An incremental step blown up

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:

Figure 1 from Enghoff et al, 2011

Figure 1. Formation rate of 4 nm aerosols versus ion concentration (Figure 1 in Enghoff et al).

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.

Figure 2. ISCCP analysis of global low cloud cover from IR-channel.

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