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UW Study: Despite 'Hiatus' In Rising Temps, Oceans Show Globe Still Warming

Laura Rauch
AP Photo
FILE - The sun shines on Point Barrow, Alaska, and the Arctic Ocean at the northern tip of the United States, July 16, 2002.

Despite widespread concern about global warming, rising air temperatures have actually slowed down dramatically over the past 15 years. This so-called “hiatus” has posed a big puzzle for climate scientists.

Researchers at the University of Washington looked deep into the oceans for answers, and found that despite the surficial evidence, climate change has not stopped. 

After several decades of marked rising, average global air temperatures have flatlined since 1999. Ka-KitTung, a professor of applied mathematics at the University of Washington, says while some climate-change deniers latched on to this finding, he never doubted the heat was going somewhere.

“Why doesn’t it warm? I think that is a great mystery,” Tung said.

To find answers, Tung and a colleague analyzed data from the oceans. They looked at temperature and salinity measurements more than a mile deep, gathered by torpedo-like Argo floats around the globe. Saltier water is denser and tends to sink, taking heat with it.

Credit Ka-Kit Tung / University of Washington
University of Washington
(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.

The researchers found that in 1999, all signs pointed to more heat sinking deep into the ocean, corresponding very closely with the time when the rapid warming of the 20th century stopped on the surface.  

Tung says the finding explains where the heat has been going – due to natural cycles of water flow from the Atlantic – and debunks the idea that warming had stopped.

“This is not denying that there’s global warming. The global warming is there, but sometimes it’s reinforced by natural cycles, and sometimes it’s partly canceled by natural cycles,” Tung said.

Based on historical data, he says this natural cycle, which is driven by the physics of melting ice, normally lasts about 30 years. This points to a possible return of rapid warming in about 15 years, although there are many complicating factors and it’s hard to predict what will happen next.

The research is published in the current issue of Science.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to