Earth Hottest It's Been in 400 Years or More, Report Says

Courtsey: http://news.nationalgeographic.com/news/2006/06/060623-global-warming.html

The last two decades of the 20th century were the hottest in 400 years and quite likely the warmest for several millennia, a leading U.S. scientific body concludes in a new report.

The National Academies' National Research Council report also said "human activities are responsible for much of the recent warming."

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The U.S. Congress had requested the report after controversy arose last year over surface-temperature reconstructions published in the 1990s by climatologist Michael Mann, now at Pennsylvania State University, University Park, and colleagues.

The reconstructions resulted in the widely cited "hockey stick" graph, which shows Earth's temperature sharply rising in recent decades after a thousand years of stability. The graph looks like a hockey stick lying on its side.

To create the graph, Mann and his colleagues pulled together temperature evidence from specimens such as tree rings, corals, and cores of sediment and ice. They had to rely on this natural evidence because thermometer records go back only about 150 years.

The graph gained prominence when the United Nations published it in a 2001 report that concluded that greenhouse gases from human activities had probably caused most of the warming measured since 1950, according to the New York Times.

Critics of the graph have said that Mann and his colleagues based it on cherry-picked and erroneous data.

Gerald North is a geoscientist at Texas A&M University in College Station. He chaired the National Research Council panel that produced the new report, which was released yesterday.

At a press conference Thursday at the National Academies headquarters in Washington, D.C., North said Mann and his colleagues prepared their data in a professional manner.

"I certainly did not see anything inappropriate. I mean, there might have been things that maybe they could have done differently, better, but … I have no cause to think there was anything inappropriate," he said.

Coffin Nailed

Mann, in an interview with National Geographic News, said the new report "nailed the coffin on the skeptics."

"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."