Hughes has studied geographical areas including Europe, Asia, and the Sierra Nevada. In addition of tree rings, Hughes uses ice cores, laminated sediments, and the historical temperature record to help understand past climates. In 1998, he was a co-author with Michael E. Mann and Raymond S. Bradley on a paper which produced the first eigenvector-based climate field reconstruction (CFR) incorporating multiple climate proxy data sets of different types and lengths into a high-resolution reconstruction of northern hemisphere temperatures. In 1999 the same team extended the method to cover 1,000 years, producing what was dubbed the hockey stick graph.
Hughes, M. K., F. H. Schweingruber, D. Cartwright and P. M.Kelly (1984). "July–August temperature at Edinburgh between 1721 and 1975 from tree-ring density and width data". Nature308 (5957): 341–344. Bibcode:1984Natur.308..341H. doi:10.1038/308341a0.