First in a two-part series.
In a recent edition of NASA's "Ask a Climate Scientist" video series, scientist Joshua Willis stands in front of a black screen, makes a few goofy faces and gives a brief answer to what has become a common question about climate science.
"A lot of people ask me: 'Has there been a pause in global warming because, like, temperatures aren't increasing as fast as they were a decade ago?'" Willis says.
"And I always say, you know, paws are for kittens and puppies, because global warming is definitely still increasing," Willis continues, smiling at his wordplay, as graphics of cute baby animals fill the screen.
It's true that Willis and nearly every other climate scientist dismiss the idea that global warming has paused. Yet the fact remains that average surface temperatures worldwide have not increased since around the turn of the century.
To the casual observer, the lack of warming at the Earth's surface, contrasted to climate scientists' insistence that the planet is still warming, might seem like a conundrum.
As scientists like Willis explain, though, most of the extra heat trapped by greenhouse gases does not warm the Earth's surface anyway.
Why do rising sea levels ignore the pause?
"Over 90 percent of the heat that we trap ... is warming the oceans," Willis said.
So as a measure of global warming, surface temperatures are not a good yardstick, because the atmosphere can only hold a small percentage of the heat that is trapped, he said.
Rather, the oceans should be the primary barometer of global climate change.
And they are certainly changing. Sea levels are going up "like gangbusters," Willis said.
The recent Intergovernmental Panel on Climate Change physical science draft report, released in late September, said it is a near certainty that rates of sea-level rise -- pushed up largely because warmer water expands -- have accelerated over the last two centuries.
The IPCC also reported it was very likely that rates of sea-level rise from 1993 to 2010 had almost doubled, from a 0.067-inch-per-year average rate for the 20th century to a 0.125-inch-per-year average rate.
To Willis and other scientists, this is a clear signal that global warming continues.
"Sea levels are still rising; the ice sheets are still melting; the oceans are still getting more acidic," Willis said. "All of that stuff is still going on just as it has, unabated."
If the heat continues to rise, where is it?
Even if they don't think global warming has paused, scientists are still interested in learning why the rate of surface warming over the last 10 to 15 years has been much slower than in the decades before, even as levels of greenhouse gases continue to increase.
If the Earth is still storing extra heat and that heat is not going into the atmosphere, it must be going somewhere else. Determining where the heat is going could lead to a better understanding of the Earth's climate system, they say.
One of the first researchers to address this question was Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research.
In 2011, Trenberth was a co-author on a paper published in Nature Climate Change that used models to show that pauses in surface temperature warming correspond to additional heat being stored deep in the ocean, below where most of our existing sensors typically measure.
Since then, Trenberth has published additional research (ClimateWire, April 8) showing that more than 30 percent of the warming in the oceans has occurred at depths below about 2,300 feet. He and his co-authors link this change to shifts in winds, especially in the Pacific Ocean, related to decadal weather patterns in the Pacific.
"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.
"The longer [the pause] goes on, the more significant it becomes," Lindzen said. At some point, he continued, models will be so clearly wrong that the public will reject them.
Is 'pause' the right scientific description?
To Michael Mann, a climate scientist at Pennsylvania State University best known for the famed "hockey stick" graph showing a dramatic warming in surface temperatures from 1,000 years ago to the present, even framing the slowdown in surface temperature increases as a "pause" can be misleading, because global warming has continued.
Mann agreed with other researchers that the oceans are likely taking up a certain amount of heat and that the recent small volcanic eruptions probably also have played a role in keeping surface warming at bay.
"The problem isn't that we cannot explain the temporary slowdown in warming -- the problem is that there are so many explanations for it, we're not yet sure what the true role is for each," he wrote in an email.
Mann also pointed out that surface temperatures, even if their rate of increase has slowed, still fall within the range of IPCC model projections.
Both Santer and Trenberth agreed that models could probably improve their representation of natural variability, solar cycles, and cooling factors like volcanic eruptions and aerosols.
But picking a period of a decade or so where one part of the Earth's climate system fails to warm and using it to discredit all of climate science is a fallacious argument, and one driven by those with an agenda to discredit climate scientists, the researchers say.
Especially when over longer periods of time, as Mann's hockey stick graph demonstrates, the warming signal is so clear.
"Cherry-picking isn't allowed. You can't look at one highly unusual 15-year period and say, 'This is my yardstick for measuring climate models,'" Santer said.