Where did global warming go? The deep ocean, experts say

The deep oceans have recently been soaking up much of the excess heat trapped under the ever-thickening blanket of greenhouse gases that humans pump into the atmosphere, according to a recent study.
By John Roach, Contributing Writer, NBC News

The deep oceans have recently been soaking up much of the excess heat trapped under the ever-thickening blanket of greenhouse gases that humans pump into the atmosphere, according to a recent study.

The finding may help explain why the pace of global warming at the surface has slowed in recent years compared to the 1990s, a phenomenon that has left members of the climate science community scratching their heads.

"The warming at the surface hasn't stopped, but it has been less than most of the climate models have been predicting," David Pierce, a climate researcher with the Scripps Institution of Oceanography, explained to NBC News. "So the question is: Where is that extra heat going?"

Kevin Trenberth and colleagues at the National Center for Atmospheric Research reanalyzed ocean temperature records between 1958 and 2009. They found that about 30 percent of the extra heat has been absorbed by the oceans and mixed by winds and currents to a depth below about 2,300 feet.

Oceans are well-known to absorb more than 90 percent of the excess heat, but its presence in the deep ocean "is fairly new, it is not there throughout the record," Trenberth said during a teleconference with reporters on Thursday. "So the question is: What happened to produce this?"

To find out, the team used a model that accounts for variables including ocean temperature, surface evaporation, salinity, winds and currents, and tweaked the variables to determine what causes the warming at depth.

"It turns out there is a spectacular change in the surface winds which then get reflected in changing ocean currents that help to carry some of the warmer water down to this greater depth," Trenberth said. "This is especially true in the tropical Pacific Ocean and subtropics."

The change in winds and currents, he added, appears related to a pattern of climate variability called the Pacific Decadal Oscillation which in turn is related to the frequency and intensity of the El Niño/La Niña phenomenon, which impacts weather patterns around the world.

The oscillation shifted from a positive stage to a negative stage at the end of the extraordinarily large El Niño in 1997 and 1998. The negative stage of the oscillation is associated more with La Niñas, which is when the tropical Pacific Ocean is cooler and absorbs heat more readily, Trenberth explained.

"So, some of this heat may come back in the next El Niño event … but some of it is probably contributing to the warming of the overall planet, the warming of the oceans. … It means that the planet is really warming up faster than we might have otherwise expected," he said.

While this ocean mixing has been suggested by some of the models scientists use to simulate the global climate, the new study is the first to re-analyze the observational record to get at an answer, noted Pierce, who was not involved in the study.

This new work, he said, should compel the climate science community to incorporate the mixing into the full suite of models, which in turn could improve climate forecasts in the 5- to 10-year time frame most relevant to planning agencies.

"What people are getting more and more interested in is what's the actual trajectory going to be …this sort of exchange between the surface and the deep they found in this paper really affects the actual trajectory you'll see," explained Pierce.

For example, knowing when the Pacific Decadal Oscillation will switch back to the warm phase could benefit planners on the U.S. West Coast. That's because sea level rise there has been suppressed for the past two decades, Joshua Willis, a project scientist at NASA's Jet Propulsion Laboratory, noted during the teleconference.

"In California," he said, "I like to say we are running a sea level deficit of about 6 centimeters and over the next 10 or 20 years we'll probably make that up and then some."

The findings were reported online earlier this month in the journal Geophysical Research Letters.

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.

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