One unusual hypothesis we’ve covered several times in the past is the idea that cosmic rays—charged particles from beyond our solar system that constantly bombard the Earth—play a significant role in controlling Earth’s climate. The claim is that these cosmic rays are an important part of cloud formation, helping create the condensation nuclei that seed clouds by ionizing molecules, encouraging them to glom together. Because the solar magnetic field deflects many of the cosmic rays, the amount reaching the Earth, and thus the clouds on Earth, fluctuate with solar activity.

The hypothesis that this can explain climate changes has two key proponents: Henrik Svensmark and Jasper Kirkby. Recent studies associated with each of them have continued stirring the academic pot this hypothesis has been simmering in.

The secret ingredient

Kirkby organized an experiment (appropriately acronymed the “CLOUD” project) using a CERN particle accelerator to test the mechanism behind the cosmic ray hypothesis. Energetic particles from the accelerator are channeled into a precisely controlled chamber where any ultra-fine particles that form can be measured. Many teensy particles like soot or airborne ocean salt can form cloud condensation nuclei, but about half form from condensed droplets of sulfuric acid.

Early results confirmed that the experimental “cosmic rays” could increase the formation of particles, although the ones that formed must subsequently grow much, much larger before they can act as condensation nuclei. But the results also hinted at something somewhat unexpected. First, the rate at which particles formed was much lower than we know they form in the atmosphere. Second, though great care had been taken to prevent contaminants from entering the CLOUD chamber, some rogue nitrogen compounds found their way into virtually all of the particles.

In a new paper in Nature, the CLOUD team explores those puzzles. They intentionally added the simple nitrogen-containing organic compounds (called amines) to the chamber to see what would happen when more than just a few uninvited molecules were present.

It was thought that amines might have a role in the formation of these particles, but their importance wasn’t well understood. That sulfuric acid in the particles comes from the reaction of sulfur dioxide, hydroxide, and water in the atmosphere. In order for these clumps of sulfuric acid to grow, a helper needs to keep the molecules in the clump from popping back into the gas phase. Ammonia is known to be an important one and had previously been included in the CLOUD experiment. But amines can perform this job as well.

Adding just a few parts per trillion of an amine (roughly the concentration you can find in the atmosphere) raised the rate of particle formation in the CLOUD chamber to 1,000 times that seen in earlier experiments. That brought the rate up to what we observe in the atmosphere. This implies that amines are much more important than previously recognized.

About half the amines in the atmosphere come from natural (non-anthropogenic) sources, but the rest come from raising livestock. Since the concentration can vary over time or from place to place, amines could explain some of the variability in cloud formation. It’s also possible that other compounds can perform the same role if amines are lacking.

The researchers note that one technique for scrubbing CO 2 from emissions relies on the use of amines, which would increase the amount of amines we put into the atmosphere. We’ll want to figure out what impact that could have on cloud behavior before relying on that technology.

But what about the cosmic rays that motivated the experiment in the first place? In these conditions, they were largely irrelevant. As long as some amine was added to the chamber, there was no apparent difference in the rate of particle formation whether the experimental cosmic rays were on or off. Ionizing some of the molecules simply didn’t make a difference unless there was very little particle formation going on.

In a CERN press release, Jasper Kirkby qualified that “our measurements leave open the possibility that the formation of aerosols in the atmosphere may also proceed with other vapors, for which the effect of cosmic rays may be different.” However, since sulfuric acid is the dominant player for cloud formation, this significantly limits the potential influence of cosmic rays on Earth’s climate.

Jeffrey Pierce, a researcher at Colorado State who has studied the cosmic ray hypothesis, found the CLOUD study’s results interesting. “One thing that they are showing is that when amines are around, the effect of changes in cosmic rays on new-particle formation is suppressed,” Pierce told Ars. “In earlier studies by my group and other groups, we found that even if cosmic rays modulated nucleation rates substantially (e.g. a 10 percent change in cosmic rays leads to a 10 percent change in nucleation), we found that the change in [cloud condensation nuclei] was quite small (<1 percent). If cosmic rays modulate particle formation by even less, this effect would be even smaller.”

Extra sulfur on the side

“Of course,” Pierce added, “all of our previous studies assumed that the cosmic rays had no effect on the amount of condensable material (e.g. [sulfuric acid] vapors) that may grow particles to [condensation nuclei] sizes.”

Henrik Svensmark’s latest study shows that this assumption may not be valid. Svensmark has his own cloud chamber at the Danish National Space Institute (the acronym here is SKY2). Rather than hooking up to a particle accelerator, he relies on the cosmic rays that penetrate right through the building, as well as some radioactive cesium that can be brought in to crank up the ionizing radiation.

In this experiment, Svensmark and his colleagues added water vapor, ozone, and sulfur dioxide to the tank, allowing an ultraviolet light to drive the reaction that creates sulfuric acid. The number and size of particles that resulted were measured. The goal was to find out whether additional ionizing radiation affected the size of the particles that formed.

When the cesium was added (which represents a much larger change than would naturally be seen in cosmic rays), the number of particles of all sizes increased. That takes more than just adding more particles at the bottom end of the size scale. The researchers tried that by fitting the chamber with a device that constantly created an incredibly fine mist of sulfuric acid. While this resulted in more of the smallest particles, it didn’t translate into many larger particles.

The researchers think that the ionizing radiation may be helping to increase the sulfuric acid vapor surrounding the particles. If you create more small particles at the expense of the amount of vapor that’s available, it becomes more difficult for the particles to grow—less “food” is available. It could be that cosmic rays can boost the production of sulfuric acid from sulfur dioxide, which would be a different link between cosmic rays and clouds than has previously been proposed.

“This study certainly has gotten my attention, and I think the results might be important in the atmosphere, though obviously more work needs to be done in relating the effects to a range of typical atmospheric conditions,” Pierce told Ars. “I've been trying to think of global model tests that would allow us to bound the potential effects of this chemistry on [condensation nuclei], which could at least tell us if the mechanism is too weak even in the most favorable (but still realistic) situation or tell us ‘wow, there really might be something going on here.’”

While the claim that fluctuating cosmic rays are behind major climate changes has never found the supporting evidence it needed (a couple of recent studies have found no correlation between incoming cosmic rays and cloudiness in recent decades, for example), there are still a few interesting stones left unturned. Even if the cosmic rays themselves prove to be unimportant, research like the CLOUD project at CERN helps us learn more about the basic physics and chemistry underlying cloud formation.

Nature, 2013. DOI: 10.1038/nature12663 (Open Access)

Physics Letters A, 2013. DOI: 10.1016/j.physleta.2013.07.004 (About DOIs).