A significant shift of Pacific climate occurred in the mid-1970s with effects that extended globally. One view is that this change was entirely natural and a product of internally-generated decadal variability of the Pacific climate system. However, during the mid-1970s there was also a significant increase of global temperature and changes to a number of other quantities that have been associated with changes of external forcings, particularly increases of greenhouse gases from the burning of fossil fuels. We analyze an un-forced control run from a global coupled climate model as well as 20th century simulations with changes in external forcings to show that the 1970s climate shift had a contribution from changes in external forcing superimposed on what was likely an inherent decadal fluctuation of the Pacific climate system. Thus this inherent decadal variability delayed to the 1970s what would have been a forced climate shift in the 1960s.