Observations of climate change often report a “missing heat” – a hiatus in the global warming, which went unaccounted for; but now, a new study concludes that the heat absent from the Earth’s surface for more than a decade is plunging deep in the north and south Atlantic Ocean, and this is part of a naturally occurring cycle.
(Top) Global average surface temperatures, where black dots are yearly averages. Two flat periods (hiatus) are separated by rapid warming from 1976-1999. (Middle) Observations of heat content, compared to the average, in the north Atlantic Ocean. (Bottom) Salinity of the seawater in the same part of the Atlantic. Higher salinity is seen to coincide with more ocean heat storage.
Credit: K. Tung / Univ. of Washington
Climate change deniers have used this argument for more than a decade: if the world is actually heating up, then why is it heating up so little? Where is all the heat? An increasing number of research is starting to show that while we are mostly focusing temperature measurements on the atmosphere, the deep oceans are heating up more than ever. Now, a new study conducted by University of Washington confirms that, showing that temperatures in the deeper part of the Atlantic Ocean have witnessed a massive warming process.
“Every week there’s a new explanation of the hiatus,” said corresponding author Ka-Kit Tung, a UW professor of applied mathematics and adjunct faculty member in atmospheric sciences. “Many of the earlier papers had necessarily focused on symptoms at the surface of Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause.”
Indeed, there have been several theories regarding the hiatus, and while several of them probably have valid points to some degree, the deep ocean seems to be accounting for the big part of the missing heat.
In order to reach this result, they studied a slow moving current in the Atlantic which carries heat between the two poles. What they found was an increase in its speed, drawing heat down by almost a mile (1.5 km).
“The finding is a surprise, since the current theories had pointed to the Pacific Ocean as the culprit for hiding heat,” Tung said. “But the data are quite convincing and they show otherwise.”
Tung and his co-author Xianyao Chen of the Ocean University of China, who was a UW visiting professor last year used recent deep-sea observations 6,500 feet (2,000 meters) depth. The data show an increase in heat sinking around 1999, when the rapid warming of the 20th century stopped.
“There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans,” Tung said. “After 30 years of rapid warming in the warm phase, now it’s time for the cool phase.”
Basically, the cycle starts when saltier, heavier water starts to sink deeper. The heat has an opposite, but smaller effect (hotter water tends to be lighter); when the surface water sinks, it takes with it the heat to the deeper parts of the ocean.
“When it’s heavy water on top of light water, it just plunges very fast and takes heat with it,” Tung said. Recent observations at the surface in the North Atlantic show record-high saltiness, Tung said, while at the same time, deeper water in the North Atlantic shows increasing amounts of heat.
To make things even complicated, climate change might add a big extra variable: sweet water from the melting glaciers could upset this circuit.
“We are not talking about a normal situation because there are so many other things happening due to climate change,” Tung concludes.
Story Source: The above story is based on materials provided by University of Washington.
