| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Skywatcher Cosmic Weather
They sound like details from those old Flash Gordon films, not forces in the real world. But strangely enough, cosmic rays do exist. During the explosion that transforms a dying star into the pyrotechnic flash of a supernova, these subatomic particles are hurled into the universe at the speed of light and constantly pepper our planet like so much invisible confetti. Scientists once considered them just another harmless component of the energetic particle swarm drifting through the universe. But recent evidence suggests that cosmic rays do much more than simply collide with Earth. Taken together, the energy they add to our planet’s atmosphere may be powerful enough to trigger ice ages. The first evidence linking cosmic rays and climate seemed, at the time, to have more to do with the sun. In the 1780s, British astronomer William Herschel noted that years during which dark spots appeared on the face of the sun would bring bountiful wheat harvests. His findings set in motion centuries of speculation about how changes in the sun affect weather here on Earth. In the late twentieth century, scientists realized that sunspots are actually the symptoms of an unusually vigorous stage in the sun’s rhythm, when extra energy streams from its incandescent surface. But only about one percent more solar energy arrives in the atmosphere during these power surges. Satellite data show that such a tiny boost in wattage isn’t capable of warming the atmosphere enough to affect weather. Disappointed, scientists turned their attention back to the sun, source of their first and only climate clue. Decades of solar observations had by then revealed a whole new dimension to the sun’s interactions with Earth. At the peak of its activity, the sun kicks up such a firestorm that the river of energy and particles it streams toward Earth gains even greater reach and power. The surging solar wind sent the Skylab satellite to a premature crash landing in 1979, and scientists watched a breezy electromagnetic bow wave envelop the entire planet in 2000. Besides swamping satellites and providing a year of extraordinarily vibrant northern lights, the solar wind also disrupted the cosmic environment that normally surrounds Earth. Like a stiff breeze scattering water droplets from a sprinkler, at full power the solar wind sends the stream of space particles on a curving detour around the planet. Among the types of particles that go missing from Earth’s atmosphere during sunspot years are cosmic rays. But exactly how these rays affect weather has remained somewhat of a mystery. The most likely answer would be via clouds. Among the first to find evidence of this kind were Henrik Svensmark and Eigil Friis-Christensen of the Danish Meteorological Institute in Copenhagen. In 1997, they reported a correlation between cloudy days and high cosmic ray concentrations. The influx of high energy particles, they suggested, was encouraging clouds to form. Scientists have now deduced a plausible mechanism by which cosmic rays could spark cloud formation. In 2002, atmospheric scientist Fangqun Yu of the State University of New York, Albany, reported in the Journal of Geophysical Research that by ionizing tiny particles in Earth’s atmosphere, the energy of cosmic rays helps these particles combine and grow large enough for water droplets to condense around. Because the extra blanket of cloud cover reflects more solar heat than it traps, the planet cools down. “People didn’t think that cosmic rays had enough energy to affect the climate itself; the energy they deliver is small compared to the amount of energy delivered by sunlight,” Yu says. “But now, if cosmic rays can affect clouds, that amplifies their signal to a level that may indeed affect climate on Earth.” Just this October, researchers at the Max-Planck Institute of Nuclear Physics in Germany found more data to support Yu’s theory. During scouting flights, they observed charged particle clusters capable of forming cloud condensation nuclei. The clusters, they say, were likely formed by cosmic rays. Yu’s mechanism for growing cloud nuclei may be put to the test in the Stanford Linear Accelerator’s atom smasher within the next few years. “There’s no knob in the atmosphere to change the intensity of the rays and see what their effect on Earth is,” says Jasper Kirkby, a particle physicist with CERN in Switzerland. “But with particle accelerators, we can make perfect reproductions of cosmic rays that we can measure and turn on and off to test the precise effects of ionization in cloud formation.” Linking cosmic rays to changes in global temperature is one thing. But saying they are responsible for the ice ages is entirely another. Solar activity, which controls the flow of cosmic rays to Earth like a spigot, waxes and wanes on a predictable eleven-year cycle; a series of ice ages occurs every 145 million years or so. Now Nir Shaviv of the Racah Institute of Physics in Jerusalem, Israel, has found evidence to suggest that the cosmic ray environments Earth encounters in its travels through the galaxy could be responsible for triggering planetary deep freezes. From a vantage point light years outside our galaxy, the Milky Way resembles a giant pinwheel made of stars and spiralling dust clouds. As far more stars are found inside each curving blade than outside, most supernovae must occur within the arms. When our planet is within these arms, Shaviv realized, it probably encounters far more cosmic rays. Calculations show that Earth travels in and out of these arms every 143 million years or so—a pace that roughly mirrors the 145-million-year timescale between the ice ages. To bolster his theory, Shaviv needed a way to measure and compare the variations in cosmic ray intensity that Earth was exposed to during its travels. Unable to leave the solar system himself, Shaviv interrogated the only space travelers available: meteorites. As they zip through space, these vagrants are exposed to the region’s prevalent concentration of cosmic rays. The energy of the rays breaks down iron atoms on the meteoroids’ surfaces in a process known as spallation damage. Falling to Earth essentially halts spallation because so few cosmic rays penetrate deep into our atmosphere. In a study recently published in the journal Physical Review Letters, Shaviv estimated the Earth’s exposure to cosmic rays over time by examining 42 iron meteorites up to 2.2 billion years old. He first calculated the age of the space rocks using isotope ratios, and then determined where in the galaxy (inside or outside of a spiral arm) the Earth was likely located when they fell. He hen obtained rough estimates of the concentrations of cosmic rays present in these areas by measuring the extent of each meteorite’s spallation damage. The results were striking. He found that meteorites Earth collected during its passage through the spiral arms sustained up to ten percent more cosmic ray damage than others. That kind of cosmic ray variation, Shaviv suggests, could alter global temperatures by as much as 15 percent—easily enough to turn the ice ages on or off. “Everything I checked seemed to fit together like pieces in a puzzle,” Shaviv says. As farfetched as the idea might sound, Kirkby says, Shaviv’s billion years of supporting data make a compelling argument. “If I were a police inspector, I’d say there was so much circumstantial evidence to connect cosmic rays and Earth’s climate that there’s something to it.”
Kathleen M. Wong is Senior Editor of California Wild |
