OSLO, Norway (Reuters) — The sun’s energy output has barely varied over the past 1,000 years, raising chances that global warming has human rather than celestial causes, a study showed on Wednesday.
Researchers from Germany, Switzerland and the United States found that the sun’s brightness varied by only 0.07 percent over 11-year sunspot cycles, far too little to account for the rise in temperatures since the Industrial Revolution.
“Our results imply that over the past century climate change due to human influences must far outweigh the effects of changes in the sun’s brightness,” said Tom Wigley of the U.S. National Center for Atmospheric Research.
Most experts say emissions of greenhouse gases, mainly from burning fossil fuels in power plants, factories and cars, are the main cause of a 0.6 Celsius (1.1 Fahrenheit) rise in temperatures over the past century.
A dwindling group of scientists says that the dominant cause of warming is a natural variation in the climate system, or a gradual rise in the sun’s energy output.
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