Do you ever worry that people will take your findings and use them to support unwarranted or even harmful conclusions?
I would be happy to kill the project if I could find out that there was something that didn’t fit or that I no longer believed in it. When we started, it was just a simple hypothesis based on a correlation, and correlations are, of course, something that could be quite dubious, and they could go away if you get better data. But this work has only strengthened itself over the years.
What first made you suspect that changes in the sun are having a significant impact on global warming?
I began my investigations by studying work done in 1991 by Eigil Fiin-Christensen and Knud Lassen Fiin-Christensen. They had looked at solar activity over the last 100 years and found a remarkable correlation to temperatures. I knew that many people dismissed that result, but I thought the correlation was so good that I could not help but start speculating—what could be the relation? Then I heard a suggestion that it might be cosmic rays, changing the chemistry high up in the atmosphere. I immediately thought, “Well, if that is going to work, it has to be through the clouds.”
That was the initial idea. Then I remembered seeing a science experiment at my high school in Elsinore, in which our teacher showed us what is called a cloud chamber, and seeing tracks of radioactive particles, which look like small droplets. So I thought to myself, “That would be the way to do it.” I started to obtain data from satellites, which actually was quite a detective work at that time, but I did start to find data, and to my surprise there seems to be a correlation between changes in cosmic rays and changes in clouds. And I think in early January 1996, I finally got a curve, which was very impressive with respect to the correlation. It was only over a short period of time, because the data were covering just seven years or something like that. So it was almost nothing, but it was a nice correlation.
How exactly does the mechanism work, linking changes in the sun with climate change on Earth?
The basic idea is that solar activity can turn the cloudiness up and down, which has an effect on the warming or cooling of Earth’s surface temperature. The key agents in this are cosmic rays, which are energetic particles coming from the interstellar media—they come from remnants of supernova explosions mainly. These energetic particles have to enter into what we call the heliosphere, which is the large volume of space that is dominated by our sun, through the solar wind, which is a plasma of electrons, atomic nuclei, and associated magnetic fields that are streaming nonstop from the sun. Cosmic-ray particles have to penetrate the sun’s magnetic field. And if the sun and the solar wind are very active—as they are right now—they will not allow so many cosmic rays to reach Earth. Fewer cosmic rays mean fewer clouds will be formed, and so there will be a warmer Earth. If the sun and the solar wind are not so active, then more cosmic rays can come in. That means more clouds [reflecting away more sunlight] and a cooler Earth.
Now it’s well known that solar activity can turn up and down the amount of cosmic rays that come to Earth. But the next question was a complete unknown: Why should cosmic rays affect clouds? Because at that time, when we began this work, there was no mechanism that could explain this. Meteorologists denied that cosmic rays could be involved in cloud formation.
You and a half-dozen colleagues carried out a landmark study of cosmic rays and clouds while working in the basement of the Danish National Space Center. How did you do it?
We spent five or six years building an experiment here in Copenhagen, to see if we could find a connection. We named the experiment SKY, which means “cloud” in Danish. Natural cosmic rays came through the ceiling, and ultraviolet lamps played the part of the sun. We had a huge chamber, with about eight cubic meters of air, and the whole idea was to have air that is as clean as you have over the Pacific, and then of course, to be able to control what’s in the chamber. So we had minute trace gases as you have in the real atmosphere, of sulfur dioxide and ozone and water vapor, and then by keeping these things constant and just changing the ionization [the abundance of electrically charged atoms] in the chamber a little bit, we could see that we could produce these small aerosols, which are the basic building blocks for cloud condensation nuclei.
So the idea is that in the atmosphere, the ionization is helping produce cloud condensation nuclei, and that changes the amount and type of clouds. If you change the clouds, of course, you change the amount of energy that reaches Earth’s surface. So it’s a very effective way, with almost no energy input, to change the energy balance of Earth and therefore the temperature.
