If you're going to question the understanding of our climate that has built through more than a century of observation and experiment, you have to do two things. One is to show that this understanding is fundamentally limited in some way. The second is to provide an alternate explanation for the patterns in the climate.

One of the ideas that has seemingly fit the bill is a connection between radiation and clouds. Ionizing radiation from cosmic rays and other sources could create ions in the atmosphere, which could then draw in water molecules. These would form the building blocks of clouds, which would then influence the climate. The amount of radiation reaching the atmosphere does change, in part because the Sun's magnetic field, which protects the Earth from incoming radiation, varies over time.

Put these ideas together, and variations in the Sun's magnetic field could have an influence on the climate. It's a nice idea, but the latest paper from CERN should finally put it to rest. (But of course, it probably won't.)

The idea of a cosmic ray-cloud influence on the climate has a few things going for it. For one, there are some indications that past climate shifts were accompanied by changes in solar activity. Obviously, that would include the light the Sun sent our way, but the changes in incoming radiation wouldn't be sufficient to drive the amount of change that occurred. This leaves open the prospect that the Sun has additional influences on the climate, such as through the strength of its magnetic field.

We already know that the magnetic field can protect us from ionizing radiation. So, if the ions that result can form clouds, then the Sun's activity can influence cloud cover. A few people found a connection between the incoming radiation and cloud cover, and we already knew clouds could influence the climate. For some people on the Internet, that was enough to argue that the Sun was driving everything, and humans have an insignificant influence on the climate.

Of course, that was always the best case for the solar influence. More careful looks at the connection between incoming radiation and cloud cover generally found no correlation at all; meanwhile, temperatures kept rising even as solar activity dropped. Plus clouds have a complex relationship to climate, reflecting sunlight or insulating the planet, depending on their location and structure.

Finally it was never entirely clear that radiation could drive cloud formation in the first place.

That last point is where CERN, the people who built the Large Hadron Collider, stepped in. They're very adept at creating radiation in a controlled environment and figured they could get to the bottom of how that radiation could influence cloud formation. Preliminary results looked promising, but it later turned out that the presence of even trace chemical contaminants could completely swamp the influence of the radiation.

Now, the same team is back with a follow-up paper that delves into the big picture of cloud formation. After years of testing, the team had exposed water vapor to a variety of conditions: different temperatures, the presence of various chemicals like sulfuric acid and ammonia, and the presence or absence of radiation. In each condition, the number of particles that water condensed on were measured.

This allowed the team to build a model of what is going on in the actual atmosphere. If you feed the conditions of the atmosphere to the model—different levels of chemicals and radiation at a given temperature—it can predict the level of cloud droplet formation that occurs. The model was compared to real-world measurements, and it turned out to be fairly accurate.

The authors then started pulling different factors out of the model and seeing how well it worked. This allowed them to identify which factors had the biggest influence on cloud formation.

Most of the effect on the formation of cloud particles is driven by sulfuric acid, ammonia, or molecules from a biological source. While ions are often included in these particles, they aren't major drivers of this process. "The relatively weak dependence on ion concentrations," the authors conclude, "indicates that, for the processes studied, variations in cosmic ray intensity do not significantly affect climate via nucleation in the present-day atmosphere."

Really, given all the other information that has come in, this should be the end of the line for the idea that cosmic rays are controlling our climate. But many academics have a hard time giving up on ideas they're fond of. And, in this case, there's a segment of the public that's anxious to believe them.

Science, 2016. DOI: 10.1126/science.aaf2649 (About DOIs).