Now and then new results appear that suggest that the idea of cosmic ray influence on clouds and terrestrial climate does not work. “Sun-clouds-climate connection takes a beating from CERN” is the latest news story which is based on a new paper from the CLOUD collaboration at CERN .
It is important to note that the new CLOUD paper is not presenting an experimental result, with respect to the effect of cosmic ray generated ions on clouds, but a result of numerical modeling. CLOUD is using their experimental measurements to estimate the typical nucleation of various aerosols of small size (1-3 nm). However, for an aerosol to affect clouds (and climate) it must first grow to 50-100 nm, to become cloud condensation nuclei (CCN). CLOUD then uses a numerical model to estimate the effect of cosmic rays on the growth process, and finds that the response of cosmic rays on the number of CCN over a solar cycle is insignificant.
This type of numerical modeling is by no means new, and neither is the result that ions in these models apparently do not affect cloud formation. We have known this for about 7 years. For example the CLOUD results, with respect to cosmic rays and clouds, are very similar to the conclusions of Pierce and Adams from 2009  where they also use a numerical model to grow small nucleated aerosols to CCN, and also find only a small change in CCN as a function of ion changes. In fact this result has been found a number of times in similar models. The argument for the lack of response to ions is the following: In the presence of ions additional small aerosols are formed, but with an increase in the number of aerosols, there is less gas to each particle, and they therefore grow slower. This means that the probability of being lost to larger particles increases, and fewer survive.
So why, in contrast to the above, do I think that the cosmic rays cloud idea is still viable? The reason is that we have tried to answer the same question (do ion-nucleated aerosols grow to CCN) without using models — and get very different results.
In 2012 we tested the growth of nucleated aerosols to CCN in our laboratory and found that when no ions were present the response to increased nucleation was severely damped, in accordance with the above mentioned models; but with ions present, all the extra nucleated particles grew to CCN sizes, in contrast to the numerical model results . Now it may be that the conditions we have in the experiment are not as in the real atmosphere. There are complex processes in the real atmosphere that that we cannot include, whose effect may change the experimental result, as we have been told many times.
It is therefore fortunate that our Sun makes natural experiments with the whole Earth. On rare occasions “explosions” on the Sun called coronal mass ejections, results in a plasma cloud passing the Earth, with the effect that cosmic rays flux decreases suddenly and stays low for a week or two. Such events, with a significant reduction in the cosmic rays flux, are called Forbush decreases, and are ideal to test the link between cosmic rays and clouds. Finding the strongest Forbush decreases and using 3 independent cloud satellite data sets (ISCCP, MODIS, and SSM/I) and one dataset for aerosols (AERONET), we clearly see a response to Forbush decreases. These results suggest that the whole chain from solar activity, to cosmic rays, to aerosols (CCN), to clouds, is active in the Earths atmosphere. From the MODIS data we even see that the cloud microphysics is changing according to expectations.
Figure 1 display the superposed signal in clouds (blue curve), based on the above three satellite datasets, in the days following the minimum in cosmic rays of the 5 strongest Forbush decreases (red curve). The delay in the minimum of the two curves is due to the time it takes aerosols to grow into CCN. A Monte Carlo simulation was used to estimate the significance of the signal, and none of 104 random realizations gave a signal of similar size. Please see our latest paper from 2016 for further evidence .
Figure 1: Statistical common disturbance in clouds (1 Principal component) based on three cloud satellite data sets (ISCCP, MODIS and SSM/I) superposed for the five strongest Forbush decreases (blue) curve. Red curve is the change in (%) of cosmic rays superposed for the same five events. The thin lines are 1-3 standard deviations. Adapted from .
Finally, there are a large number of studies showing that past climate changes are closely correlated to variations in cosmic rays. For example, the energy that goes into the oceans over 11 years solar cycle is of the order 1-1.5 W/m2, which is 5-7 times too large to be explained by solar irradiance variations . Therefore something is amplifying the solar cycle, and “cosmic rays and clouds” is a good candidate to explain the observed forcing.
In conclusion, observations and experiments go against the above mentioned numerical model result. As I see it, something is missing in the prevailing theory. A solution to this problem is still worth pursuing.
 E. M. Dunne et al., Global atmospheric particle formation from CERN CLOUD measurements, (2016), DOI: 10.1126/science.aaf2649
 J. R. Pierce, P. J. Adams, Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? Geophys. Res. Lett. 36, L09820 (2009).
 H. Svensmark, M. B. Enghoﬀ, and J. O. P. Pedersen, Response of Cloud Condensation Nuclei (> 50 nm) to changes in ion-nucleation, Physics Letters A, 377, 2343–2347, (2012). https://dl.dropboxusercontent.com/u/51188502/CCN_Svensmark_PhysicsLettersA.pdf
 J. Svensmark,M. B. Enghoff, N. J. Shaviv, and H. Svensmark, The response of clouds and aerosols to cosmic ray decreases, J. Geophys. Res. Space Physics, 121, 8152–8181, (2016), doi:10.1002/2016JA022689. https://dl.dropboxusercontent.com/u/51188502/Forbush_long_JGR_rev3_nored.pdf
 N. J. Shaviv, ‘Using the oceans as a calorimeter to quantify the solar radiative forcing’ J. Geophys.Res., 113, 2156 (2008)