Myth: Our current global warming is due primarily to a decrease in cosmic ray intensity.

Fact: The fact is that this theory is extremely new, and has only recently been considered possible. There are still many questions left to be answered and more work to be done on quantifying its effect before such a statement could have any validity. Before we get into the details of why this is, I’d like to explain the theory itself a little better.

Basically, this theory, researched by a scientist named Henrick Svensmark, claims that the Sun indirectly affects our climate. The first thing to understand is that clouds generally have a cooling effect on the planet, through an effect called albedo: they reflect sunlight back into space. Next, we have cosmic rays, which are "energetic particles originating from space that impinge on Earth’s atmosphere". Cosmic rays primarily consist of protons, and they originate from outside our solar system. The Sun also emits rays, called solar cosmic rays, which have much lower energy than cosmic rays. When there is an increase in solar activity, the increased solar cosmic ray intensity causes a decrease in the other cosmic rays, called the Forbush decrease.

According to the cosmic ray flux theory, cosmic rays, upon entering our atmosphere, produce small particles called aerosols, which aid in cloud formation. So, more cosmic rays means more clouds and a greater cooling effect. However, this theory’s proponents claim that we are currently in a period of increased solar activity, which is causing less cosmic rays to enter our atmosphere. This means less clouds and therefore, a warming effect.

From the outset, Svensmark’s research has been met with criticism. His problems started when he claimed a correlation between cosmic ray intensity and global cloud cover based on limited data:

One of the biggest criticisms levied against Svensmark was that he had used data from a satellite that did not measure total global cloud cover. When other researchers plugged in a more comprehensive data set, the correlations Svensmark found between cosmic radiation and the Earth’s global cloud cover broke down.

So Svensmark tweaked his theory: Though clouds at middle and high levels are unaffected by cosmic rays, he said, low-level cloud formation was still highly correlated.

Criticism lingered.

Criticism has lingered because, while he’s found a correlation with low-level clouds, his experiments do not reflect real-world conditions. In his experiments, he has taken a mixture of water, sulphur dioxide, ozone, and air, and bombarded it with high energy UV light. According to Eli Rabett, this experiment contained about "5000 times more SO2 than in places where clouds actually form" and "more ozone than in the ozone layer". There is also concern over the use of high energy UV, which according to Gavin Schmidt at RealClimate, "never penetrates the lower troposphere." This is quite a problem, considering Svensmark’s supposed correlation is with low-level cloud formation. Schmidt also listed several additional questions that still need to be answered:

First, the particles observed in these experiments are orders of magnitude too small to be Cloud Condensation Nuclei (CCN). In the press release, this is why they talk about the ‘building blocks’ of CCN, however, aggrandisation of these small particles is in no sense guaranteed (Missing step #1). Secondly, the focus is on low clouds over the ocean. However, over the ocean, there are huge numbers of condensation nuclei related to sea salt particles. Thus to show that the cosmic ray mechanism is important, you need to show that it increases CCN even in the presence of lots of other CCN (Missing step #2). Next, even if more CCN were made, you would need to show that this actually changed cloud cover (or optical thickness etc.) (Missing step #3). And given that change in cloud properties, you would need to show that it had a significant effect on radiative forcing - which despite their hand waving, is not at all well quantified (even the sign!) (Missing step #4). Finally, to show that cosmic rays were actually responsible for some part of the recent warming you would need to show that there was actually a decreasing trend in cosmic rays over recent decades - which is tricky, because there hasn’t been (see the figure) (Missing step #5).

I’d like to clarify here that the purpose of this article is not to discredit Svensmark’s research. The study is peer-reviewed, and is for that reason, at least feasible. For all we know these questions could be answered in the future, with additional research. The point I want to make is that this is a very new theory. Many skeptics have latched onto this theory, claiming that it explains 20th century warming, and therefore debunks anthropogenic global warming. On the contrary, Svensmark’s work has created more questions than it has answered, and is therefore nowhere near solid enough to warrant such bold claims.