Other researchers have focused on the Holocene as well, notably Michael Mann, a Penn State University climatologist, and his colleagues. But their reconstructions have taken the record back only about 1,500 years.
The new work, using different thermometer stand-ins, or proxies, not only reaches results similar to these previous efforts covering the recent past. It also accounts for natural variations in climate over longer time scales in ways that suggest rising temperatures will exceed the range of natural fluctuations. The long-term variations would include changes in Earth’s orbit, for instance.
Based on the reconstructed temperatures records, natural variability over the study's time span accounts for roughly 1 degree C from coldest to warmest compared with the current climate, observes David Anderson, branch chief for the Paleoclimate Program in at the National Oceanic and Atmospheric Administration's National Climatic Data Center office in Boulder, Colo.
"If you go grab the mount of warming expected just within the next 80 years, that's more like 3 degrees," says Dr. Anderson, who was not a member of the study team – three times the change one would expect from natural variability alone, and all in the warm direction.
According to the reconstruction, global average temperatures increased by about 0.6 degrees Celsius (1 degree Fahrenheit) from 11,300 to 9,500 years ago. Temperatures remained relatively constant for about 4,000 years. From about 4,500 years ago to roughly 100 years ago, global average temperatures cooled by 0.7 degrees C.