In 1999, a circulation pattern in the Pacific Ocean changed suddenly, which some scientists say made global temperatures plateau and led to the global warming "hiatus."

A new study, published this month in Geophysical Research Letters, lends weight to this theory. Using records stretching back to 1791, the study finds that a switch in the Pacific Decadal Oscillation or PDO has always been accompanied by changes in temperature in the north and south Pacific Ocean. It also affects the ocean heat content.

So how does this lead to the hiatus? The PDO is a switch, like its more famous cousin, the El Niño Southern Oscillation. Periodically, as dictated by nature's variability, the switch flips into a different phase.

In 1999, the PDO entered the cool phase. The ocean along the equator cooled while the northern and southern parts (the subtropics) of the Pacific became hotter. And the heating spread down to a depth of 700 meters in the subtropics.

The extra ocean heat is coming from the atmosphere, said Braddock Linsley, an oceanographer at Lamont-Doherty Earth Observatory at Columbia University and lead author of the study.

"Because the ocean is in contact with the atmosphere, there's heat exchange between the atmosphere and the surface ocean," he said. "It seems that about 90 percent of the heat that should be in the atmosphere right now with all that extra CO2 [humans have emitted since 1999] has gone into the ocean."

That has lead to a temporary pause in global temperature rise from climate change.

Looking backward to a hotter future
Scientists are only now coming to understand the PDO and the Interdecadal Pacific Oscillation, which is the wind-and-ocean circulation that extends across the entire Pacific Ocean of which the PDO is a part. One of the reasons is that pre-1950s data from the Pacific is sketchy and discontinuous. To understand the periodicity of Earth's circulation patterns, measurements going back centuries or millenia are needed.

To get around this requirement, Linsley and his colleagues constructed a historical record of the PDO using fossils. Specifically, they used coral fossils to reconstruct ocean temperatures back when the organisms were alive. The scientists collected corals from three regions—Fiji, Tonga and Rarotongo—in the southern Pacific and built a composite record of sea surface temperature for the region stretching back to 1791.

They found that sea surface temperatures have been switching in step with the PDO and the ocean heat content.

The switches have been happening consistently every 20 to 25 years.

That means that the PDO may enter a warm phase in the next five years, Linsley said. And with that, the observed hiatus will end and global temperatures will rise without reprieve.

Kevin Trenberth, a climatologist at the National Center for Atmospheric Research, said the study's reconstruction of temperature is consistent with the instrumental measurements made more recently.

Trenberth, however, believes that the PDO may switch sooner than the historical records may suggest. In fact, it may have already shifted, he said in an email.

In April 2014, the PDO moved to a positive phase. Whether this is a temporary change remains to be seen. The signs so far have been ominous—2014 was the warmest year on record (ClimateWire, Jan. 9).

If it is permanent, "it is logical to suggest that the winds and ocean currents change accordingly and switch us into a new regime where heat is not buried so deeply, and we jump to the next level in global warming," Trenberth said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500