Mann then attended Yale University, intending to obtain a PhD in physics, and received both an MS and an MPhil in physics in 1991. His interest was in theoretical condensed matter physics but he found himself being pushed towards detailed semiconductor work. He looked at course options with a wider topic area and was enthused by Ph.D. adviser Barry Saltzman about climate modelling and research. To try this out he spent the summer of 1991 assisting a postdoctoral researcher in simulating the period of peak Cretaceous warmth when carbon dioxide levels were high, but fossils indicated most warming at the poles, with little warming in the tropics. Mann then joined the Yale Department of Geology and Geophysics, obtaining an MPhil in geology and geophysics in 1993. His research focused on natural variability and climate oscillations. He worked with the seismologist Jeffrey Park, and their joint research adapted a statistical method developed for identifying seismological oscillations to find various periodicities in the instrumental temperature record, the longest being about 60 to 80 years. The paper Mann and Park published in December 1994 came to conclusions similar to those from a study developed in parallel using different methodology and published in January of that year, which found what was later called the Atlantic multidecadal oscillation.</ref>