“It’s contentious,” said IPCC panelist Shang-Ping Xie, a professor of climate science at the Scripps Institution of Oceanography at UC San Diego. “The stakes have been raised by various people, especially the skeptics.”

Though scientists don’t have any firm answers, they do have multiple theories. Xie has argued that the hiatus is the result of heat absorption by the Pacific Ocean — a little-understood, naturally occurring process that repeats itself every few decades. Xie and his colleagues presented the idea in a study published last month in the prestigious journal Nature.

The theory, which is gaining adherents, remains unproved by actual observation. Surface temperature records date to the late 1800s, but measurements of deep water temperature began only in the 1960s, so there just isn’t enough data to chart the long-term patterns, Xie said.

Scientists have also offered other explanations for the hiatus: lack of sunspot activity, low concentrations of atmospheric water vapor and other marine-related effects. These too remain theories.