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.
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:
- 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.)?
- How much do aerosol nucleation rates change due to changes in ion formation rates?
- How much do CCN concentrations change due to changes in aerosol nucleation rates?
- 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  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
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.
Non-thunderstorm clouds, however, interrupt the electric current because gas-phase ion concentrations within clouds are very low making the clouds very resistive to electric current flow. Charge builds up on the top and bottom of the cloud much like charged plates in a capacitor. Cosmic rays may affect this charge build up through changing the resistance of current flow in the clear atmosphere; however, the strength of this effect is still not well known.
This may have an effect on the cloud properties by enhancing the collision rate between cloud droplets and between aerosols and cloud droplets. Often in clouds, liquid water drops will exist even when temperatures are well below 0ºC (freezing point of water). Collisions between the charged aerosols with these supercooled cloud droplets may enable the freezing of these droplets, which could lead to cloud invigoration due to the heat released from freezing or enhanced precipitation (clouds consisting of both liquid drops and ice crystals are more effective at generating precipitations than clouds containing only one phase drops/crystals). These effects, however, are all still very uncertain.
Figure 6. The enhancement of droplet freezing by collisions with charged aerosol is an essential component of the near-cloud mechanism, but is not well understood.
The uncertainties in the near-cloud mechanism far exceed those of the clear-sky mechanism (it is not even clear whether a change in the cosmic-ray flux would lead to more or less cloud cover through the near-cloud mechanism). However, it remains an interesting potential connection between cosmic rays and clouds that needs to be explored if we are to understand how cosmic rays may affect clouds.
While reported observed correlations between cosmic rays and clouds are suggestive of effects of cosmic rays on clouds, cosmic rays rarely change without other inputs to the Earth system also changing (e.g. total solar irradiance or solar energetic particle events, both also driven by changes in the sun, but distinct from cosmic rays). Thus, we must understand the physical basis of how cosmic rays may affect clouds. However, it is clear that substantially more work needs to be done before we adequately understand these physical connections, and that no broad conclusions regarding the effect of cosmic rays on clouds and climate can (or should) be drawn from the first round of CLOUD results. Finally, there has been no significant trend in the cosmic ray flux over the 50 years, so while we cannot rule out cosmic-ray/cloud mechanisms being relevant for historical climate changes, they certainly have not been an important factor in recent climate change.
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