Abstract
Abstract. The response of clouds to sudden decreases in the flux of galactic cosmic rays (GCR) – Forbush decrease events – has been investigated using cloud products from the space-borne MODIS instrument, which has been in operation since 2000. By focusing on pristine Southern Hemisphere ocean regions we examine areas where we believe that a cosmic ray signal should be easier to detect than elsewhere. While previous studies have mainly considered cloud cover, the high spatial and spectral resolution of MODIS allows for a more thorough study of microphysical parameters such as cloud droplet size, cloud water content and cloud optical depth, in addition to cloud cover. Averaging the results from the 22 Forbush decrease events that were considered, no statistically significant correlations were found between any of the four cloud parameters and GCR, when autocorrelations were taken into account. Splitting the area of study into six domains, all of them have a negative correlation between GCR and cloud droplet size, in agreement with a cosmic ray – cloud coupling, but in only one of the domains (eastern Atlantic Ocean) was the correlation statistically significant. Conversely, cloud optical depth is mostly negatively correlated with GCR, and in the eastern Atlantic Ocean domain that correlation is statistically significant. For cloud cover and liquid water path, the correlations with GCR are weaker, with large variations between the different domains. When only the six Forbush decrease events with the largest amplitude (more than 10% decrease) were studied, the correlations fit the hypothesis slightly better, with 16 out of 24 correlations having the expected sign, although many of the correlations are quite weak. Introducing a time lag of a few days for clouds to respond to the cosmic ray signal the correlations tend to become weaker and even to change sign.
Highlights
The magnitude of the Sun’s contribution to 20th century climate variations has been the subject of some controversy, and many possible mechanisms have been suggested
In the whole domain between 0◦ S and 40◦ S (TOT), we note that the cloud droplet size (CER) has a rather large negative correlation with galactic cosmic rays (GCR), and the sign of that correlation is consistent with Table 2
The correlation between GCR and Cloud Optical Depth (COD) is negative, meaning that a reduction in cosmic ray flux leads to an increase of cloud albedo, which is opposite to what Table 2 suggests
Summary
The magnitude of the Sun’s contribution to 20th century climate variations has been the subject of some controversy, and many possible mechanisms have been suggested. A link between the flux of ionizing galactic cosmic rays (GCR), modulated by solar activity, and global cloud cover was proposed by Svensmark and Friis-Christensen (1997). They proposed that the GCR flux stimulates the formation of cloud condensation nuclei (CCN) in the atmosphere, and that the higher CCN concentrations at times of high GCR fluxes would lead to increased cloud cover and a cooling of the Earth’s climate. High and statistically significant correlations between GCR and low cloud cover were presented, based on data for the period 1983–1994 from the International Satellite Cloud Climatology Project (ISCCP), using infrared sensors only. Numerous reassessments were subsequently published (e.g., Kristjansson and Kristiansen, 2000; Udelhofen and Cess, 2001; Kristjansson et al, 2002; Laut, 2003; Damon and Laut, 2004), questioning both the physical and statistical basis for the earlier conclusions on cause and effect. Kristjansson et al (2002, 2004), adding new data up to the year 2001 to the ISCCP time series, showed that the ISCCP
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