For years, experts sought to determine why temperature readings taken from weather balloons did not show the same increases as readings on the ground.
The resulting mismatch has fuelled scepticism of global warming.
Researchers at Yale University in the US say exposed instruments on the balloons may be the problem.
Weather balloons are sent up around the world twice a day and older versions of the balloons used temperature probes that were exposed.
The result, say the researchers, was higher readings on balloons sent up in daytime because of the exposure to sunlight.
After correcting for the problem, the researchers estimate there has been a global temperature increase of 0.2C (0.4F) per decade for the last 30 years.
"Unfortunately, the warming is in an accelerating trend - the climate has not yet caught up with what we've already put into the atmosphere," said lead author Steven Sherwood of Yale.
"This has muddied the interpretation. There are steps we should take, but it seems that shaking people out of complacency will take a strong incentive."
The research was funded by the US National Oceanic and Atmospheric Administration (Noaa). The findings were published in an online issue of the journal Science.
