San Francisco Chronicle, United States
July 25 2005
Ice ages linked to galactic position
Study finds Earth may be cooled by movement through Milky Way’s
stellar clouds
Keay Davidson, Chronicle Science Writer
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.
Another hypothesis
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.
Blueprint of the Milky Way galaxy
A sketch of our disc-shaped galaxy as seen from above. The “spiral
arms” are vast, arc-shaped clouds of stars. As our sun orbits the
galaxy, it occasionally passes through a spiral arm. Inside a spiral
arm, some theorists believe, our solar system is exposed to unusually
intense showers of cosmic rays that trigger cloud formation and,
perhaps, ice ages on Earth. Our solar system is presently located in
a small “spur” of clouds called the Orion Arm, located between two
larger arms known as the Perseus and Carina-Sagittarius spiral arms.