Average global temperatures have been essentially flat for nearly the past 17 years, even as heat-trapping carbon dioxide from burning fossil fuels has accumulated in the atmosphere. Now a new study published in Science reports that the current "pause" in man-made global warming is the result of currents in the North Atlantic burying the extra heat in the deep ocean. The researchers postulate that this phenomenon flips back and forth between hot and cold phases every 20 to 35 years. When it flips out of the current cold phase, average temperatures will begin to rise steeply.
The BBC reports:
The researchers say that there was another hiatus between 1945 and 1975 due to this current taking down the heat, that led to fears of a new ice age.
From 1976 though, the cycle flipped and contributed to the warming of the world, as more heat stayed on the surface.
But since the year 2000, the heat has been going deeper, and the world's overall temperatures haven't risen beyond the record set in 1998. …
A key element in this new understanding is the saltiness of the water. The waters in the Atlantic current coming up from the tropics are saltier because of evaporation. This sinks more quickly and takes the heat down with it.
Eventually though, the salty water melts enough ice in Arctic waters to lower the saline level, slowing down the current and keeping the heat near the surface.
"Before 2006 the saltiness was increasing, this indicated that the current was speeding up," said [principal researcher] Prof [Ka-Kit] Tung [from the University of Washington].
"After 2006, this saltiness is diminishing but it's still above the long-term average. Now it is slowly slowing down.
"Once it gets below the long-term average, then it is the next period of rapid warming." …
"We probably may have another 10 years, maybe shorter as global warming itself is melting more ice and ice could flood the North Atlantic, but historically we are in the middle of the cycle."
The U.N Intergovernmental Panel on Climate Change 2013 Physical Sciences report suggests that the current temperature slow-down will soon end and states, "It is more likely than not that internal climate variability in the near-term will enhance and not counteract the surface warming expected to arise from the increasing anthropogenic forcing." In other words, when the warm-up resumes it will soar. By how much? The IPCC report projects, "The global mean surface temperature change for the period 2016-2035 relative to 1986-2005 will likely be in the range of 0.3°C to 0.7°C." This implies increases of 0.15°C to 0.35°C per decade.
Hat tip Ken Constantino.