Despite the fact that humans keep pumping greenhouse gases into the atmosphere, in the last decade the Earth has not warmed as much as scientists might expect.
While climate change skeptics tend to trumpet this fact as evidence that the Earth is not warming, researchers know that more heat is being trapped in the Earth's systems. The difficulty is puzzling out where that heat going if it is not showing up in increased temperatures on the surface.
Scientists looking into this problem have posed various explanations. These include decreased water vapor in the stratosphere, which could cool the Earth, and increased sulfur emissions from China blocking heat.
A paper released yesterday in Nature Climate Change, though, adds to a case that a number of researchers are now making: that the past decade's missing warmth is in the ocean.
Virginie Guemas, the first author on the paper and a researcher at Catalan Institute of Climate Science in Spain, said her group was able to show that most of the missing heat, at least in the early part of the decade, was being stored in the top 700 meters of the tropical Pacific and Atlantic oceans and in the north Atlantic Ocean.
The researchers did this by using data from ocean observations and putting them into a European climate model, which they ran retrospectively, seeing whether it could predict the heat going into the oceans.
"This is quite important in the current debate because it means this global warming slowdown is mainly due to natural variability," Guemas said.
Previous work by scientists at the National Center for Atmospheric Research in Boulder, Colo., has also pointed to the oceans as a sink for excess heat.
Explanations for 'hiatus periods'
They have posed the idea of 10- to 15-year-long "hiatus periods" in which the temperature of the surface does not rise but the heat goes to the ocean instead.
Gerald Meehl is a senior scientist at NCAR and a lead author on the 2011 paper in Nature Climate Change modeling what happened during such hiatus periods. He showed that in the time periods when the Earth's surface temperatures are not rising, the ocean layer between 300 and 700 meters deep and another layer, below 700 meters, take up that heat.
"We're looking at the same phenomenon," said Meehl, responding to the recent paper by Guemas and others.
Robert Kaufmann, a researcher at Boston University who co-authored a 2011 paper published in Proceedings of the National Academy of Sciences, offered some different explanations for where the heat has gone.
Kaufmann found the oceans explanation interesting and worth investigating but said, "I'm not willing yet to abandon some of the other papers, including mine, that the authors cited as possible explanations" for where the heat has gone.
The statistical model Kaufmann and his colleagues put together to explain the lost heat proposed it was due to a natural decline in the amount of solar radiation hitting the Earth, the switch from an El Niño to a La Niña in 1998 and increased sulfur emissions in China blocking radiation.
That model, he said, has continued to perform accurately as it has been updated with new data, another reason to think it might be a good explanation.
Kevin Trenberth, another NCAR scientist, published a paper online last week in the journal Geophysical Research Letters that used ocean observations to show that the excess heat is going into the ocean, similar to the recent Nature Climate Change paper by Guemas.
In an interview, Trenberth pointed out that from a climate scientist's perspective, what global warming really means is that humans' output of greenhouse gases is trapping more energy in the Earth's climate system, not just that air temperatures are rising at the Earth's surface.
In fact, he added, most of that energy does not normally translate into increased air temperatures, but it goes into various parts of the climate system.
Will the oceans' heat 'come back to haunt us'?
Mostly it's absorbed by the oceans, and some goes into melting sea and land ice, or increasing evaporation and clouds. Just a small amount of it goes into increasing surface temperatures, and right now it's even less than scientists would expect.
Trenberth's recent paper shows a significant amount of the heat going into the deep ocean. "Since about 2000, about 30 percent of the heat has gone below 700 meters [deep] according to our analysis."
He said this does not contradict the Guemas analysis.
"Quite a bit [of heat] has gone between 300 and 700 meters, which seems to be the focus of this Nature Climate Change article," he said. "What our article suggests is, well that's true, but some of it has actually gone below 700 meters as well."
In fact, Trenberth's paper states that the warming of the ocean below 700 meters is "unprecedented," a fact that may eventually affect the ocean's ability to absorb gases like carbon dioxide, he said.
It could also lead to more stratification, or separation between layers, in the ocean. That lack of mixing can create dead zones with negative ecological consequences, he noted.
And not all that heat will stay in the ocean, either.
"Some of it may come back to haunt us," Trenberth said, as the ocean gets rid of the heat during something like an El Nino event or more hurricanes and tropical storms, which feed on higher ocean temperatures.
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500. E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.
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