A slowdown in the rate of manmade global warming is due to natural cooling cycles in the Pacific and Atlantic Oceans, but that effect is likely to reverse itself in the next few decades, climate scientists say.
The slowdown doesn't mean the planet is cooling, they emphasize; it's still warming, but at a slower rate in the last 10 to 15 years than in previous decades.
Some climate skeptics have fastened on what's being called a "hiatus" or "pause" in the warming rates to argue against human-caused climate change.
However, the authors of a new study are describing the phenomenon as a "false pause" and say the natural climate cycles in the oceans will reverse themselves in coming decades, bringing back an accelerating warming.
The researchers have analyzed actual temperature readings from 1880 to 2010 and combined them with a number of climate models they say show the observed slowdown is a result of the timing of large ocean circulation cycles, known as multidecadal oscillations, in both the Pacific and the Atlantic.
These large circulations of water across the Pacific and Atlantic and the rest of the globe's seas can cause the oceans, particularly in their upper layers, to experience natural cycles of warming and cooling, they explain.
Normally the cycles cancel each other out, they point out -- when the northern Pacific is warming, the northern Atlantic is cooling, and vice versa, so they offset their impacts on atmospheric temperatures -- but those cycles don't exactly match in their timing and magnitude.
For the last 10 years, the level of northern Pacific cooling has been larger than that of warming in the northern Atlantic, leading to an "offset [of] anthropogenic warming over the past decade," the researchers report in the journal Science.
However, that offset will probably reverse, increasing human-caused warming in the near future, they say.
"I think probably the biggest thing that people should understand is there is randomness in the climate system," says study leader Byron A Steinman, a professor of environmental and earth sciences at the University of Minnesota.
"The recent slowdown in no way invalidates the idea that continued burning of fossil fuels will increase Earth's surface temperature and pose a substantial burdens on human society," he says.
Although the researchers say they are confident in their prediction of a reversal of the slowdown followed by accelerated warming, they acknowledge it is difficult to pin down exactly when in the next few decades that may happen, adding that their study was not designed to generate a precise timeline.
However, historical data suggests the ocean patterns are "very close to a turning point," says study co-author Michael Mann, director of Pennsylvania State University's Earth System Science Center.