A climate change mystery is coming into sharper focus.
A new study released in the journal Science explains why despite the rise of heat-trapping gases in the atmosphere, global warming has slowed over the last 15 years. Some scientists point to the cooling effect of volcanoes or changes in solar activity.
Lead author and University of Washington applied mathematics professor Ka Kit Tung suggests massive movement of heat from shallow to deep regions of the ocean is responsible.
“From the energy standpoint, the warming does not just warm the surface. It should warm the whole ocean column," said Tung. "There is evidence that the integrated temperature of the whole ocean column has been increasing even through the current period of pause.”
Global warming heat stored in Atlantic, Southern oceans
Ninety-three percent of the excess heat caused by global warming gases is stored in the oceans. Tung used underwater sensors to measure the temperature at various depths in the water column over the last three decades of the 20th century. Climate models indicated that the Pacific Ocean was hiding the heat.
But observations showed the heat is sinking deep in the Atlantic and Southern Oceans, part of a natural 30-year cycle. Tung said the current phase, with cooler temperatures at the ocean surface, started in 1999 when the rapid warming of the last century slowed down. “And we are currently in the middle of this 30-year period where more heat is going into the ocean, Tung said, “That’s why what [heat] remains near the surface has not been much, not much warming.”
Salty seawater triggers heat migration into ocean depth
Tung's study finds that the excessive heat now stored in the ocean depths moves like a conveyor belt between the north and south poles. The warm water becomes saltier as it travels through the subtropics because there is more evaporation of surface water at those latitudes.
Salty seawater is heavier and sinks, making the sea surface - and the air above it - cooler. Tung said that accounts for the warming pause we’re experiencing now. Temperatures will rise, however, when the ocean cycle flips to its warm phase. “The rate of surface temperature increase will be just as high, or even higher, than what we experienced in the last three decades of the 20th century. But the actual temperature that it starts with is now the highest temperature, but it is going to go up very rapidly. So the next phase of accelerated warming is going to be very damaging.”
The current global warming slowdown, Tung predicts, could last a decade or longer.