engleski

Sudden Cosmic Ray Decreases: No Change of Global Cloud Cover

According to the cosmic ray cloud connection hypothesis, changes in the galactic cosmic ray (GCR) intensity cause significant changes in the Earth's cloud cover with important consequences for our understanding of climate driving processes. Svensmark and Friis-Christensen (1997) who analyzed one solar cycle reported that global cloud cover changed in phase with the GCR flux by 2-3%. They estimated that the radiative forcing of GCR during one solar cycle is comparable to the radiative forcing induced by the increase in greenhouse gases since 1750. This initiated heavy debates in the scientific community and many critics. However, up to date no really conclusive results have been published. Later analyses made by Marsh and Svensmark (2003) indicated that the correlation holds only for low clouds (0-3.2km) at low latitudes. In our approach we use the same cloud data and assumptions as previous authors, but instead of analyzing cosmic ray changes over the 11-year solar cycle we concentrate on rapid changes, so-called Forbush decreases (Fds) which are comparable in amplitude but last only about a week. This offers the opportunity to investigate the hypothesis in much more detail and with much better stringency. For each of the six selected Fds extensive Monte Carlo simulations were carried out to calculate the global change in ion production rate in the atmosphere with a spatial resolution of 5° x 5° at three different altitudes. For each grid cell (altogether 71534 grid cells) ion production change was compared with corresponding ISCCP D1 cloud data change allowing for time lags ranging from 0 to 10 days. The most often considered but still uncertain underlying physical mechanism involves GCR induced formation of ions followed by cloud condensation nuclei formation. According to the mentioned mechanism it should be expected less than usual cloud cover about 1-2 days after a Fd event. Detailed correlation analysis of these ionization data with the corresponding cloud data does not show any indication for the proposed link between cosmic ray intensity and global cloud cover at any altitude. Therefore, we find no response of global cloud cover to Forbush decreases.

Forbush decrease ; cosmic rays ; cloud cover ; climate

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