New results from the University of Aarhus in Denmark and the Danish National Space Institute allegedly show that particles from space create cloud cover, according to a recent press release. And the Physics World magazine (May, 2011) report that the

researchers say this is the best experimental evidence yet that the Sun influences the climate by altering the intensity of the cosmic-ray flux reaching the Earth’s surface.

Quite spectacular claims! So let’s see what is the source of this information.

The basis for the statements was a recent paper published in GRL by Enghoff et al. The key points in the paper are stated as: (a) Cosmic rays increase nucleation rate, (b) A particle beam is not needed, for experiments, and (c) Ions are important for atmospheric nucleation rate. But where is the link to real clouds?

The word ‘cloud’ is mentioned in the paper. In the introduction:

Aerosol and cloud research is one of the most critical frontiers of climate science [Shindell et al., 2009; Bodenschatz et al., 2010] and the direct radiative forcing and indirect cloud albedo forcing from aerosols remain the dominant uncertainty in the radiative forcing of the atmosphere

The word ‘cloud’ is also found in some of the titles of the publications in the citation list. And that’s it.

OK, so this study is really about a laboratory experiment. The results presented are of the impact of ionization on the formation rate of aerosols with size ~4 nanometers (nm), as shown in the figure below:

Enghoff et al claim that the figure shows a clear contribution from ion-induced nucleation and considers this to be the (quoted from the paper’s abstract)

first unambiguous observation of the ion-effect on aerosol nucleation using a particle beam under conditions that resemble the Earth’s atmosphere.

Not so fast!

There is a significant amount of aerosol formation taking place with no ionization (“background levels”), and when the sample air in the experiment was replaced, this caused a large jump (seen as a shift along the vertical axis) in the formation rate (the different colours in the figure above). This clearly shows that the formation rate is also highly sensitive to factors other than ionization.

The figure further shows that the relationship between the ionization and the aerosol formation is not all that strong. There is a relationship, but there is substantial scatter and the slopes of the best fit are not very steep.

So let’s look at the clouds data. Is there a clear 11-year cycle in the cloud cover? The figure below presents the global low cloud cover from the ISCCP project. Perhaps there is an 11-year cycle embedded in the evolution since 1983, perhaps not. However, the variations in the evolution are clearly not dominated by 11-year cycles.

We see the same thing in a study by Harrison & Stephenson (2006), which provides an additional clue based on diffuse light, which is light scattered by clouds. They do find a link between cosmic galactic rays (GCR) and the diffuse light (figure below), suggesting a link between the GCR and cloudiness. But when you look at the scatter plot, the link is not very visible (if you ignore the fitted lines). The reason for this is that this link is very weak.

There has been a number of studies on the relationship between solar activity and earth’s climate, suggesting there is a solar signal. But the solar influence seems to be weak. GCR don’t come from the sun, but are charged particles from distant galaxies and stars that are modulated by the solar magnetic field. The solar magnetic field is closely linked to solar activity.

Figure 1 here may be consistent with a weak relationship between GCR and clouds, due to a substantial scatter, weak trend (not very steep slope), and the pronounced effect of changing the air supply.

It is conceivable that turbulent mixing of the air will produce small pockets of air with higher formation rates, in a same fashion as air with different impurities did in the experiments of Enghoff et al.

However, there is another reason why there really is a weak link from Enghoff et al.’s results to clouds. The experiment conducted by Enghoff et al. examined the formation of ultra-fine aerosols with size of 4 nanometers. But clouds need particles of the size approximately 10000 nanometers (10 micrometers) to form cloud drops for air that is barely supersaturated, according to the Köhler curve (which is central to cloud micro-physics).

What happens between the stage where 4nm aerosols form and the stage where they become act as CCN cloud drops (with a ~10 micrometer radius) is unknown (see Figure 4 for a depiction of these two stages). The aerosols must grow and become an order of million times larger in terms of their initial volume.

It is interesting to note the way the word “climate” appears in the GRL paper (3 times: once in the introduction and twice in the titles in the reference list) and the Danish press release (15 times, counting phrases such as ‘climate chamber’ and ‘climate researcher’). The press release claims that the results

substantiate the connection between the Sun’s magnetic activity and the Earth’s climate

and that

there is much to indicate that climate models must hereby take cosmic radiation into consideration.

It is difficult to explain any long-term change in our climate in terms of the hypothesis that GCR affect clouds, having an effect on the albedo, and ultimately the temperatures. For starters, we see no evidence for any long-term change in the sun in the last 50 years (light blue and red curves in Figure 5 below) or in the GCR measurements (grey symbols in Figure 5).

So my take on this, is that the paper only really shows that the nature of the ionizing particles is not important for the ion-induced component of the nucleation. Does this imply that the cloud experiment at CERN is necessary? I wonder. But key point (c), that

ions are important for atmospheric nucleation rate