by Judith Curry

Sixteen years into the mysterious ‘global-warming hiatus’, scientists are piecing together an explanation. – Jeff Tollefson

Nature has a News and Views piece entitled Climate Change: The Case of the Missing Heat (complete article is available online). Some excerpts:

Average global temperatures hit a record high in 1998 — and then the warming stalled. For several years, scientists wrote off the stall as noise in the climate system: the natural variations in the atmosphere, oceans and biosphere that drive warm or cool spells around the globe. But the pause has persisted, sparking a minor crisis of confidence in the field. Although there have been jumps and dips, average atmospheric temperatures have risen little since 1998, in seeming defiance of projections of climate models and the ever-increasing emissions of greenhouse gases.

JC comment: My, my. Recall David Rose’s article in Oct 2012 on the pause, claiming that global warming had stopped, and the pushback against Rose’s article. The pause has now gone mainstream with this article in Nature, even in the IPCC did its level best to downplay it.

Climate sceptics have seized on the temperature trends as evidence that global warming has ground to a halt. Climate scientists, meanwhile, know that heat must still be building up somewhere in the climate system, but they have struggled to explain where it is going, if not into the atmosphere. Some have begun to wonder whether there is something amiss in their models.

JC comment: Now, no one understands the cause of the pause, but climate scientists say the heat is hiding in the ocean. My next post will be on ocean heat content, so I’m not getting into this here. The competing explanation (the ‘denier’ one, I guess since I don’t hear mainstream climate scientists mentioning this) is that the heat never made it into the system, possibly related to changing cloud patterns or properties that reflected more solar radiation.

But none of the climate simulations carried out for the IPCC produced this particular hiatus at this particular time. That has led sceptics — and some scientists — to the controversial conclusion that the models might be overestimating the effect of greenhouse gases, and that future warming might not be as strong as is feared. Others say that this conclusion goes against the long-term temperature trends, as well as palaeoclimate data that are used to extend the temperature record far into the past. And many researchers caution against evaluating models on the basis of a relatively short-term blip in the climate.

JC comment: Size matters here, i.e. the length of the hiatus. Depending on when you start counting, this hiatus has lasted 16 years. Climate model simulations find that the probability of a hiatus as long as 20 years is vanishingly small. If the 20 year threshold is reached for the pause, this will lead inescapably to the conclusion that the climate model sensitivity to CO2 is too large. Further, 20 years is approaching the length of the warming period from 1976-2000 that is the main smoking gun for AGW.

This has led skeptics – and some scientists – . . . Rather scary that Nature does not seem to acknowledge that skepticism is one of the norms of science, and regards ‘skeptics’ and ‘scientists’ are mutually exclusive groups.

“If you are interested in global climate change, your main focus ought to be on timescales of 50 to 100 years,” says Susan Solomon, a climate scientist at the Massachusetts Institute of Technology in Cambridge.

JC comment: People are interested in climate change on all sorts of time scales, including decadal. Solomon should have stated that if you are interested in the climate response to a long-term secular buildup of greenhouse gases, then your main focus should be timescales of 50-100 years. I agree with this. And if you look at the last 100 years, you have that other inconvenient pause to explain: 1940-1975. With the reduction in sensitivity to aerosol forcing, the aerosol explanation for this earlier pause no longer holds up. Stadium wave dynamics can explain both the 1940-1975 and the current hiatus; a further inference is that warming of 1976-2000 was enhanced by natural climate variability.

This variation in ocean temperature, known as the Pacific Decadal Oscillation (PDO), may be a crucial piece of the hiatus puzzle.

JC comment: I certainly agree that the PDO is probably a crucial piece of the puzzle, but one of the quickest ways to get labeled as a ‘denier’ has been to argue that the PDO in its warm phase contributed to the 1976-2000 warming.

“You can’t keep piling up warm water in the western Pacific,” Trenberth says. “At some point, the water will get so high that it just sloshes back.” And when that happens, if scientists are on the right track, the missing heat will reappear and temperatures will spike once again.

JC comment: Well that is an interesting ‘forecast.’ If this is natural internal variability, e.g. the stadium wave (which includes the PDO), then you would expect warming to resume at some point (I’ve argued this might be in the 2030’s). This would make the hiatus 30+ years (similar in length to the pevious hiatus from 1940 to 1975). This is long enough to invalidate the utility of the current climate models for projecting future climate change.

And about the missing heat reappearing, well stay tuned for my next post on ocean heat content.

Other blog posts

Here are two relevant blog posts worth looking at.

Bob Tisdale has a post at WUWT discussing the Nature article that focuses on the PDO.

Fabius Maximus has an extensive article Scientists explore causes of the pause in global warming, perhaps the most important research of the decade, which includes a survey of some recent papers.

Public Opinion

Chris Mooney has an article in Mother Jones entitled Global-Warming Denial Hits a 6-Year High. Excerpts:

The obvious question is, what happened over the last year to produce more climate denial?

According to both Anthony Leiserowitz of Yale and Ed Maibach of George Mason, the leaders of the two research teams, the answer may well lie in the so-called global warming “pause”—the misleading idea that global warming has slowed down or stopped over the the past 15 years or so. This claim was used by climate skeptics, to great effect, in their quest to undermine the release of the UN Intergovernmental Panel on Climate Change’s Fifth Assessment Report in September 2013—precisely during the time period that is in question in the latest study.

As Maibach’s colleague Anthony Leiserowitz of Yale adds, it isn’t as though those who were already convinced about global warming became less sure of themselves over the last year. Rather, the change of views “really seems to be happening among the ‘don’t knows,'” says Leiserowitz. “Those are the people who aren’t paying attention, and don’t know much about the issue. So they’re the most open-minded, and the most swayable based on recent events.”

Journalists take heed: Your coverage has consequences. All those media outlets whotrumpeted the global warming “pause” may now be partly responsible for a documented decrease in Americans’ scientific understanding.

Well, if the scientists don’t understand the cause of the pause, and the public is aware of the pause, then exactly what are we to conclude about the public understanding of climate change? Maybe that the public is not sufficiently ‘sophisticated’ to believe climate model projections that are running much warmer than observations for the past decade?