Nearer Neighbors Have Greater Influence

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Science  13 Jul 2001:
Vol. 293, Issue 5528, pp. 177
DOI: 10.1126/science.293.5528.177a

Clouds help to control climate by reflecting or absorbing solar radiation, thereby affecting Earth's albedo. Cloud droplet formation depends on the presence of condensation nuclei, which can be produced by ionizing radiation. Is cloud cover modulated by changes in the intensity of ionizing galactic cosmic rays (GCRs)? Udelhofen and Cess analyzed cloud cover anomalies over the United States from 1900 to 1987, as well as the output of a model of cloud formation. They found a significant correlation, in both observations and model data, between cloud cover and the 11-year cycle of solar variability, and a strong time-series spectral peak at 11 years. The cloud cover variations were in phase with the solar cycle but not with GCRs, and the model did not reproduce these variations when a constant solar forcing was used. These observations indicate that variations in atmospheric heating caused by solar forcing, but not by GCR changes, are responsible for the recorded changes in cloudiness. — HJS

Geophys. Res. Lett. 28, 2617 (2001).

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