Cosmic Rays and Cloud Nucleation
Jeffrey Pierce, Dalhousie University, Halifax, Nova Scotia
Evidence of a correlation between GCRs and climate via their influence on cloud cover has been
debated, but insight into potential underlying physical mechanisms is providing a better understanding of
the types of studies required to better quantify any impact. According to Jeffrey Pierce, there are two
potential GCR-cloud-climate pathways:
1. GCRs enhance aerosol nucleation rates and cloud condensation nuclei concentrations
through ionization of gases. These changes modify cloud formation, cloud amount, and subsequently, the
shortwave radiation reaching the surface.
2. GCRs impact precipitation through the modification of near-cloud electrification with
subsequent impact on the freezing of supercooled liquid droplets. Processes in this category could alter
the global electrical circuit with potential but as yet unknown mechanisms.
Pierce noted that the first mechanism has been studied in far greater detail. International Satellite
Cloud Climatology Project (ISCCP) cloud analysis has suggested a 2 percent absolute change in cloud
amount over the solar cycle, which corresponds to a 6 percent relative change. 30 Although there is a 5-20
percent change in GCR-induced ionization in the troposphere over the solar cycle, this results (due to a
number of dampening factors) in a smaller increase in nucleation rates, an even smaller increase in cloud
condensation nuclei, and finally, a still smaller change in cloud amount. Thus it appears that the ion-
aerosol clear-sky mechanism is too weak to explain the observed cloud changes, even with favorable
assumptions for model inputs. Pierce asserted that a number of controlled experiments are necessary to
better assess both the ion-aerosol hypothesis and the near-cloud hypothesis. 31
30 K.S. Carslaw, R.G. Harrison, and J. Kirkby, Cosmic rays, clouds and climate, Science 298:1732-1737, 2002.
31 See also discussion by Daniel Baker, above.