Cosmic Ray Levels and Cloud-forming effects on temperature

March 12, 2010 by jason

I have previously written an article on why Cosmic ray levels do not directly affect climate change (ie. why the changes in radiative energy reaching earth do not directly affect the earth's temperature). I thought it might now be relevant to address a slightly more complex issue - the indirect effects of Cosmic ray levels on global temperature.

The reason I divided this topic into two separate articles, was because the first (the direct effect of cosmic ray levels on the earth's climate is basically an open and shut case - cosmic ray levels do not directly affect the climate). To illustrate why the story of cosmic ray levels is a little more complex, I will quote a danish paper titled Influence of Cosmic Rays on Earth's Climate, where the author states that

Source: Influence of Cosmic Rays on Earth's Climate

This paper goes on to discuss the relevance of solar cycle length on temperature, which I will cover in a later article. In this article I will only discuss the indirect effects of cosmic ray levels on global temperature.

Effects of Cloud Cover

The first question in need of answering, is whether cloud cover can affect global temperature at all. There is no question that clouds affect the weather, but to answer whether cloud cover can have a significant long-term effect on the earth's global climate, or whether it is just a bystander, we can look at this graphic:

Scatter plot showing total monthly cloud cover against monthly global surface air temperature from July 1983 until June 2009 (Source: The International Satellite Cloud Climatology Project)

From this we can see a clear correlation between cloud cover and global temperature. Based on the linear fit coefficient of determination, cloud cover explains 26% of temperature variation. Whether cloud cover actually drives these changes is not evident from the graph, but we can at least conclude there is some relationship between the two.

How Cosmic Ray Levels affect climate

To answer the question of whether cosmic ray levels can affect the world temperature by affecting cloud cover, we don't really need to look farther than a plot of global temperatures against cosmic ray levels over the past 40,000 years (for more on how this is done, see my description of how historic information about cosmic ray levels is obtained). By taking data from the EPICA Ice Core records we can obtain the following plot of global temperature levels against cosmic ray levels for the past 40,000 years:

Plot of D18O concentrations (temperature levels) against the inverse of Be10 concentrations (indicator for the cloud forming tendency of cosmic ray levels) over the past 40,000 years (Data Source: EPICA Ice Core Data)

What is initially striking about this graph is that the fitted lines seem to correlate for most of the period. However, if you look at the left extreme of the graph, where large and rapid changes in cosmic ray levels occur, we should expect to see a noticeable change in temperature. It seems like only small and drawn out changes in cosmic ray levels would affect temperature, which doesn't make a lot of sense. There is one particular point of interest though, indicated in the graph with an arrow, where a strong correlation does seem to occur. Based on the above graph alone however, it does seem to be difficult to ascertain whether there is a correlation. To graphically illustrate whether or not there is a correlation, we can plot each temperature/cosmic ray level point for a given period of time, as points on a scatter plot:

Scatter plot of D18O concentrations (temperature levels) against Be10 concentrations (indicator for cosmic ray levels) over the past 40,000 years (Data Source: EPICA Ice Core Data)

From this we can see a general trend, that extremely low cosmic ray levels tend to produce higher temperatures (this is indicated by the dense cluster of points at high D18O levels and low Be19 concentrations), but higher cosmic ray concentrations do not really affect climate at all. This is presumably because particularly low cosmic ray concentrations significantly reduce cloud cover and thus force an increase in temperature. Why particularly high concentrations of cosmic ray levels would not significantly decrease temperature (by increasing cloud cover) is unknown.