A NASA research department has admitted that the calculations it used to show an increase in U.S. temperatures were flawed, after a campaign by an amateur meteorologist using his blog.

Climatologists at NASA's Goddard Institute for Space Studies in New York have been forced to revise estimates after research by Stephen MacIntyre, who published his findings on his Climate Audit blog.

As a result of his calculations, which he e-mailed to NASA, scientists at the agency now say that 1934, not 1998, was the warmest year in the United States since records began to be kept in 1880.

• Click here to see the revised NASA data table.

They also accept that five of the 11 warmest U.S. years on record occurred before 1940, and that only one was in the 21st century.

The revelations have been pounced on by the fringe group of researchers and pundits who deny that man-made global warming is taking place.

However, the Goddard Institute claimed that the differences in the recalculated temperature — at one-hundredth of a degree in the U.S. and one-thousandth of of a degree all over the world — were so insignificant as to have no impact on the overall global-warming trend.

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The Met Office, which calculates Britain's temperature patterns, contended that the findings do not impact on the U.K.

David Parker, a Met Office climate scientist, told Times Online that no British data would have to be reassessed.

McIntyre, a Canadian former mining executive, has devoted his blog to campaigning that NASA's temperature-keeping records are wrong.

"I come from a background where you have to announce bad results," he said. "They might not like the fact that they made a small embarrassing error, but if it was me, I'd have announced the results and put the best spin on it that I could.

"I would not have left myself open to the suggestion that I was not being forthcoming."

In a posting on his blog, McIntyre wrote that NASA records for the hottest 10 years on file had been dramatically changed in the U.S. as a result of his research.

"Four of the top 10 are now from the 1930s: 1934, 1931, 1938 and 1939, while only 3 of the top 10 are from the last 10 years (1998, 2006, 1999)," he wrote. "Several years (2000, 2002, 2003, 2004) fell well down the leaderboard, behind even 1900."

[According to the revised figures, 1939 and 1954 tie for 10th place at 0.85 degrees Centigrade above the 125-year average, or mean. Four of the top 10 (or 11) have occurred since 1990, and the years since 1998 show consistent increases of at least 0.44 degrees above mean, the only such cluster in the table. Click here to see a graph of the revised numbers.]

The Goddard Institute claimed that the cause of the error was a switch to a new data-collection system in 2000. This led to an incorrect assumption that the old and new methods matched, which was proved to be untrue.

According to latest figures, 1934 is now the hottest year on record in the U.S. at 1.25 C higher than mean. 1998, the previous front-runner, is now second at 1.23 C, followed by 1921 at 1.15 C.

The old system put 1998 first, with 1.24 C above normal, with 1934 at 1.23 C. Next was 2006, now relegated to fourth place, which was placed at 1.23 C but is now 1.13 C.

In a memo circulated to interested parties admitting to the error, seen by Times Online, the scientist James Hansen, the director of the agency's Goddard Institute for Space Studies, said that there was "no need to read further unless you are interested in temperature changes to a tenth of a degree over the U.S. and a thousandth of a degree over the world."

• Click here to read Hansen's memo (pdf format).

"Recently it was realized that the monthly more-or-less-automatic updates of our global temperature analysis had a flaw in the U.S. data," he admitted.

He added that the 1934 and 1998 temperatures had been "practically the same" in any case, "the difference being much smaller than the uncertainty" — and that he had been "besieged by rants" on the Internet and faced calls to resign.

"For two days I have been besieged by rants that I have wronged the President, that I must 'step down', or that I must 'vanish'," he wrote. "Hmm, I am not very good at magic tricks."

Parker, who is one of the scientists in charge of calculating the U.K.'s temperatures, said: "It [the new data] appears to make the last few years not as warm in the U.S. as initially calculated. But not the rest of the world."

He added: "The figures have slight significance for U.S. temperatures, but the U.S. only covers 2 percent of the world's surface, so there is very little significance globally."