And yet, Mann said, "you can go back nearly 2,000 years and the conclusion still holds—the current warmth is anomalous."
"The burst of warming over the past one to two decades takes us out of the envelope of natural variability."
The study will appears today in the journal Proceedings of the National Academy of Science.
Hockey Stick Graph
The hockey stick graph has become a lightning rod for criticism of the idea that the planet is warming mainly due to human-made greenhouse gases.
(Explore an interactive map of global warming's effects.)
Many critics contend that tree rings are unreliable temperature gauges, because temperature is not the only factor that affects the rings.
The controversy led to hearings in the U.S. Congress over the methods Mann and colleagues used in the 1998 study.
However, a 2006 report from the National Research Council—a private, nonprofit scientific institution that advises the U.S. government—supported the hockey stick study while detailing the major uncertainties.
(Related: "Earth Hottest It's Been in 400 Years or More, Report Says" [June 23, 2006].)
In centuries past, isolated regions have warmed up from time to time, such as during the so-called Medieval Warm Period, when Europe experienced warmer temperatures from about A.D. 900 to 1400.
"But what's unique about modern warming is that essentially the whole globe is warming up in tandem," Mann said.
"The so-called hockey stick it's alive and well."
Climatologist Gabriele Hegerl of the University of Edinburgh in Scotland is "pleasantly surprised" by the new study.
"Being able to get essentially [the] same result without tree ring data shows that what we are seeing is not something specific to tree rings," Hegerl said, "but a real temperature response."
SOURCES AND RELATED WEB SITES