Over the past 50 years, the mean global temperature rise was 0.12 degrees Celsius (0.2 degrees Fahrenheit) per decade, slowing to an average 0.05 C (0.09 F) per decade over the past 15 years.
Half of the slowdown could be attributed to volcanic eruptions, whose particles reflect sunlight, and a bigger-than expected drop in heat from the Sun’s changing activity cycle, said a summary of the report.
The other half is attributed to a “cooling contribution from internal variability”.
Laurent Terray with the French computer modelling agency Cerfacs said the term is used to explain a shift in the way heat is distributed between land, sea and air.
Still unclear is what causes the variation or determines its duration.
“We know that this kind of episode, of a decadal length or thereabouts, can occur once or twice a century,” said Terray.
“If it (the present one) continues for two more decades, we may start to think that the computer models are underestimating internal variability.”