Most of Earth's clouds get their start in deep space. That's the surprising conclusion from a team of researchers who argue that interstellar cosmic rays collide with water molecules in our atmosphere to form overcast skies.
As common as clouds are on Earth, the processes that produce them are not well understood. Scientists think particles of dust or pollen can serve as nuclei for water droplets, which in turn gather by the trillions into clouds. That would help explain how clouds form over urban areas: Fine particles called aerosols are emitted from the exhaust pipes of millions of vehicles and work their way into the atmosphere, where they are thought to attract water molecules. But it doesn't explain how clouds formed in preindustrial society--or how they form today over vast stretches of rainforest and ocean.
That's where cosmic rays come in. The idea goes like this: High-speed cosmic ray particles--protons and neutrons of still-mysterious origins that travel at nearly the speed of light--collide with water molecules in the atmosphere, stripping away electrons from those molecules and converting them into electrically charged ions. The ions then begin attracting other water molecules, which eventually form clouds.
The theory seems to hold water in the lab. In 2006, physicist Henrik Svensmark of the Technical University of Denmark in Copenhagen and colleagues produced aerosols artificially in an atmospheric chamber by bombarding water molecules with a particle beam. "More ions resulted in more aerosols," Svensmark says.
In the new study, Svensmark's team wanted to see if the idea also worked in the real world. The researchers focused on a phenomenon known as a Forbush decrease. Here, a massive storm on the sun's surface flings a superhot fog of particles, called a coronal mass ejection, past Earth, temporarily shielding our planet from cosmic rays. If cosmic rays really do contribute to cloud formation, Svensmark and colleagues hypothesized, then cloud cover should dip during Forbush decreases.
And indeed that's what Svensmark's team found. When the researchers examined cloud data collected by weather satellites over the past 22 years and compared them with 26 Forbush decreases, they discovered that, for the five strongest events, the water-droplet content of Earth's clouds decreased by an average of 7%. It's like bare patches forming in a field, says Svensmark, whose team reports its findings this month in Geophysical Research Letters. The cloud patterns eventually returned to normal, he says, but they took weeks to do so. "We're now convinced that aerosols are affected by the Forbush decrease," Svensmark says.
Geoscientist Jón Egill Kristjánsson of the University of Oslo, Norway, calls the findings "astonishing." He and other researchers have searched for years for relationships between Forbush decreases and cloud formation and have found nothing, or they have found significant relationships "only in very remote locations." If the data can be confirmed by other observations, he says, "Svensmark's new results would greatly strengthen the case for a cosmic ray-cloud connection."
Svensmark argues that the findings suggest a link between cosmic rays and climate change. Because clouds bring rain and reflect light from the sun, fewer clouds would mean a warmer Earth. But Kristjánsson isn't willing to go that far. Monitoring instruments "over the last 50 years or so show either no trend or a slightly upward trend" in cosmic rays hitting Earth, he notes. According to Svensmark's theory, that would mean either no increase in cloud formation or a slight increase--neither of which would warm the world.