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Global-mean temperature (C) and CO2 (ppm) 1971–2012, with hiatus highlighted (Image courtesy Scripps).

New research by climate scientists Shang-Ping Xie from University of Hawaiʻi at Mānoa and Yu Kosaka of Scripps Institution of Oceanography at UC San Diego attributes the attenuation of a worldwide temperature increase to a cooling of eastern Pacific Ocean waters, one that counteracts the warming effect of greenhouse gases. The study, Recent global-warming hiatus tied to equatorial Pacific surface cooling, appeared online in the journal Nature on August 28, 2013.

When the climate cycle that governs ocean cooling reverses and begins warming again, researchers predict that the planet-wide march toward higher temperatures will resume with vigor. The study does not consider when the reversal might happen, but it brings scientists closer to understanding how to look for signs of it.

Prior to 2000, global temperatures had risen at a rate of 0.13° C per decade since 1950. The hiatus has transpired while levels of carbon dioxide, the main greenhouse gas produced by human activities continued a steady rise, reaching 400 parts per million for the first time in human history in May 2013.

The disconnect led some climate watchers to speculate that increases in the concentration of carbon dioxide are not as strongly coupled to global warming, even though the heat-trapping properties of carbon dioxide have been identified for more than a century.

Climate scientists conclude, however, that natural variability in the form of eastern Pacific Ocean cooling is behind the hiatus. Innovative computer modeling methods to simulate regional patterns of climate anomalies allowed them to see global warming in greater spatial detail, revealing where it has been most intense and where there has been no warming or even cooling.

“Specifically the model reproduced the seasonal variation of the hiatus, including a slight cooling trend in global temperature during northern winter season,” said Xie, a meteorology professor at UH Mānoa’s International Pacific Research Center and the first Roger Revelle Chair in Environmental Science at Scripps. “In summer, the equatorial Pacific’s grip on the Northern Hemisphere loosens, and the increased greenhouse gases continue to warm temperatures, causing record heat waves and unprecedented Arctic sea ice retreat.”

The National Science Foundation, the National Basic Research Program of China, and the NOAA Climate Program Office supported the research.

For a full description of the study, go to the Scripps news website or visit the UH Mānoa press release.

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