"All other things being equal, adding more greenhouse gases to the atmosphere will have a warming effect on the planet," Curry said. "However, all things are never equal, and what we are seeing is natural climate variability dominating over human impact."

Curry isn't the only one to suggest flaws in established climate models. IPCC vice chair Francis Zwiers, director of the Pacific Climate Impacts Consortium at the University of Victoria in Canada, co-wrote a paper published in this month's Nature Climate Change that said climate models had "significantly" overestimated global warming over the last 20 years — and especially for the last 15 years, which coincides with the onset of the hiatus.

The models had predicted that the average global surface temperature would increase by 0.21 of a degree Celsius over this period, but they turned out to be off by a factor of four, Zwiers and his colleagues wrote. In reality, the average temperature has edged up only 0.05 of a degree Celsius over that time — which in a statistical sense is not significantly different from zero.