“The Sun makes fantastic natural experiments” Henrik Svensmark says, “that allow us to test our ideas about its effects on the Earth’s climate.” Most dramatic are the events called Forbush decreases. Ejections of gas from the Sun, carrying magnetic fields, can suddenly cut the influx of cosmic rays coming to the Earth from exploded stars.
CERN’s CLOUD experiment has just published a paper on the effect of cosmic rays on aerosols that can affect cloud formation. I’ll have something to say on that shortly, but Nigel Calder gives a good writeup on other experimental evidence for cosmic rays affecting cloud formation. It is a bit long, but an important read for anyone who wants to understand Dr. Svensmark’s hypothesis. The executive summary: Svensmark demonstrates experimental evidence for GCR influence on cloud formation, and explains the reason critics failed to find such evidence.