WITH 2014 likely to be declared the world's hottest year on record, the last thing the planet needs is a climate shift to turbo-charge the global warming already under way.
While it's an early call, a measure of surface temperature differences in the Pacific shifted to a positive reading in the five months of November, according to the US National Oceanic and Atmospheric Administration – the longest such run in almost 12 years.
Known as the Pacific Decadal Oscillation, the El Nino-like pattern typically lasts 15-30 years and is understood to operate as an accelerator on global surface temperatures during its positive phase – and a brake during its negative phase – as the ocean takes up fluctuating amounts of the extra heat being trapped by rising greenhouse gas emissions.
"It certainly could be an early sign of a change but you'd probably want to see another year or two before it's a genuine phase shift," Matthew England, a professor at the University of NSW's Climate Change Research Centre, said. "This could be the start of another ramping up of warming."
The last positive phase of the PDO, also known as the Interdecadal Pacific Oscillation, ran from about 1978 to 1998, a period of a rapid increase of surface temperatures. Since then, temperature increases have flattened out, despite an increase in greenhouse gases, as oceans have taken up more of the excess heat.
Even with the negative-PDO drag - worth about 0.2 degrees in global average air temperatures a decade - 2005 and 2010 both topped the 1998 record annual global temperatures. The first 11 months of 2014 were the warmest on record and with ocean temperatures remaining exceptionally warm, this year is very likely to set a fresh high, according to NOAA, the Bureau of Meteorology and other agencies.
In positive-PDO periods, the tropical Pacific is relatively warm and north of about 20 degrees latitude, it should be cool, said Shayne McGregor, a UNSW research fellow, said: "It's definitely consistent with what we've seen in tropics."
"During a positive PDO phase, you'd expect temperatures to keep climbing again as they did in the 1980s and 1990s," Dr McGregor said, adding that as PDOs are measured by rolling 11-year averages, it will be a while before any shift becomes clear.
Cai Wenju, a principal scientist with the CSIRO, agreed it will take time before any index shift is clear. Still, the next positive phase would likely bring faster warming at the surface and worsening drought conditions for Australia and much the region, he said.
"When it is positive, it tends to give us less rainfall because convection is shifting away from the western Pacific," Dr Cai said, adding that floods worsen in Ecuador, Peru and elsewhere on the Pacific's eastern rim.
While 0.2 degrees either way can set new global records or stall the march higher of surface temperatures – about 0.05 currently separates the five warmest years – over the longer term, global warming will swamp even interdecadal fluctuations, Professor England said.
"If we stay with relatively flat temperatures for another five years, it means nothing for global warming projections for the end of the century," Professor England said, noting temperatures may then be 4-7 degrees hotter on current greenhouse gas trajectories.
"The chance of that tiny 0.2-degree decadal signature overwhelming the greenhouse warming becomes vanishingly small."
