The study, which also found the magnitude of the global temperature rise to be unmatched in 4,000 years, suggests that the current warming trend cannot be explained by naturally occurring temperature fluctuations.
Man-made snow coats a ski run next to barren ground under a chairlift at Shawnee Peak ski area in Bridgton, Maine, Jan. 2012. Over the past century, global average temperatures appear to have risen faster than at any time since the end of the last ice age 11,300 years ago, and perhaps longer.
Robert F. Bukaty/AP
EnlargeOver the past century, global average temperatures appear to have risen faster than at any time since the end of the last ice age 11,300 years ago, and perhaps longer. Meanwhile, the magnitude of the increase has been unmatched in at least the past 4,000 years.
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Researchers say those are the implications of a new study that uses natural stand-ins for thermometers to trace temperature trends back to the beginning of the current warm, interglacial period. Significantly, the study?s findings suggest the current warming trend cannot be explained by some forms of naturally occurring temperature variability, a lingering issue in the debate over the impact of human activity on global warming. [Editor's note: The reference to temperature variability has been revised.]
The main trigger for the current warming trend, especially since the middle of the last century, has been rising emissions of heat-trapping carbon dioxide as people burn fossil fuels and change land-use patterns, researchers say.
Although other so-called paleoclimate records reach farther back into geological time, the team focused on the Holocene epoch, in which human civilizations emerged and evolved.
"To our knowledge, based on this reconstruction, the rate of change today is unprecedented" in the Holocene, says Shaun Marcott, an atmospheric scientist at Oregon State University who led a team formally reporting the results in Friday's issue of the journal Science. Indeed, it may be unprecedented in the past 22,000 years, he adds, when previous paleoclimate research he and his colleagues have conducted is taken into account.
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.
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