The role of cosmic radiation in climate change has been discussed in various publications. On this internet site I put a small article with some comments on it.
The main reasoning is as follows: When the sun is active, the solar wind causes to some extent cosmic radiation from outer space to be deflected away from the solar system. It is postulated that cosmic radiation enhances cloud formation (at higher altitudes). This would lead to some cooling on Earth. So: If this would be the case a more active sun would lead to higher temperatures.

Recently I found some interesting data on the internet (see data sources at the bottom) on the amount of cosmic radiation in time. This enables one to see whether a correlation between solar activity / cosmic radiation and the temperature on Earth could be found.

figure1: Level of cosmic radiation vs. global temperature anomaly [°C]

In figure 1 the level of cosmic radiation is plotted against the global temperature anomaly (GISS, monthly data). No significant correlation can be observed (r² = 0.0536). Only a very slight negative trend can be discerned, but this is not statistically significant.

figure2: Level of cosmic radiation vs. timeline of global temperature anomaly [°C]

Figure 2 shows the timeline. If there would be a (very weak) correllation between cosmic radiation hidden in the "natural noise" of the temperature signal on Earth, a tendency towards slightly tilted (in a "NW-SE" orientation) timelines should be observable. This is not the case. Most scatter is in a much more vertical (or even "SW-NE") direction.

Finally one can take a look at the individual signals. In figure 3 the temperature anomaly on Earth (vs. 1951-1980) is plotted against time. This shows a clear trend (r² = 0.547). This is not the case for the amount of cosmic radiation (r² = 0.005). However, the role of the solar activity can be observed. The solar maxima (1958-59, 1969-71, 1990-92, 2000-2003) can be discerned quite well.

figure3: Global temperature anomaly [°C] for 1953-2006

figure4: Level of cosmic radiation for 1953-2006

Conclusion

There is hardly an appreciable correlation between the amount of cosmic radiation and the temperature on Earth. At most it can give rise to a minor contribution. Claims that the current temperature rise on Earth can be explained by solar activity / cosmic radiation levels cannot be upheld.

Data sources:

The Cosmic ray data come from this directory:
http://www.env.sci.ibaraki.ac.jp/ftp/pub/WDCCR/STATIONS/climax/cardformat/
The card data format is explained here:
http://www.env.sci.ibaraki.ac.jp/ftp/pub/WDCCR/READ.ME
Hourly data were converted into monthly means.

The NASA GISS monthly mean land and sea surface temperature data are here:
http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts+dSST.txt
The December 2006 version was used.