Mystery of the Missing Heat: Upper ocean has cooled slightly in recent years, despite warming climate

Sid Perkins

Between 2003 and 2005, the top layers of the world's oceans cooled slightly, but scientists aren't sure where the heat went.

According to climate data gathered worldwide, 2003, 2004, and 2005 are three of the five warmest years since reliable record keeping of global air temperatures began more than a century ago. However, oceanographic surveys suggest that on average, the upper 750 meters of the world's ice-free oceans cooled about 0.03°C during that 3-year period.

This cooling reverses an oceanic-warming trend observed since the 1950s, oceanographer John M. Lyman and his colleagues report in the Sept. 28 Geophysical Research Letters. Between 1993 and 2003, the average temperature of the upper layers of the icefree ocean rose about 0.09°C, they note.

The newly documented cooling occurred throughout the top 750 m of ocean and seems to have extended to deeper waters as well, says study coauthor Josh K. Willis, an oceanographer now at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Data used in the new analysis were gathered by buoys tethered in deep water, instruments towed by or dropped from ships, and an armada of robotic probes, says Lyman, who's at the National Oceanographic and Atmospheric Administration (NOAA) laboratory in Seattle.

While the top layers of the ocean have cooled slightly overall, some limited areas have warmed, says coauthor Gregory C. Johnson, also of NOAA in Seattle. The cooling trend, as well as its patchiness, probably results from variations in climate cycles such as the Pacific Decadal Oscillation, he notes.

"Even within a long-term warming trend, you can have short-term drops in [ocean] temperature due to year-to-year variability," says Lyman.

Scientists are working to identify where the heat went. One possibility: It may have moved to the deepest layers of the ocean. The cooling of surface waters would cause them to contract, triggering a small drop in sea level, says Willis. But satellite data suggest that sea level is still rising. So, the missing heat may have gone deep, causing waters there to expand and prevent a decline in sea level. However, "it's hard to envision a way to put that much heat down deep so quickly," says Willis.

In another scenario under consideration, the missing heat may have radiated into space. However, satellite observations don't support that notion, says Bruce A. Wielicki, a physicist at NASA's Langley Research Center in Hampton, Va. Yet another possibility is that the heat warmed some of the waters in polar regions and promoted melting of the ice cover there, he notes.

"We have a few more pieces to unravel" about where the heat has gone, comments Sarah T. Gille, an oceanographer at Scripps Institution of Oceanography in La Jolla, Calif. "It's a real conundrum."

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Letters:

I was surprised you didn't mention the effect of salinity in ocean water in this article. Warming climate has melted much of the glaciers, bringing fresh water into the North Atlantic. That water isn't dense enough to sink and carry on the conveyor belt that usually brings warm currents from the tropics. This slowing of the conveyor belt happened during the Little Ice Age, and apparently, it's happening again.

Bella C. Chiu
Arlington, MA

Isn't it likely that the accelerated breakup of polar ice holds the key to the missing heat? Massive amounts of heat are absorbed by the solid-to-liquid phase change when ice melts, and recent observations have shown a striking reduction of ice thickness.

David Roper
Grand Junction, CO

Perhaps the heat is going into the atmosphere and is the origin of what we measure as "global warming."

Paul Etzler
Cedar City, UT

As far as scientists can tell, top-layer cooling of the world's oceans isn't an effect of melting ice in the polar regions. Some of the heat may be warming the atmosphere, scientists say. However, much of the missing heat remains unaccounted for.—S. Perkins

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References:

Lyman, J.M., J.K. Willis, and G.C. Johnson. 2006. Recent cooling of the upper ocean. Geophysical Research Letters 33(Sept. 28):L18604. Abstract available at http://dx.doi.org/10.1029/2006GL027033.

Further Readings:

Perkins, S. 2005. Ocean-sensor project reaches milestone. Science News 167(Jan. 8):30. Available to subscribers at http://www.sciencenews.org/articles/20050108/note12.asp.

Sources:

Sarah T. Gille
Scripps Institution of Oceanography
University of California, San Diego
9500 Gilman Drive
Mail Code: 0230
La Jolla CA, 92093-0230

Gregory C. Johnson
NOAA/Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Building 3
Seattle, WA 98115-6349

John M. Lyman
NOAA/Pacific Marine Environmental Laboratory
7600 Sand Point Way NE
Building 3
Seattle, WA 98115-6349

Bruce A. Wielicki
NASA Langley Research Center
Mail Stop 420
21 Langley Boulevard
Hampton, VA 23681-2199

Josh K. Willis
NASA Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109

From Science News, Volume 170, No. 14, September 30, 2006, p. 213.