Error in NASA climate data sparks debate

Due to an error in calculations of mean U.S. temperatures, 1934, not 1998 as previously reported, is the hottest year on record in the United States. NASA scientists contend that the error has little effect on overall U.S. temperature trends and no effect on global mean temperatures, with 2005 still the hottest year worldwide by far, followed by 1998. The data corrections have added new fuel to the climate change debate, however — and could spell more public relations woes for NASA.

The Goddard Institute for Space Studies (GISS) at NASA measures long-term changes in global surface temperatures using raw data collected at thousands of stations around the world (called the Global Historical Climatology Network, or GHCN). The raw temperature data are then corrected to account for a number of factors, including differences in the time of day of measurements between stations, and differences between rural stations and urban stations (which tend to be hotter, due to the so-called "urban heat island" effect).

On Aug. 4, however, the well-known climate change skeptic and former mining executive Steven McIntyre — who previously challenged climatologist Michael Mann's 1998 finding that temperatures have increased rapidly since 1900 A.D., compared with the previous thousand years, forming a distinctive "hockey stick" temperature pattern — observed a strange jump in the U.S. data occurring around January 2000. He sent an e-mail to NASA about his observation, and the agency responded with an e-mail acknowledging a flaw in the calculations and thanking him for his help, he says. By Aug. 7, he says, the agency had removed the incorrect U.S. data from the GISS Web site and replaced it with corrected numbers for all 1,200 stations.

The issue didn't end there, however. The corrections made almost no difference to global temperature trends, NASA reported, while U.S. mean annual temperatures from 2000 to 2006 were all reduced by about 0.15 degrees Celsius. Most significantly for climate change skeptics, however, the year 1934 now edges out 1998 as the hottest year in the United States.

McIntyre wrote about his findings in his blog ClimateAudit, dubbing the incorrect data a "Y2K" error and setting off a heated back-and-forth debate that gained traction in the blogosphere. In addition to noting the altered U.S. data, McIntyre also cast doubts on NASA's methods of collecting data and on its transparency, claiming that the old data should have been kept up on the Web site for comparison, and NASA should have alerted the public to the changes. Furthermore, he says, he had asked repeatedly to see the "source code" NASA uses to calculate its numbers, and had been repeatedly denied. "Certainly I think the way they handled it was inappropriate," he says. "I've got experience in public companies and if you have some bad news or adverse results you have to announce them."

Climate scientists, however, are asserting that the uproar over the data corrections is nothing more than a tempest in a teapot. NASA GISS scientist Jim Hansen, who helped devise the algorithm used to correct for the various climate factors, wrote in an Aug. 10 e-mail that the errors were introduced when the U.S. stations switched between two different datasets in 2000, with the faulty assumption that the second dataset also included the necessary corrections, an error that was recognized and fixed, Hansen said. Acknowledging that 1934 now appears to have been slightly hotter than 1998 in the United States, he noted that the difference in the mean between the two years, of 0.02 degrees Celsius, was and always had been smaller than the uncertainty, although their relative positions are now flipflopped. Globally, however, the changes had no effect on rankings, and 1998 was still by far the warmest year on record before 2005, he says. "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.

Gavin Schmidt, a NASA GISS scientist who created the blog RealClimate along with Michael Mann, posted in an Aug. 10 column that the furor is "ado over nothing." He says that despite the fact that the corrections don't alter the global trends, he has fielded hundreds of comments by confused and sometimes irate posters on his blog about the issue. "There are two factors that make it an interesting story," he says. "One is the little guy telling NASA that something is wrong — that has a lot of resonance. And then there's the more politicized issue, which is, 'how can we twist this to prove global warming is fake?'"

Although some people who have learned of the data errors are genuinely confused, Schmidt says, others "are being deliberately manipulative." The question of NASA releasing its source code is a case in point, he says, as both the raw data and the correction algorithms are actually freely available from NASA, and therefore anyone wanting to check NASA's numbers has all the necessary information to reproduce its results. As for McIntyre's question about whether NASA is concealing something by overwriting the old online data with the new, he says, "the whole analysis gets redone every month. That's a completely standard procedure."

McIntyre's Web site, meanwhile, is receiving more hits than ever as the controversy expands, and went down for a few days this week to move to a new server to accommodate the extra traffic. McIntyre says his goal is to push NASA to be more "forthcoming" about its adjustments. "What they should have done, what I would have done in their shoes, is say, 'we acknowledge this particular error, we don't think there are others, but we've put it all online and any interested parties can look at it,'" he says. "If they'd done that, they would have avoided much of the present controversy."

Schmidt, however, sees the situation differently. "If you can reframe this as a freedom of speech issue, or a nondisclosure issue, you can get people to say it's an outrage," he says. "That kind of stuff is a deliberate political tactic. There is a very vocal group of people who so desperately wish that global warming would just go away that any of these tactics are fair game."

Carolyn Gramling

Links:
Intergovernmental Panel on Climate Change  
"Hockey stick climate study faces scrutiny," Geotimes, September 2006 
NASA GISS temperature data  
ClimateAudit blog 
RealClimate blog 

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