Correcting Ocean Cooling

by Rebecca Lindsey November 5, 2008

On a Thursday evening in February 2007, Josh Willis stood in front of his laptop, his wife cajoling him to get ready to go out to dinner. He looked with a sinking feeling at the map he had just made. Willis, a scientist at NASA’s Jet Propulsion Laboratory, specializes in making estimates of how much heat the ocean stores from year to year.

Photograph of Josh Willis

Josh Willis is an oceanographer at NASA’s Jet Propulsion Laboratory who specializes in sea level trends and the response of the oceans to global warming. (Photograph courtesy Josh Willis.)

“The oceans are absorbing more than 80 percent of the heat from global warming,” he says. “If you aren’t measuring heat content in the upper ocean, you aren’t measuring global warming.”

In 2004, Willis published a time series of ocean heat content showing that the temperature of the upper layers of ocean increased between 1993-2003. In 2006, he co-piloted a follow-up study led by John Lyman at Pacific Marine Environmental Laboratory in Seattle that updated the time series for 2003-2005. Surprisingly, the ocean seemed to have cooled.

Not surprisingly, says Willis wryly, that paper got a lot of attention, not all of it the kind a scientist would appreciate. In speaking to reporters and the public, Willis described the results as a “speed bump” on the way to global warming, evidence that even as the climate warmed due to greenhouse gases, it would still have variation. The message didn’t get through to everyone, though. On blogs and radio talk shows, global warming deniers cited the results as proof that global warming wasn’t real and that climate scientists didn’t know what they were doing.

Maps of change in heat content in the oceans from 1993 through 2003, and 2003 through 2005.

Josh Willis and his colleagues concluded that the world’s oceans gained heat in the decade from 1993 to 2003 (top). However, a follow-up study for the years 2003 to 2005 showed a surprisingly large decrease in heat content—about 5 times as large as the previous decade’s warming (bottom). Areas that warmed are red, while areas that cooled are blue. Note that the scale of each map is different: the 1993–2003 map ranges from -12 to +12 watts per square meter, while the 2003–2005 map ranges from -60 to 60.

Willis initially interpreted the cooling as a temporary fluctuation, a “speed bump” on the road to global warming. (Maps by Robert Simmon, based on data from Josh Willis and John Lyman.)

That February evening, Willis says, he was updating maps and graphs with the data that had become available since the 2006 ocean cooling paper was published. He was preparing for a talk he had been invited to give at the National Center for Atmospheric Research in Boulder, Colorado. The topic was “Ocean cooling and its implications for understanding recent sea level trends.”

He was looking at a map of global ocean temperatures measured by a flotilla of autonomous, underwater robots that patrol the world’s oceans. The devices—Argo floats—sink to depths of up to 2,000 meters, drift with the currents, and then bob up to the surface, taking the temperature of the water as they ascend. When they reach the surface, they transmit observations to a satellite. According to the float data on his computer screen, almost the entire Atlantic Ocean had gone cold. Unless you believe The Day After Tomorrow, Willis jokes, impossibly cold.

Photograph of an Argo float surfacing.

Argo floats are aquatic robots that measure ocean temperature, pressure, and salinity at depths of up to 2,000 meters. The floats augment satellite, ship, and buoy measurements of the ocean. (Photograph © 2004 Sabrina Speich, Argo Information Centre.)

“Oh, no,” he remembers saying.

“What’s wrong?” his wife asked.

“I think ocean cooling isn’t real.”