It’s been called the “hiatus,” “pause,” or “slowdown” and has been a favored meme of climate skeptics for years.
Despite the continued increase of greenhouse gas emissions from us, rise of global surface temperatures has been easing since 1998.
Two new studies published this week examine the origins of the“pause,” and, surprisingly, suggest that it may persist for years even in our notably warming world.
The first study, published on Feb. 26 in the journal Science, looked into likely causes. “It appears as though internal variability has offset warming over the last 15 or so years,” Byron A. Steinman, lead author of the paper and assistant professor at the University of Minnesota Duluth, told Quartz.
That internal variability is found in the natural cycles of temperature change that occur over years or even decades in the oceans, like El Niño and La Niña. There are others, like the “Atlantic multidecadal oscillation” and the “Pacific decadal oscillation,” which Steinman said are leading culprits for the warming slowdown.
The paper, which was co-authored by Michael E. Mann and Sonya K. Miller of Pennsylvania State University, found an oceanic tug-of-war between the two systems. Sometimes the ocean cycles worked together to suck heat or burp it skyward—sometimes their push-pull led to a draw.
The second study, published on Feb. 23 in the journal Nature Climate Change, took up the question of how long our warming break might last.
Chris D. Roberts and colleagues at the Met Office Hadley Center in Britain looked at the pause’s possible lifespan. Using a suite of climate models, they estimated that there is good chance, up to 25%, of it continuing until the end of the decade.
More troubling are the odds that the end of the hiatus, whenever it does happen, will be followed by a five-year period of accelerated warming. This could mean that global surface temperatures rise at twice the normal rate of 0.36°F per decade. They put the chances of that warm burst at up to 60%.
The Science study, which looked at how a warming pause is created, took over 150 models and let them age from 1850 to 2012.
This gave the researchers a tally of the random natural ripples inside the climate system (those Atlantic and Pacific temperature variations) and ones outside the system called “forcings” (atmosphere-cooling volcanic eruptions and changes in the sun’s strength over time). They then took the observed ground and ocean surface temperatures for the past 130 years and subtracted the volcanoes and the sun. That gave them a measure of the power of ocean cycles to create the warming pause.
The Nature Climate Change study looked at an “archive of 15,000 years of simulated climate” to see what a nature-made hiatus typically looks like.
When they looked at a subset of models that matched well to temperature trends in the Pacific Ocean, they found that a natural 5-year-long hiatus could occur up to 30% of the time. There was about a 10% chance of a 10-year-long warming pause. And at 20 years, the chances were about 1%.
Roberts told Quartz that this all suggests our current warming pause is unique, but, despite the low probability, it is also “very possible” that the pause could continue a few more years. And that wouldn’t be inconsistent with what we know about the effects of the heat-trapping ocean oscillations at work in the Science study.
The idea that the oceans are storing the heat that we should be feeling isn’t new. Our ability to measure that drowned heat has gotten better of the past 15 years, thanks to an ever-expanding, semi-autonomous armada of diving buoys.
Through that army, scientists have been honing in on the mystery of the hiatus by searching for the specific ways (and locations where) heat is entering the oceans. It turns out the Pacific Ocean is playing a big role where winds are helping churn the waters and suck in heat.
But sinking heat is just one side of a seesaw.
Michael E. Mann, a co-author on the Science paper, told Quartz in an email,“the Pacific Ocean has been in a natural ‘cooling’ mode, which has slowed the warming of the globe, but we expect that to reverse in the near future.”
Some even say that 2014, the hottest year on record, already marked the end of the hiatus. But Roberts of the Met Office advised caution before calling it officially off. “I would argue that we need a run of several unusually warm years to be able to definitively identify the end,” he said.
All of the researchers who spoke to Quartz about the two studies agreed that the warming pause was just that.
“Eventually we expect temperatures to ‘catch up,’ but it may take longer than five years for that to happen,” Roberts told Quartz.