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Global Warming Hiatus Tied to Cooler Temperatures in Pacific Ocean: A Study

Aug 28, 2013 05:28 PM EDT

A team of researchers believe they have identified the cause behind the much-cited slowdown in global warming that has taken place over the last decade and a half or so, pointing to cooler temperatures in the eastern Pacific Ocean as the likely cause.

Between 1950 and 2000, global temperatures rose at a rate of 0.13 degrees Celsius per decade, according to NOAA. Since then, however, this rate has dropped to roughly 0.05 degrees Celsius, Raw Story reports.

This so-called "global warming hiatus" has taken place even in the face of a steady rise of carbon dioxide, which reached 400 parts per million for the first time in human history in May. As a result, some climate watchers have speculated that increases in carbon dioxide is not as closely tied to global warming as once believed, despite its known heat-trapping abilities.

However, in the new study, published in the journal Nature, researchers used innovative computer modeling methods to simulate regional patterns of climate anomalies that then enabled them to locate regions where global warming has been especially intense and those that have remained flat, or even undergone cooling over the last several years.

In doing so, they discovered that a natural variability in the form of cooler temperatures in the eastern Pacific Ocean to be behind the hiatus.

"Specifically the model reproduced the seasonal variation of the hiatus, including a slight cooling trend in global temperature during northern winter season," Shang-Ping Xie, co-author and first Roger Revelle Chair in Environmental Science at Scripps, said in a statement. "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 study takes into account the tropical Pacific Decadal Oscillation, a larger climate cycle that includes El Niño and La Niña, quicker cycles known to cause shifts in the distribution of warm water in the equatorial Pacific Ocean.

While both El Niño and La Niña last only a few years, the Pacific Decadal Oscillation, true to its name, takes place over the course of several decades and, starting in 1998, has been undergoing its cooling phase.

While it's not clear how long this cooling phase will last (the previous one took place roughly between 1940 to 1970), the researchers warn that once it ends, the march toward increased temperatures will resume -- with vigor.

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