Dahl-Jensen, D., Mosegaard, K., Gundestrup, N., Clow, G.D., Johnsen, S.J., Hansen, A.W. and Balling, N. 1998. Past temperatures directly from the Greenland Ice Sheet. Science 282: 268-271.
What was done
Temperature measurements from two Greenland Ice Sheet boreholes were used to reconstruct the temperature history of the Greenland Ice Sheet over the past 50,000 years.
What was learned
The data revealed that temperatures on the Greenland Ice Sheet during the Last Glacial Maximum (approximately 25,000 years ago) were 23 ± 2 °C colder than at present. After the termination of the glacial period, temperatures increased steadily to a maximum of 2.5°C warmer than at present during the Climatic Optimum (4,000 to 7,000 years ago). The Medieval Warm Period and the Little Ice Age were also documented in the record, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present, respectively. After the Little Ice Age, the authors report that "temperatures reached a maximum around 1930 A.D." and that "temperatures have decreased during the last decades."
What it means
The results of this study stand in direct contrast to the predictions of general circulation models (GCMs) that consistently suggest there should have been a significant warming in high northern latitudes over the past several decades. They also show large temperature excursions over the last 10,000 years, when the air's CO2 content was relatively stable. Both of these observations raise doubts about the ability of current GCMs to accurately forecast earth's climatic response to the ongoing rise in the atmosphere's CO2 concentration.