Study finds Earth may be cooled by movement through Milky Way's stellar clouds
Keay Davidson, Chronicle Science Writer
Monday, July 25, 2005
It might sound preposterous, like astrology, to suggest that galactic events help determine when North America is or isn't buried under immense sheets of ice taller than skyscrapers. But new research suggests the coming and going of major ice ages might result partly from our solar system's passage through immense, snakelike clouds of exploding stars in the Milky Way galaxy.
Resembling the curved contrails of a whirling Fourth of July pinwheel, the Milky Way's spiral arms are clouds of stars rich in supernovas, or exploding stars. Supernovas emit showers of charged particles called cosmic rays.
Theorists have proposed that when our solar system passes through a spiral arm, the cosmic rays fall to Earth and knock electrons off atoms in the atmosphere, making them electrically charged, or ionized. Since opposite electrical charges attract each other, the positively charged ionized particles attract the negatively charged portion of water vapor, thus forming large droplets in the form of low-lying clouds.
In turn, the clouds cool the climate and trigger an ice age -- or so theorists suggest.
In that regard, researchers are finding correlations between the timing of Earth's ice ages and epochs when our solar system passed through galactic spiral arms.
The latest evidence appears in the June 20 issue of Astrophysical Journal. The article is the result of an unusual collaboration between an astronomer, Professor Douglas Gies of Georgia State University's Center for High Angular Resolution Astronomy, and a 16-year-old student at Grady High School in Atlanta, John Helsel. They report the results of their effort to determine how the sun has moved through the galaxy over the last half-billion years.
Difficult to map
By making a variety of assumptions about the rate of solar motion and the distribution of spiral arms in the galaxy -- which are difficult to map because galactic dust and foreground stars get in the way -- Gies and Helsel conclude that "the sun has traversed four spiral arms at times that appear to correspond well with long-duration cold periods on Earth."
"This," they continue, "supports the idea that extended exposure to the higher cosmic-ray flux associated with spiral arms can lead to increased cloud cover and long ice age epochs on Earth."
Gies and Helsel's article is the long-term result of a project that Helsel began working on "as a science fair project," Gies says. Gies, 50, is a neighbor of Helsel's. Gies had previously "developed a scheme to model the motion of some massive stars in the galaxy," and when Helsel approached him for guidance on the science fair project, their "conversation quickly focused on studying the sun's motion and encounters with spiral arms in the galaxy."
A veteran investigator of the galaxy-ice age hypothesis is astrophysicist and assistant professor Nir Shaviv, 33, of Racah Institute of Physics at Hebrew University in Jerusalem, who was previously a postdoctoral researcher at the California Institute of Technology. He has reanalyzed other scientists' previously published data on meteorites, which contain mildly radioactive isotopes -- fragments of atoms that were altered by cosmic-ray bombardments over millions of years while the meteorite was still hurtling through space. Based on the ages of different isotopes, he concludes the cosmic-ray bombardments were most intense during past epochs when Earth is believed to have passed through known spiral arms.
An alternate but related hypothesis of ice ages suggests that Earth occasionally passes through huge interstellar clouds of hydrogen gas. Such clouds are common in the spiral arms. According to this hypothesis, the interstellar clouds chemically soak up oxygen molecules in Earth's atmosphere, dramatically lowering the levels of the gas ozone.
Because ozone normally heats the atmosphere by trapping infrared radiation, a decline in ozone could cool Earth and "may trigger an ice age of relatively long duration," the astrophysicists Ararat Yeghikyan of Armenia and Hans Fahr of Germany proposed last year in the journal Astronomy & Astrophysics.
Many other factors involved
Galaxy-ice age theorists caution that their findings are only tentative and that many other factors also affect the timing of ice ages.
Still, their research probably has long-term practical value. That's because it could eventually help scientists to better distinguish between "normal" global climate change caused by Mother Nature's whims, such as the passage through a spiral arm, and climate change caused by humans -- such as drivers whose fossil-fuel-burning cars contribute to global warming.
Various versions of the galaxy-ice age hypothesis have kicked around the peripheries of the climatological and astronomical communities since at least the 1970s. Until recently, though, such hypotheses have received relatively scant scientific attention.
One reason for the neglect is that climate change is a dauntingly complex topic, one in which causes of any event -- even a simple rain shower in downtown San Francisco -- have innumerable short-term and long-term causes.
Another likely reason is that climatologists and astronomers are two scientific communities that rarely interact because their interests, background, training and funding sources are so different. Like most scientists, they hesitate to tread on unfamiliar intellectual turf for fear of making naive mistakes.
But climatologists and galactic astronomers have at least one thing in common: a grand sense of time. Both deal with events -- such as the comings and goings of ice ages and the slow spinning of the Milky Way -- that require them to use clocks timed in hundreds of thousands or millions of years. This gives them a common language of discourse, like a tourist to France who doesn't speak French but can crudely communicate with a chef via their common knowledge of French cuisine.
Relevant to ozone thesis
Although Yeghikyan and Fahr's proposed ozone explanation for certain ice ages differs from the cosmic-ray thesis, "I take the idea presented by Gies and Helsel as absolutely serious" and relevant to the ozone thesis, Fahr said in an e-mail. That's because passage through a spiral arm would increase Earth's exposure to the dense interstellar clouds, which are common within the arms, Fahr noted.
Other scientists view the galaxy-ice age hypothesis with cautious interest.
On the one hand, astrophysicist Erik Leitch of Caltech says the Gies and Helsel paper is "a suggestive result." It "is not unreasonable" to infer that the solar system, while passing through a spiral arm, would experience more intense cosmic ray bombardment because "the spiral arms seem to be the main sites of star formation in the galaxy, and the massive stars which become supernovae don't live long enough to travel very far out of the arms before they explode."
Therefore, Leitch said, "if you're in a spiral arm, you're much more likely to be near a massive star about to explode than if you're not" -- and hence, in turn, likelier to be exposed to intense bursts of cosmic rays.
On the other hand, Leitch warns, just because Earth occasionally passes through unusually intense showers of cosmic rays doesn't mean those showers will trigger ice ages. Regarding the Gies and Helsel paper, the proposed connection between cosmic-ray surges and cooling periods "seems more tenuous to me. ... Cosmic rays may 'seed' more cloud cover, but it's not clear to me that increased cloud cover will always lead to cooling."
According to some computer models, he explained, clouds can act not only like a sunshade but also like a blanket -- that is, clouds not only shield Earth from solar rays but also trap infrared heat radiated by the ground. It's anyone's guess whether the net effect of increased cloud cover would cool or warm the climate.
Shaviv disagrees: He is confident that low-altitude clouds "have a clear cooling effect."
Karen Aplin of the Rutherford Appleton Laboratory in Oxford, England, who has studied links between atmospheric ionization and cloud formation, observed: "The climate system is extremely complex, with many feedbacks, and it is not at all straightforward to establish that these (links between cosmic rays and clouds) exist."
In a 2001 article co-written with R.G. Harrison, Aplin "showed that ions formed by cosmic rays can make small particles, condensation nuclei, in the atmosphere," she said. There's a catch, though: "These particles are too small to act as cloud condensation nuclei. ... To trigger cloud formation, they would have to live for quite a while and grow many times bigger."
Whether they do so -- and if so, how -- remains an open question.
"The assumption that an increase in cosmic rays causes an atmospheric response, which, in turn, causes ice ages is a large one, although it's not impossible," her colleague Harrison told The Chronicle.