Willie Soon and Sallie Baliunas, Lessons and Limits of Climate History: Was the 20th Century Climate Unusual?, The George C. Marshall Institute, April 2003 (marshall.org)
Willie Soon and Sallie Baliunas, two scientists affiliated with the Harvard-Smithsonian Center for Astrophysics, examine the repeated claim that the earth's climate during the twentieth century was unusual compared with other centuries in the last millennium. They point out that the temperature of the past 1,000 years is not known well enough to allow a precise comparison with previous centuries. A worldwide set of temperature measurements has only been available since 1861, and there are large gaps in coverage.
For earlier years, paleoclimatologists use proxy measurements from tree rings, corals, and ice cores. From these they can estimate temperatures in the local areas. But Soon and Baliunas suggest these data are far too incomplete in geographic coverage and in the temperature information they convey to allow a realistic estimate of global surface temperature.
The authors point out that the research featured heavily in the Intergovernmental Panel on Climate Change that suggests a sharp increase in global temperatures in the last century "depends …