Greenland ice core "signal" characteristics: An expanded view of climate change 

Journal of Geophysical Research, 98, p. 12839-12847, 1993. 

P.A. Mayewski
Glacier Research Group, EOS/SERB, University of New Hampshire, Durham, New Hampshire 03824, USA.
L.D. Meeker
M.C. Morrison
M.S. Twickler
S.I. Whitlow
K.K. Ferland
D.A. Meese
M.R. Legrand
J.P. Steffensen
Geofysisk Afdeling, Niels Bohr Instituttet for Astronomi, Fysik og Geofysik, Københavns Universitet

The last millenium of earth history is of particular interest because it documents the environmental complexities of both natural variability and anthropogenic activity. We have analysed the major ions contained in the Greenland Ice Sheet Project 2 (GISP 2) ice core from the present to ~674 A.D. to yield an environmental reconstruction for this period that includes a description of nitrogen and sulfur cycles, volcanic emissions, sea salt and terrestrial influences. We have adapted and extended mathematical procedures for extracting sporadic (e.g., volcanic) events, secular trends, and periodicities found in the data sets. Finally, by not assuming that periodic components (signals) were "stationary" and by utilizing evolutionary spectral analysis, we were able to reveal periodic processes in the climate system which change in frequency, "turn on," and "turn off" with other climate transitions such as that between the little ice age and the medieval warm period.