 MacDonald's early skepticism regarding plate tectonics stemmed from his detailed study, with Walter Munk, of the rotation of the Earth.Since small polar motions are readily detectable on human time scales, the possibility of explaining paleomagnetic data by polar wandering seemed, for a time, to be a viable possibility; plate tectonic motions are now considered to be the correct explanation.MacDonald was a prominent early scientific advocate of action to address the threat of global warming from fossil-fuel combustion.