"It's time for the public discussion to get beyond this silly debate about the hockey stick," he said.
"The main conclusion from the study in '99 has been recognized in multiple additional studies and multiple additional lines of evidence," Mann added.
The older study was first published in 1999 in the science journal Geophysical Research Letters.
Though scientists have documented global warming in several ways—melting Arctic ice, accelerating glaciers in Greenland, increasing concentrations of greenhouse gases—the hockey stick stands out for its visual summary of global warming.
A version of the graph appears in the Al Gore global warming movie, An Inconvenient Truth. (See "Al Gore's Inconvenient Truth Movie: Fact or Hype?")
Proxy Data
Tree rings, corals, and cores of sediment and ice are so-called proxy data. Scientists rely on them to estimate temperatures during time periods before humans began collecting records with instruments such as thermometers.
(See photos on measuring climate change.)
In the 1999 study Mann and colleagues concluded from the proxy data that the warming of the past few decades was unprecedented over the last 400 years and likely over the past thousand years.
However, he said, they were cautious about temperatures prior to 1600, as the data was incomplete.
The National Research Council echoes those findings in its new report, though it particularly emphasizes the shakiness of the data prior to 1600. The council called for more research to collect better data for the years prior to 1600.
The new report's authors conclude they have "a high level of confidence that the last few decades of the 20th century were warmer than any comparable period in the last 400 years … and potentially the last several millennia."
Congressperson Sherwood Boehlert, a New York Republican and chair of the U.S. House Committee on Science, requested the study in November after Representative Joe Barton, a Texas Republican, launched an investigation into Mann and his colleagues.
In a statement issued Thursday, Boehlert said, "There is nothing in this report that should raise any doubts about the broad scientific consensus on global climate change."
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