A scientific team, headed by Vincent Courtillot, director of the Institut de Physique du Globe in Paris, has published a hypothesis that explains global warming by linking cosmic rays from the sun to cloud formation. Here is the description of the theory from Science (Jacopo Pascotti, "Daggers Are Drawn Over Revived Cosmic Ray--Climate Link," Science, vol 319, 11 January 2008, p. 144):
To illustrate how the sun and Earth's magnetic field influence climate, Courtillot's team presented a graph depicting how fluctuations in solar brightness and the strength and orientation of the geomagnetic field shifted up and down in unison with global temperatures during the past century. This was particularly apparent, they claim, from 1940 to 1970, when a decrease in solar brightness and subsequent weakening of the geomagnetic field was followed by a 0.2°C decline in average annual global temperatures. On centurial scales, Courtillot's team speculates that a higher flux of cosmic rays seeds cloud formation; more clouds would result in lower temperature. On a millennial scale, they argue, changes in Earth's inner dynamo lead to rapid shifts of our planet's magnetic dipole. Currently, the magnetic north and south poles are located near the geographic poles, funneling cosmic rays into a bone-dry lower atmosphere. According to the team, when the dipole wanders toward more humid latitudes, more cosmic rays may interact with water vapor in the lower atmosphere, influencing cloud formation.
The Science article outlines the response of climate researchers to Courtillot's argument. Climate researchers claim that Courtillot presents nothing new and that the temperature decline of 1940 to 1970 has been adequate explained in terms of emission of, e.g., sulfate aerosols.
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