by Judith Curry
A new paper is in press that sheds some light on the relationship between cosmic rays and lower tropospheric cloud cover.
Relationship of Lower Troposphere Cloud Cover and Cosmic Rays: An Updated Perspective
Ernest M. Agee, Kandace Kiefer and Emily Cornett
An updated assessment has been made of the proposed hypothesis that “galactic cosmic rays (GCRs) are positively correlated with lower troposphere global cloudiness.” A brief review of the many conflicting studies that attempt to prove or disprove this hypothesis is also presented.
It has been determined in this assessment that the recent extended quiet period (QP) between solar cycles 23-24 has led to a record high level of GCRs, which in turn has been accompanied by a record low level of lower troposphere global cloudiness. This represents a possible observational disconnect, and the update presented here continues to support the need for further research on the GCR-Cloud hypothesis and its possible role in the science of climate change.
In press, Journal of Climate. Abstract [here].
The Introduction starts with a good overview of the relation between solar activity and clouds, starting with the 1997 paper by Svensmark and Friis-Christensen.
From the Conclusions:
Several studies, as referenced here, have continued to promote the controversial cosmic ray-cloud connection hypothesis, both from the standpoint of errors in data analysis as well as scientific links that establish GCR-CCN as a viable contributor to climate change. It is also important to note (see Carslaw et al. 2002) that there are two mechanisms by which cosmic rays may affect cloud droplet number concentrations or ice particles: a) Ion-aerosol clear-air mechanism, and b) Ion-aerosol near-cloud mechanism. Recent attempts have also been made to further resolve the GCR-CCN controversy by examining the global cloudiness response to very short-term solar variations (namely Forbush Decreases that are approximately of one-week duration). Calogovic et al. (2010) have found no response in global cloud cover to Forbush Decreases at any altitude and latitude. Svensmark et al. (2009), however, have shown that Forbush decreases associated with CME passage results in lower troposphere clouds containing less liquid water. Their results, in general, show global scale influences of solar variability on both cloudiness and aerosols. The work by Harrison and Ambaum (2010) also shows a positive relationship between cloudiness and large GCR changes associated with Forbush decreases (as observed at Shetland, England). Laken and Kniveton (2011) on the other hand, fund no evidence of any relationship between liquid cloud fraction and GCRs.
It is concluded that the observational results presented, showing several years of disconnect between GCRs and lower troposphere global cloudiness, add additional concern to the cosmic ray-cloud connection hypothesis. In fact, this has been done in the most dramatic way with the measurement of record high levels of GCRs during the deep, extended quiet period of cycle 23-24, which is accompanied by record low levels of lower troposphere global cloudiness. Research on the GCR-cloud correlations must continue, particularly in view of the two physical mechanisms mentioned above (as well as the uncertainty in the reliability of the ISCCP lower troposphere cloudiness to show the proposed correlations). Finally, it is clearly known that other factors can affect mean global cloudiness besides solar variability, due to internal forcing mechanisms on different time scales (such as ENSO).
JC comment: Trying to identify a GCR signal in cloudiness is confounded by a plethora of other dynamical and thermodynamical processes that can modify cloud properties, notably the teleconnection regimes such as ENSO and the internal decadal oscillations. Both mechanistic research (such as the CERN experiment) and climatological analyses such as done in this paper are needed to support progress in understanding this issue.
Update: RealClimate has a post regarding the CERN CLOUD experiment that provides a good overview, explaining the proposed physical mechanisms for cosmic rays to influence climate.
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