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,
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.
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