A LONG-STANDING debate about the causes of global warming may finally be solved. Cosmic rays from space really could help warm the planet by changing the way clouds form in the lower atmosphere.
The theory does not contradict the widely accepted idea that greenhouse gases are the main cause of global warming. But if greenhouse gases were the sole heating mechanism, the Earth鈥檚 surface and atmosphere would heat up at the same rate 鈥� and satellite measurements show that is not what has happened. While the surface of the planet has warmed by 0.6 掳C over the past century, the lowest eight kilometres of the atmosphere have warmed little, if at all.
Some researchers have proposed that variations in levels of cosmic rays are the key to this paradox. They believe these very high-energy particles from deep space boost cloud growth by knocking electrons off atoms, forming charged ions that can trigger water droplets to condense. Fewer cosmic rays would mean fewer clouds, so the Sun could heat the Earth鈥檚 surface directly without affecting the atmosphere (快猫短视频, 11 July 1998, p 44).
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This theory is supported by measurements of the Sun鈥檚 activity. The solar wind of low-energy charged particles that stream from the Sun deflects cosmic rays. As the Sun becomes more active and the solar wind intensifies, the theory predicts fewer cosmic rays should reach the Earth, and less cloud should form. Data from the past 20 years backs this up: as the Sun has become more active, low-altitude cloud cover has dropped.
However, critics have pointed out a glaring problem with the theory: cosmic rays don鈥檛 appear to affect cloud formation at higher altitudes, even though there are more of them at such heights. Now Fangqun Yu of the State University of New York at Albany reckons he has solved the problem.
If the concentration of charged particles produced by the cosmic rays is very high, he proposes, they are much more likely to bump into one another and recombine into neutral particles, instead of persisting long enough to trigger new clouds to form. He believes that cosmic rays make no difference to high-altitude cloud formation because there are so many ions being produced that they soon get neutralised. But at lower altitudes, the ions last long enough to trigger new clouds (Journal of Geophysical Research-Space Physics, DOI: 10.1029/2001JA000248).
Yu says it helps explain why the Earth鈥檚 surface has warmed, but not the atmosphere. 鈥淏oth greenhouse gases and cosmic rays have contributed to warming,鈥� he says. Atmospheric scientist Brian Tinsley of the University of Texas at Dallas says that because cloud cover varies with latitude, Yu鈥檚 theory may also help explain why some regions have warmed more than others.