Researchers studying global warming have often been confounded by the differences between observed increases in surface-level temperatures and unchanging low-atmosphere temperatures. Because of this discrepancy, some have argued that global warming is unproven, suggesting instead that true warming should show uniformly elevated temperatures from the surface through the atmosphere. Researchers have proposed a theory that changes in cloud cover could help explain the puzzling phenomenon, but none-until now-have come up with an argument that could account for the varying heat profiles.

A study in the July 2002 issue of Journal of Geophysical Research-Space Physics, published by the American Geophysical Union, proposes for the first time that interstellar cosmic rays could be the missing link between the discordant temperatures observed during the last two decades (since recorded satellite records began in 1979). The report, by Fangqun Yu of the State University of New York-Albany, proposes that the rays, tiny charged particles that bombard all planets with varying frequency depending on solar wind intensity, may have height-dependent effects on our planet's cloudiness. Previous research has proposed a link between cosmic rays and cloud cover, has not suggested the altitude dependence of the current study.

"A systematic change in global cloud cover will change the atmospheric heating profile," Yu said. "In other words, the cosmic ray-induced global cloud changes could be the long-sought mechanism connecting solar and climate variability."

The hypothesis, if confirmed, could also shed light on the Sun's role in global warming. The amount of cosmic rays reaching Earth depends on solar winds, which vary in strength by space-weather conditions. Yu points out that indications of Earth's warming have coincided with decreased cosmic ray intensity during the 20th century. Such explanations for natural causes of global warming do not rule out human contributions to temperature change, but present the possibility that humans are not solely responsible for some of the observed temperature increases.

In addition, recent satellite data have revealed a correlation between cosmic ray intensity and the fraction of the Earth covered by low clouds. Yu proposes that the amount and charge of cosmic ray-generated ions can contribute to the formation of dense clouds by stimulating the production rate of low-atmosphere particles that make the clouds more opaque. In addition, natural and man-made differences in atmospheric chemistry, like greenhouse gas concentrations, can also affect the cosmic rays' influence on clouds, according to Yu. Such height-dependent atmospheric differences can increase the quantity of ambient particles in the lower troposphere and decrease the particles in the upper air, thus affecting the type of cloud cover.

High clouds, for example, generally reflect sunlight while lower clouds tend to retain surface energy; both effects are scientifically well established and have a significant effect on global temperatures. The data provides evidence supporting Yu's claim that cosmic ray-induced cloud changes may have warmed the Earth's surface but cooled the lower troposphere, which could provide an explanation of the Earth's varying temperature trends.

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The research was supported by the National Science Foundation.

Contact: Harvey Leifert, hleifert@agu.org, 202-777-7507, American Geophysical Union

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