Scripps Institution of Oceanography

The enigma of why mountain glacier started to retreat in the 1850s in the Rocky Mountains and elsewhere remains unresolved. The most important factor affecting climate change presumably was a change in the mode of operation of the sun one or two decades earlier, (from irregular periodicity and low output to regular periods and greater brightness). But the direct cause appears to have been the onset of drought in the 1830s. Interestingly, there is no obvious solar information in the drought narrative in Montana and southwestern Canada. The presence of tidal lines in the spectrum of the Pacific Decadal Oscillation, together with the presence of lines that could be interpreted as beat periods between solar and tidal forcing in the drought series, suggests that the energy of solar variation is preempted for interference with tidal forcing, within the system of oscillations informing precipitation patterns in the region. The suggestion is supported by the presence of a strikingly dominant 12.5-year period in the drought series, which is interpreted as a difference tone between the main sunspot cycle (at 10.8) and a tidal period at 5.8. Also, this period is close to 2/3 of the nodal tide (at 18.61).