Pacific Decadal Oscillation and other types of multidecadal variability, but for some reason never asks the obvious question: Could these natural climate fluctuations cause a change in global cloudiness? Fig. 4. Running five-month average values of the PDO index for 1900–2008. The assumption that global average cloudiness never changes has been rather easy for the IPCC to make since we do not have sufficient global observations with which to document, let alone understand, such chaotic fluctuations in climate. We would need highly accurate, global measurements of the climate system for at least fifty years to prove or disprove the existence of natural mechanisms of climate change. Unfortunately, those measurements do not exist. Consequently, the IPCC can correctly claim there is virtually no published research to support natural sources of long-term climate change.