CCN for most clouds (e.g. we know that cloud cover has not more than doubled due to human-generated emissions). Therefore, it is unlikely to generate a ~6% change in cloud cover (reported in observations of clouds with 11-year solar cycle and after Forbush decreases) from less than a 1% change in CCN.

Clear-sky hypothesis summary

In summary, the clear-sky hypothesis is driven by 5-20% changes in ion formation rates in the troposphere. These ion changes would need to drive changes in cloud cover by several percent to account for reported correlations. While uncertainties in processes remain, it appears unlikely to me (and most other scientists working on aerosol-cloud interactions who’ve shared their thoughts on this hypothesis with me) that this mechanism will be strong enough to greatly change clouds. I would not go so far to say that the case is closed on this mechanism, but if it is to be important there must be some amplification factor in one (or more) of the questions described above that we are currently unaware of. Thus, it will be exciting to see what the future CLOUD experiments (or other controlled experiments) show regarding questions #3 and #4.

Ion-aerosol near-cloud hypothesis

The ion-aerosol near-cloud hypothesis has received less attention than the clear-sky hypothesis; however, there is still active research being done on it. The near-cloud hypothesis has to do with the global electric circuit (see the figure below).

Figure 5. Schematic showing how cosmic rays modulate the global electric circuit and may affect the charging around clouds.

Thunderstorms create a charge separation with positive ions at the top of the cloud and a negative ions at the bottom (this negative charge gets discharged through lightning to the ground). The positive charge at the top of the cloud moves through the conductive upper atmosphere to the ionosphere giving the ionosphere a positive charge. The difference in charge between the ionosphere and the Earth’s surface drives an electric current from the ionosphere to the surface. The resistance of the atmosphere to current flow depends on the ion concentrations (more ions = less resistance). Thus, when more cosmic rays enter the atmosphere, electricity flows more quickly through the atmosphere.

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