The Great Global Warming Slowdown

As public resistance to the basic tenets of human-caused climate change subsides, new battles are emerging in the climate wars.

One such battle reached a pinnacle last month in the lead up to the release of the Intergovernmental Panel on Climate Change’s fifth assessment report. While the report concluded that global warming is “unequivocal” and, with 95 percent certainty, that human activity is the “dominant cause,” many media outlets were buzzing about a different story.

“Climate scientists struggle to explain warming slowdown” read a Reuters headline in April. “Global warming has slowed,” declared The Economist in the opening sentence of “A cooling consensus” in June. “Leaked draft of climate report struggles with drop in warming” reported in August.


Global average temperature anomaly (C)

On September 27, the date of the report’s release, made the slowdown the focus of its coverage of the event. “UN climate change report dismisses slowdown in global warming” read the headline plastered on the front page of the popular news website. “The planet has largely stopped warming over the past 15 years, data shows — and a landmark report released Friday by the U.N.’s climate group could not explain why the mercury has stopped rising,” the article stated.

With annual climate negotiations scheduled to begin in just over a month in Warsaw, Poland, the pressure was on for the climate science community to address the slowdown. 


The slowdown in surface temperature warming is old news for Kevin Trenberth, Distinguished Senior Scientist at the National Center for Atmospheric Research in Boulder, Colorado.

In November of 2009, one month before the highly anticipated United Nations Climate Change Conference in Copenhagen, Denmark, a collection of emails between British and American climate scientists was stolen from the University of East Anglia’s Climate Research Unit. In what came to be known as ‘Climategate,’ some of the emails were claimed to expose a cover-up by climate scientists.

One of the most widely distributed sound bites came from one of Trenberth’s emails: “The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t.”

The excerpt, taken out of context according to Trenberth, was used to imply that climate scientists were deliberately hiding data that showed that global warming had slowed, or even stopped.

“It is amazing to see this particular quote lambasted so often,” Trenberth wrote in an article published on NCAR’s website in 2009. “It stems from a paper I published this year bemoaning our inability to effectively monitor the energy flows associated with short-term climate variability. It is quite clear from the paper that I was not questioning the link between anthropogenic [human-caused] greenhouse gas emissions and warming, or even suggesting that recent temperatures are unusual in the context of short-term natural variability.”

While the ‘Climategate’ conspiracy has been largely debunked, research suggests that the media frenzy had a noticeable impact on public opinion. A study by researchers from Yale University concluded that “nationally representative surveys conducted in 2008 and 2009 found significant declines in Americans’ climate change beliefs, risk perceptions, and trust in scientists” and “the results demonstrate that Climategate had a significant effect on public beliefs in global warming and trust in scientists.”

An Apparent Hiatus in Global Warming?

Since the ‘Climategate’ fiasco, Trenberth has been a vocal critic of efforts to sow doubt in the public mind on the basic tenets of human-caused climate change. Earlier this year, he was awarded the 2013 American Geophysical Union’s Climate Communications Award for his “longstanding work in explaining climate to the media and public, and his dedication to education and outreach.”

Earlier this month, Trenberth and NCAR colleague John Fasullo published a research article in the American Geophysical Union’s new journal Earth’s Future. The article, “An apparent hiatus in global warming?,” summarized NCAR research on the cause of the slowdown in surface temperature warming and its implications for climate science going forward.


The Pacific Ocean covers about 46 percent of earth’s water surface and one-third of the planet’s total surface area, making it larger than all land area combined.

“It’s not as if global warming has stopped, it’s just that it’s being dumped into different parts of the climate system,” said Trenberth. Their research found that while surface temperatures have slowed, the deep oceans are warming at unprecedented rates. According to Trenberth, this has everything to do with natural variations in the Pacific Ocean, which he dubs the Pacific Decadal Oscillation. During a positive phase of the PDO, heat moves from the Pacific Ocean into the atmosphere, causing surface temperatures to increase. During a negative phase, the ocean absorbs heat from the atmosphere, burying it in the deep ocean. Trenberth believes we’re currently in a negative phase of the PDO and that it’s only a matter of time until we switch back to the positive phase and surface temperature warming resumes.

“If you look at the past record you can see that,” said Trenberth. “There’s often not that much change in the global mean temperature for several years, and then suddenly it jumps by three-tenths of a degree Celsius, but then it never really goes back down to the previous level.”

Forward Thinking

In its most recent report, the IPCC recognized Trenberth’s ocean heat redistribution research along with a laundry list of other possible explanations for the slowdown in surface temperature warming. In addition, the report left open the possibility that IPCC climate models may have overestimated the warming effect of human-caused greenhouse gas emissions. A recent study published in the journal Nature found that 114 of the 117 climate models studied overestimated the amount of surface temperature warming, calling the IPCC’s methodology into question.


Observed globally averaged combined land and sea surface temperature anomaly 1850-2012 (IPCC)

“IPCC has thrown down the gauntlet – if the pause continues beyond 15 years (well it already has), they are toast.” said Judith Curry, professor and chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

But Trenberth is wary of labeling climate models as ‘predictions.’ “It’s a misrepresentation of what the climate science has said in the past and the IPCC has said to claim that the hiatus is not predicted,” he said. “The system was never designed to predict it in the first place.”

Regardless, proponents of government policies to mitigate greenhouse gas emissions could encounter a dismissive public if surface temperature warming doesn’t pick up in the coming years. “There’s no way around the fact that this reprieve for the planet is bad news for proponents of policies,” declared The Economist earlier this year. “The reality is that the already meager prospects of these policies, in America at least, will be devastated if temperatures do fall outside the lower bound of the projections that environmentalists have used to create a panicked sense of emergency.”

Even Trenberth expects the surface temperature warming hiatus to continue. “It’s going to be around for a bit longer yet,” he said. But he also points out that “global mean surface temperature is just one small part of what’s going on with the planet and with global warming.”

Despite the slowdown in surface temperature warming, global sea level rise, a consequence of melting sea and land ice and warming oceans, has shown no sign of letting up. According to Trenberth, this is a clear indication that global warming continues unabated.

Trenberth is also wary of those who are trying to use the slowdown in surface temperature warming as an excuse to put off taking action to mitigate greenhouse gas emissions. “It’s mainly 40-50 years and beyond that the behavior that we adopt now will have a big influence in terms of the greenhouse gases,” he said. “This is why scientists are really worried, because by the time you realize we’ve got a problem Houston, we need to do something about it, it’s actually too late to do something about it, we’ve got to live with the consequences for the next 50 years or beyond.”


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