Atmospheric researchers studying global warming have sought for years to determine why readings taken from weather balloons didn't show the same increases as readings on the ground. The difference has fueled skeptics of global warming.
Now, researchers at Yale University say exposed instruments on the balloons may be the problem.
Weather balloons are sent up around the world twice a day — at local times equivalent to noon and midnight Greenwich mean time — and older versions of the balloons used temperature probes that were exposed. The result was higher-than-normal readings on probes sent up in daytime because of the sunlight exposure.
In more recent years, however, new probes were developed that were shielded from the light.
The result, while readings were rising on the ground they were not doing so up in the air, since the extra solar heating was no longer warming the probes.
After correcting for the problem, the researchers estimate there has been a global temperature increase of 0.4 degree Fahrenheit, 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, associate professor of geology and geophysics at 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 findings are published in Friday's online issue of the journal Science.
The research was funded by the National Oceanic and Atmospheric Administration.