A naturally-occurring shift to a warmer Pacific Ocean caused global temperatures to spike in the previous three years and end the “hiatus” in global warming, according to scientists.
The UK’s Met Office reports a shift to a positive, or warmer, Pacific Decadal Oscillation (PDO) coincided with the three warmest years on record globally — 2014, 2015 and 2016.
Met Office scientists said the PDO shift caused global temperatures to increase back to levels seen in the 20th Century. The implication being that natural forces still play a dominant role in global climate.
“Yes, strong positive values of the PDO were the main reason we have seen record high global temperatures the past few years,” climatologist Judith Curry told The Daily Caller News Foundation.
“We are now starting to see the PDO head into negative territory since July,” Curry told TheDCNF. “A cool phase of the PDO was largely responsible for the warming hiatus seen in the early years of the 21st century”
On top of the warm PDO, a strong El Niño began to warm the tropical Pacific in 2015, helping to end the “hiatus” in warming. Overlaying all of this was a warm PDO phase. Both are naturally-occurring phenomena.
“I am pleased to see that these multidecadal oscillations are receiving more attention and credit for causing variations in global temperature,” Curry said.
“From my perspective, these large-scale ocean oscillations are the intrinsic climate signal, with external forcing [CO2] projecting onto these natural oscillations,” she said.
The PDO shift warmed the tropics, the U.S. west coast and ended the 15-year “hiatus” or “slowdown” in global warming, the Met Office reported. During that time, global average surface temperature displayed little to no warming.
PDOs are like long-term El Niños and can last for decades. A positive PDO brings cool waters to “the interior North Pacific and warm along the Pacific Coast,” says the National Oceanic and Atmospheric Administration (NOAA), while a negative phase brings the opposite.
“There is no question that the PDO index has had large positive values from January 2014 to present, and this is simply a reflection of the pattern of sea surface temperature anomalies in the North Pacific Ocean,” Nate Mantua, a NOAA ecologist who tracks PDO shifts, told TheDCNF.
Scientists aren’t quite sure what exactly causes the PDO patterns, and they aren’t able to predict them very well either. Warm phases are associated with colder and drier winters, while cold phases tend to have warmer, wetter winters.
“So the PDO pattern has been in a positive phase for almost three years now, and that coincides with the run of record-warm global mean surface temperatures,” Mantua said.
Curry also said the PDO seemed to be heading into a negative phase since July, which has persisted into August. If it persists long enough, it could coincide with a shift to a cool Atlantic Multidecadal Oscillation (AMO).
“Last time we had negative PDO and AMO was the late 1960s to early 70s,” Curry said.
Forecasters also predict a La Niña to form this winter, meaning the Pacific Ocean will be cooler than normal.
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