"We've found some of the missing energy in the deeper parts of the ocean, and that's the part that relates to the hiatus," he said. "What has happened in the last decade or so is more heat is going into the ocean."
A study published yesterday in the journal Science bolsters that idea. It uses fossil data to reconstruct past temperatures in the Pacific Ocean. That research shows that the middle depths of the ocean, between about 1,500 and 3,300 feet deep, have warmed 15 times faster in the past 60 years than at any time during the past 10,000 years.
Other research has also pointed to the Pacific as a storehouse for additional heat (Greenwire, Aug. 28).
This period of slow surface warming is not unique, Trenberth added. There have been times of lower surface temperatures in the past, like from 1977 to 1986 and from 1987 to 1996.
After each of those nearly decadelong spans without surface warming, temperatures rapidly jumped up again, continuing their inexorable upward trend, Trenberth said.
Volcanoes, aerosols, computer models and other mysteries
Benjamin Santer, a climate scientist at Lawrence Livermore National Laboratory, sees research into the pause in surface warming as a sort of scientific mystery that helps researchers better understand how the Earth's climate works.
Santer thinks there are a few reasons for the surface warming slowdown, including natural climate variations, which include extra heat going into the ocean, which, as he pointed out, has continued to warm.
"There's this rich internal climate variability, so it's easily possible to get a short 10- or 15-year period with little or no [surface] warming, even with human-released greenhouse gases," Santer said.
He's also working to see whether factors outside of climate variability, such as small volcanic eruptions, air pollution or even errors in the measurements of atmospheric temperatures, may be playing a role.
Santer believes one of the reasons climate models may on average predict more surface warming than has actually occurred is that they are leaving out the cooling effects of small volcanic eruptions over the past decade, which reflect more heat out of the atmosphere.
They may also underrepresent cooling from aerosol pollutants from industrial activities.
"If model simulations leave out important cooling influences that have affected the real world over the hiatus period, then you are going to get the wrong answer," Santer said.
Potent ammunition for climate contrarians
Santer expressed frustration that scientists and others who are skeptical of global warming had used the pause in temperature increases as evidence to say climate change is not happening.
In fact, testimony by William Happer, a physicist at Princeton University who pointed out the pause in warming to members of Congress, spurred some of Santer's research into the topic.
Happer believes that carbon dioxide does have a warming effect on the atmosphere but that its effect has been wildly overestimated by climate models.
Happer first noted the slowdown in surface warming in 2005, he said.
After the 1998 El Niño, which had a warming effect, and a subsequent La Niña, which had a global cooling effect, "it never really started warming again," Happer said. "It just sort of settled down to a flat plateau which we are still in. ... And by now it's completely at odds with some of the models."
To Happer, the lesson of the slowdown in surface temperatures is that models greatly overestimate the role of CO2 in warming the atmosphere.
Richard Lindzen, a professor of meteorology at the Massachusetts Institute of Technology and another prominent critic of climate models, said the recent slowdown in surface warming demonstrates how climate models fail to simulate natural variability.