Could cosmic rays have helped cause global warming?

Physicist Henrik Svensmark, director of the Center for Sun-Climate Research at the Danish National Space Center, looks through a small cloud chamber. Courtesy of the Danish National Space Center

For the past four years, physicist Henrik Svensmark has been making clouds inside a box. He mixed a few atmospheric gases, and then bombarded them with UV rays and heavy electrons.

But Svensmark has managed to make more that just clouds—his results are making quite a stir within the mainstream climate science community.

These cloud-chamber reactions prove his controversial idea that particles from faraway exploding stars can spark cloud formation in the Earth’s lower atmosphere. This phenomenon may be a significant factor in global warming, he says.

Svensmark’s theory involves cosmic rays, which are streams of fast-moving, high-energy particles emitted from exploding stars far outside of our solar system. These particles move about in interstellar space, sometimes for millions of years.

Some rays get through to the Earth’s atmosphere where they “generate a shower of new, secondary particles,” Svensmark said. These secondary particles then trickle down toward the planet.

When cosmic rays react with the air, they form radioactive elements called isotopes. By analyzing layers of these isotopes in tree rings or ocean sediment, scientists can piece together the amount of cosmic radiation that has reached the Earth over thousands of years.

Ten years ago, Svensmark and his colleagues reported that this historical record of cosmic radiation correlated strongly with satellite data on the Earth’s cloud cover.

“It made a lot of attention at the time,” Svensmark said, “because no one could think of any explanation for why cosmic rays would form clouds.”

Svensmark proposed that as cosmic rays fell through the Earth’s atmosphere, they formed “condensation nuclei.” Water vapor molecules could then stick to the nuclei and eventually form clouds. More clouds would reflect more sunlight, and the Earth’s temperatures would drop. In contrast, fewer cosmic rays would mean fewer clouds and a warmer climate.

Previous studies suggested a decrease in the cosmic radiation reaching Earth since the beginning of the twentieth century, causing Svensmark to hypothesize that “most of the warming during the twentieth century can be explained by a reduction in low cloud cover.” 

At the time, most mainstream climatologists rejected the idea, arguing that human carbon production was the primary factor behind twentieth century warming.

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.

“I was told over and over again,” Svensmark said, “that this was only a theory, based on doubtful correlations, and that I had no real experiment to back it up.”

But in the Oct. 4 issue of the Proceedings of the Royal Society A, Svensmark reports the first experimental evidence that his theory is at least feasible. Working at the Danish National Space Center in Copenhagen, Svensmark recreated the gas chemistry of Earth’s atmosphere in a plastic chamber and used UV lamps to simulate the Sun’s rays. Cosmic rays from above also penetrated the chamber while instruments measured how these rays interacted with the gas mixture.

Electrons released by the rays rapidly catalyzed reactions, creating tiny, stable clusters of sulfuric acid and water molecules—the building blocks of cloud condensation nuclei. Svensmark’s chamber neatly demonstrated exactly how cosmic rays could make clouds.

“Now after studying this for 10 years, I think it’s completely certain that solar activity”&mdashwhich affects how much cosmic radiation reaches the Earth—“is changing the Earth’s climate,” Svensmark said.

Still, most of today’s leading climate scientists remain unconvinced.

Fangqun Yu, an atmospheric physicist at the State University of New York, Albany said that Svensmark’s work shows that cosmic radiation could lead to small sulfuric acid-water clusters.

“[But,] there exists no solid evidence that similar processes still occur as clusters grow bigger,” he said.

And even if this mechanism was shown to occur in the atmosphere, other climatologists say this wouldn’t mean it caused our current state of global warming. 

“There’s no trend whatsoever in the cosmic ray record from 1952 to now that can explain the recent warming,” said Rasmus Benestad, a senior scientist at the Norwegian Meteorological Institute.

Moreover, Benestad said, the theory doesn’t take into account one major fact: Nighttime warming trends are greater than daytime warming trends. If Svenmark’s theory was correct, he said, then warming during the day would predominate.

“The whole cosmic ray-cloud connection is a result of sloppy science, some of it downright deceptive,” said Raymond Pierrehumbert, a climatologist at the University of Chicago.

Though there may be “some remote chance” that cosmic rays played a role in past climate change, when it comes to the twentieth century, Pierrehumbert said “the observed cloud record is too short and complicated to be a reliable test” of the cosmic ray hypothesis. 
Others scientists say the complicated nature of cloud cover is exactly why Svensmark may be right.

“All calculations about the greenhouse effect and global warming assume cloud cover isn’t changing,” said Richard Muller, a physicist at the University of California, Berkeley, but it’s uncertain whether this assumption is correct.

The question of cloud cover, Muller said, “is the most critical unanswered question in the entire global warming debate.”

Originally published October 11, 2006


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