The Abstract states:
Close passages of coronal mass ejections from the sun are signaled at the Earth's surface by Forbush decreases in cosmic ray counts. We find that low clouds contain less liquid water following Forbush decreases (FDs), and for the most influential events the liquid water in the oceanic atmosphere can diminish by as much as 7%. Cloud water content as gauged by the Special Sensor Microwave/Imager (SSM/I) reaches a minimum around 7 days after the Forbush minimum in cosmic rays, and so does the fraction of low clouds seen by the Moderate Resolution Imaging Spectroradiometer (MODIS) and in the International Satellite Cloud Climate Project (ISCCP). Parallel observations by the aerosol robotic network AERONET reveal falls in the relative abundance of fine aerosol particles which, in normal circumstances, could have evolved into cloud condensation nuclei (CCN). Thus a link between the sun, cosmic rays, aerosols, and liquid-water clouds appears to exist on a global scale.
The paper concludes:
Our results show global-scale evidence of conspicuous influences of solar variability on cloudiness and aerosols. Irrespective of the detailed mechanism, the loss of ions from the air during FDs reduces the cloud liquid water content over the oceans. So marked is the response to relatively small variations in the total ionization, we suspect that a large fraction of Earth's clouds could be controlled by ionization. Future work should estimate how large a volume of the Earth's atmosphere is involved in the ion process that leads to the changes seen in CCN and its importance for the Earth's radiation budget. From solar activity to cosmic ray ionization to aerosols and liquid-water clouds, a causal chain appears to operate on a global scale.
Svensmark, H., T. Bondo, and J. Svensmark (2009),
Cosmic ray decreases affect atmospheric aerosols and clouds,
Geophysical Research Letters, doi:10.1029/2009GL038429, in press.
(accepted 17 June 2009)
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