Empirical evidence for a nonlinear effect
of galactic cosmic rays on clouds
Galactic
cosmic ray changes have been suggested to affect weather and climate, and new
evidence is presented here directly linking galactic cosmic rays with clouds.
Clouds increase the diffuse solar radiation, measured continuously at UK
surface meteorological sites since 1947. The ratio of diffuse to total solar
radiation – the diffuse fraction - is used to infer cloud, and is
compared with the daily mean neutron count rate measured at Climax, Colorado
from 1951-2000, which provides a globally-representative indicator of cosmic
rays. Across the UK, on days of high cosmic ray flux (above 3600´102neutron counts.hr-1,
which occur 87% of the time on average) compared with low cosmic ray
flux, (1) the chance of an overcast day increases by (19 ± 4) %,
and (2) the diffuse radiation fraction increases by (2 ± 0.3) %.
During sudden transient reductions in cosmic rays (e.g. Forbush events),
simultaneous decreases occur in the diffuse fraction. The diffuse radiation
changes are therefore unambiguously due to cosmic rays. Although the
statistically significant non-linear cosmic ray effect is small, it will have a
considerably larger aggregate effect on longer timescale (e.g. century)
climate variations when day-to-day variability averages out.