A cosmic ray-climate link and cloud observations

TL;DR

This study comprehensively examines satellite and ground-based data to investigate the hypothesized link between cosmic rays and cloud cover, finding no robust evidence at global scales but possible weak regional effects.

Contribution

It provides a detailed analysis of satellite and ground data, clarifying the absence of a strong cosmic ray-cloud link and highlighting the limitations of current datasets and methods.

Findings

No evidence of a solar-cloud link in satellite data

Weak regional correlations suggest possible local effects

Current datasets may lack sensitivity to small signals

Abstract

Despite over 35 years of constant satellite-based measurements of cloud, reliable evidence of a long-hypothesized link between changes in solar activity and Earth's cloud cover remains elusive. This work examines evidence of a cosmic ray cloud link from a range of sources, including satellite-based cloud measurements and long-term ground-based climatological measurements. The satellite-based studies can be divided into two categories: 1) monthly to decadal timescale correlations, and 2) daily timescale epoch-superpositional (composite) analysis. The latter analyses frequently focus on high-magnitude reductions in the cosmic ray flux known as Forbush Decrease (FD) events. At present, two long-term independent global satellite cloud datasets are available (ISCCP and MODIS). Although the differences between them are considerable, neither shows evidence of a solar-cloud link at either long or short timescales. Furthermore, reports of observed correlations between solar activity and cloud over the 1983 to 1995 period are attributed to the chance agreement between solar changes and artificially induced cloud trends. It is possible that the satellite cloud datasets and analysis methods may simply be too insensitive to detect a small solar signal. Evidence from ground-based studies suggests that some weak but statistically significant CR-cloud relationships may exist at regional scales, involving mechanisms related to the global electric circuit. However, a poor understanding of these mechanisms and their effects on cloud make the net impacts of such links uncertain. Regardless of this, it is clear that there is no robust evidence of a widespread link between the cosmic ray flux and clouds.