Researchers at the US' National Oceanic and Atmospheric Administration may have overturned one of the key weapons in the armoury of climate-change sceptics.
Data from weather balloons in the 1970s has long puzzled scientists, because it appears to contradict computer models of global warming. Most models have a strong link between the temperature of the air, and that of the Earth. But atmospheric data showed little or no change in atmospheric temperatures since the 1970s, despite warming at ground level.
However, it seems that this data was not subject to proper analysis, the scientists say, and the impressions it gives - that temperatures have remained roughly constant in the intervening decades - is misleading.
"Even though models predict a close link between atmospheric and surface temperatures, there has been a large difference in the actual measurements," said study leader Steven Sherwood, an associate professor of geology and geophysics at Yale.
"This has muddied the interpretation of reported warming," he told Reuters.
The problem lies in the placement of the temperature sensors. Early on in research, the sensors were in exposed positions, often in direct sunlight. Just as standing in the sun on a warm day feels warmer than standing in the shade, this led to temperature readings that were too high.
In later research, the sensors were changed to reduce this warming effect. However, the discovery that the earliest data is likely to be misleadingly warm does help explain the discrepancies between the models of the lower atmosphere and the data. It suggests that the temperatures are likely to have risen, after all.
The research team says that once the warming effect is accounted for, there has most likely been an increase in temperature of 0.2 degrees Celsius, each decade for the last thirty years.
The research, published in the journal Science, shows that "we can't hang our hats on the old balloon numbers", as Sherwood puts it.
Reuters reports that two additional studies published in Science support the findings of Sherwood and his team. ®