The differences that account for those five rating points are quite small, however - only about 0.036 degrees Fahrenheit (0.02 degrees Celsius) - so the results do not disagree significantly, said UAH principal research scientist Roy Spencer.

Jean Baptiste Joseph Fourier, a French natural philosopher who lived between 1768 and 1830, was as agile at dodging the guillotine as calculating mathematics. He survived four appointments with Monsieur Guillotin's humane killing invention during the French Terror. He lived to compose the 1824 paper titled "General Remarks on the Temperature of the Terrestrial Globe and Planetary Spaces," in which he described an invisible dome of carbon dioxide that absorbed the sun's heat and helped warm Earth. He wrote that the question of global temperature is one of the most remarkable and interesting in all of science - and one of the most difficult.

In 1896, Nobel-Prize-winning chemist Svante Arrhenius recognized that CO2 concentrations in the atmosphere were increasing, and calculated the effect a doubling of CO2 would have on Earth's temperatures. Arrhenius prepared an estimate about twice the modern level for this "climate sensitivity" - not bad for the pre-computer era, when all his calculations had to be done on paper. Understandable, he called it the most tedious work he had ever done in his life.

"Average global temperature" is one of the foundations on which concerns about global warming are built. Goddard scientists put the average global temperature in2000 at 57.8 degrees F (14.4 degrees C). This is up about 0.9 degrees F (0.5 degrees C) since 1950.

The Goddard group was able to calculate global temperatures back to the late 1800s, using data from thermometers on the surface. The UAH scientists took its data back only to 1976, and looked at temperatures in the troposphere - the lower atmosphere - as measured by microwave sounding units on satellites.

The warmest temperatures in all these measurements appear among the last 15 years.

The Goddard data show the four warmest, in order were, 1998, 2002, 2003 and 2004. The UAH data show 1998, 2002, 2003 and 1987 - though as Spencer noted, the temperature difference between 1987 and 2004 is negligible.

To obtain surface temperatures, Goddard scientists sampled readings from more than 7,000 individual stations. To compute an average, they took "the warmest and coolest temperatures in a day, and calculate the temperature that is exactly in the middle of those high and low values," a NASA news release said. They repeat the process for each station.

There is some bias in this otherwise straightforward process, however. Most temperatures station are located in developed nations and on land - which means they are taking fewer readings from the southern hemisphere, where is less land. They also are fewer readings over the open ocean.

Moreover, some scientists have been concerned surface records may be contaminated by the urban-heat-island effect. Because many temperature stations are placed in cities, it is possible the extraneous heat from cars, buildings and other sources are contaminating the record.

"There are enough data points that you can remove those in urban stations," NASA research scientist Drew Shindell told UPI's Climate. "It's something we have a good handle on and can remove from the data."

TheUAH data are taken from satellites monitoring the troposphere, above the surface. These data have their own problems, however. For instance, orbital deterioration of the satellites originally contaminated the result, though scientists said they have addressed that question.

Satellites also pick up a cooler signal from the stratosphere. Most climate models expect the troposphere and the surface to warm at roughly the same rates. The UAH results have not shown that. The Huntsville team measured an increase of 0.144 degrees F (0.08 degrees C) per decade, while the NASA surface measurements found 0.216 degrees F (0.12 degrees C) per decade. Some critics take the differences in the data to mean global climate change is not significant.

Several recent papers re-examined the satellite data and tried to subtract out the stratospheric cooling, resulting in closer agreement to the surface temperature trends.

The UAH team has not yet adjusted their analysis based on these papers, because they do not think the other analyses are correct.

"The only adjustment we've made that we think is justified is the orbital decay," Spencer told Climate. "There are some recent papers that we are responding to in the scientific literature, but we don't see any reason to make any adjustments based on those papers. Nevertheless, I am in the process of redoing the recalibration on the satellite record, based on what we've learned in the last few years."

Spencer would not speculate on what that recalibration might show.

"There are many decision that made in analyzing the satellite data," he said. "Depending on what decisions are made, you get a different answer. The same is true of the surface data."

Despite the absolute differences, the data correspond reasonably well on some issues. Both show definite degrees of global warming, and both pinpoint the last 15 years as the warmest, and even name the same hottest years.

"We're hoping the new calibration will clear up some of the disagreement from some of the groups," Spencer said. "We always have the right answer - until we get the next right answer."

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