Cause for ‘The Pause’ #38 – Cause of global warming hiatus found deep in the Atlantic Ocean

Anthony Watts:

From the University of Washington  and the department of Trenberth’s missing heat comes a claim that we’ll have to wait another 15 years for global warming to resume. Sounds like a goalpost mover to me.

The Oceans that Slowed 21st Century Global Warming
Why did the rapid global warming that characterized the latter part of the 20th century slow down over the last 15 years or so? Many different theories have been proposed, but a new study suggests that a massive movement of heat from shallow surface waters to deep regions of the Atlantic and Southern Oceans — but not the Pacific Ocean, as many researchers had predicted — might be responsible. Xianyao Chen and Ka-Kit Tung analyzed data from profiling floats, or oceanographic sensors that can move vertically throughout the water column, and traced the pathways that heat has taken through the world’s oceans since the turn of the 21st century. The oceans are capable of storing about 90% of the world’s surface heat content, and the researchers suggest that most of the excess heat that would have otherwise continued to fuel global warming is currently stored in the basins of the Atlantic and Southern Oceans.

(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

The researchers also suggest that a sudden shift in salinity that corresponded with the slowdown of global warming at the beginning of the 21st century may have triggered this migration of heat to deeper waters. Historically, similar events have lasted 20 to 35 years, according to Chen and Tung. Consequently, the researchers suggest that global warming will pick back up in 15 more years or so, when heat returns to the surface waters.

Article #11: “Varying planetary heat sink led to global-warming slowdown and acceleration,” by X. Chen at Ocean University of China in Qingdao, China; X. Chen; K.-K. Tung at University of Washington in Seattle, WA.

================================================================

Following rapid warming in the late 20th century, this century has so far seen surprisingly little increase in the average temperature at the Earth’s surface. At first this was a blip, then a trend, then a puzzle for the climate science community.

More than a dozen theories have now been proposed for the so-called global warming hiatus, ranging from air pollution to volcanoes to sunspots. New research from the University of Washington shows that the heat absent from the surface is plunging deep in the north and south Atlantic Ocean, and is part of a naturally occurring cycle. The study is published Aug. 22 in Science.

Subsurface warming in the ocean explains why global average air temperatures have flatlined since 1999, despite greenhouse gases trapping more solar heat at the Earth’s surface.

“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 the Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause.”

The results show that a slow-moving current in the Atlantic, which carries heat between the two poles, sped up earlier this century to draw heat down almost a mile (1,500 meters). Most of the previous studies focused on shorter-term variability or particles that could block incoming sunlight, but they could not explain the massive amount of heat missing for more than a decade.

“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 co-author Xianyao Chen of the Ocean University of China, who was a UW visiting professor last year, used recent observations of deep-sea temperatures from Argo floats that sample the water down to 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.

Read more