The pace of global warming is likely to quicken in coming years as natural processes in the Pacific switch from serving as a brake to an accelerator, placing the planet on course to exceed a landmark level within a decade, according to a new paper.
The Interdecadal Pacific Oscillation (IPO), a cycle that lasts 10-30 years and affects how much heat is absorbed in the Pacific, started to switch to its positive or "warm" phase since 2014. During positive periods, the central ocean is relatively warm compared with surface waters at higher latitudes.
The paper by Melbourne University researchers, published this week in Geophysical Research Letters, argues the preceding negative phase - lasting from 2000-2014 - may have provided a "temporary buffer" for surface temperature increases and "cushioned the impacts of global warming on extreme events, such as heatwaves".
"A turnaround of the IPO to its positive phase could initiate a period of accelerated warming over the next one or two decades," Ben Henley and Andrew King state. "This would likely lead to the Paris target of 1.5 degrees (warming since pre-industrial times) being surpassed within the next decade."
"The negative phase might have lulled us into a false sense of security - the planet appears temporarily to not be warming as fast," Dr Henley, an associate of the ARC Centre of Excellence for Climate System Science, told Fairfax Media.
Almost 200 nations signed up to the Paris accord in late 2015, agreeing to keep temperature rises between 1.5 to 2 degrees to prevent dangerous climate change. President Donald Trump this week postponed a meeting of advisers that was expected to decide whether the US will exit the climate deal.
The IPO is less well known and studied than the El Nino Southern Oscillation, a five to seven year cycle that also has a planet-wide influence on the weather. A strong El Nino in 2015-2016 helped boost background warming, making 2015 and then 2016 the hottest years on record.
Depending on the data used, global surface temperatures have warmed more than a degree compared with the baseline readings for 1850-1900. (See chart below showing the increase in temperatures and the IPO phases.)
The 1.5-degree lower end of the Paris target could be breached by 2026 - in the absence of other cooling events such as an increase in volcanic eruptions - if greenhouse emissions continue to track at the high end of model forecasts and the IPO switches, the paper found.
"This doesn't signal a failure in the Paris agreement but highlights how close we are to 1.5 degrees and the urgency of turning it around as soon as possible," Dr Henley said.
Malte Meinshausen, director of Melbourne University's Climate and Energy College, said the current unprecedented back-to-back coral bleaching covering as much as two-thirds of the Great Barrier Reef, even at current warming levels, was a reminder at what is at stake.
"With 2 degrees warming, we likely do not see much coral reef cells surviving," Professor Meinshausen said. "Already at 1.5 degrees warming, the chance are slim for many - as we unfortunately witness right now."
The IPO has switched several times since the warming phase identified for the period from 1925 to the mid-1940s. Even if the IPO stayed in its "cool" phase, the 1.5-degree warming mark would likely be surpassed by 2031, Dr Henley said.
(See chart below showing how model projections indicate when the 1.5-degree mark will be reached)
"That's really at the cutting edge - it's exactly what we're trying to work out," Dr Henley said.