Low Clouds and Cosmic Rays

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Science  15 Dec 2000:
Vol. 290, Issue 5499, pp. 2033
DOI: 10.1126/science.290.5499.2033b

If the study of global climate change were a card game, one of the wild cards would be the role of clouds. Clouds are a primary influence on the energy budget of Earth's surface and atmosphere because of their effects on the reflection and absorption of solar radiation and their trapping of outgoing long-wave radiation. Clouds differ in their radiative properties, however, and the complexity of cloud formation is greater than our understanding of all of the factors that control their distribution and composition. Solar cosmic rays may influence global cloud cover because they can ionize atmospheric particles and thus create condensation nuclei for cloud droplet formation. The terrestrial cosmic ray flux depends on solar output and is modulated by Earth's magnetic field; both of these quantities are known to vary.

Marsh and Svensmark have measured global average monthly cloud anomalies for lower, middle, and upper troposphere, and correlated them with changes in the cosmic ray flux. They found, surprisingly, that cloud cover at altitudes of less than 3.2 kilometers covaries with cosmic ray fluxes from 1980 to 1995, but no correlation was seen for higher altitude clouds. If this relation is systematic, cosmic ray variability could have a significant effect on the evolution of climate. — HJS

Phys. Rev. Lett.85, 5004 (2000).

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