From New Scientist, July 20 Issue
Magazine section: Frontiers    

Journey through milky way keeps us cool

New Scientist vol 175 issue 2352 - 20 July 2002, page 20

IN THE search for what triggers ice ages, everything from space dust to Earth's wobbly orbit has been blamed. But a much bigger mover and shaker could be to blame for these icy spells - the Milky Way itself.

Our Solar System rotates around the Milky Way's core at twice the speed of its four spiral arms, meaning we pass through each of them on our journey round the Galaxy. But the trip is made perilous by cosmic rays, deadly high-energy particles that stream out from exploding supernovae. The Milky Way's arms are peopled by dense clusters of short-lived stars going supernova and belting out these cosmic rays.

Astrophysicist Nir Shaviv of the Hebrew University in Israel wondered if meteorites landing on Earth from space might carry traces of the Solar System's movements. Radioactive potassium would be formed in the meteorites as they're blasted by cosmic rays before reaching the safety of Earth's atmosphere. The stronger the radioactive signal, the closer our Solar System probably was to the spiral arms and their cosmic rays.

After analysing the isotope ratios in 42 iron meteorites up to 2.2 billion years old, Shaviv discovered that Earth apparently passed through the Milky Way's arms roughly once every 143 million years. This time period coincides with a cycle that affects the number of ice ages.

According to Shaviv, a 10 per cent change in the cosmic rays reaching Earth would alter global temperatures by 1.3 °C. Depending on how near or far away Earth is from a galactic arm, the planet's temperature would change by up to 15 °C, easily enough to stop or start an ice age. His results will appear in the journal Physical Review Letters.

Cosmic rays seem to cool the Earth rather than heat it up. And while there is no widely accepted explanation for the phenomenon, we do know that the rays ionise the atmosphere. That might mean they create enough dust to seed clouds that would prevent sunlight from warming the planet.

Shaviv emphasises that his theory doesn't replace other explanations for ice ages, especially those for shorter cycles acting over the 20,000 to 100,000-year time frame. But research suggesting that the Sun's magnetic fluctuations can trigger ice ages (New Scientist, 15 June, p 6) may back him up, since such activity can block cosmic rays.

Atmospheric physicist Henrik Svensmark of the Danish Space Research Institute in Copenhagen says the results are convincing. What's needed now, he says, is a better understanding of how cosmic rays might affect climate.

Charles Choi

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