Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.-N., Henderson, K. and Mashiotta, T.A. 2003. Tropical glacier and ice core evidence of climate change on annual to millennial time scales. Climatic Change 59: 137-155.
The authors analyzed decadally-averaged δ18O records derived by them and their colleagues from three Andean and three Tibetan ice cores, demonstrating that "on centennial to millennial time scales atmospheric temperature is the principal control on the δ18Oice of the snowfall that sustains these high mountain ice fields," after which they produced "a low latitude δ18O history for the last millennium" that they use as a surrogate for air temperature. For the Guliya Ice Cap (35.28°N, 81.48°E), this work revealed several instances throughout prior centuries when average decadal temperatures were greater than that experienced over the last few decades of the 20th century.