by Judith Curry

Bjorn Stevens has published two interesting and important papers in the last few weeks, which have placed him squarely in the cross-fire of both the scientific and public debates on climate change.

Paper #1

This paper was discussed in Nic Lewis recent post Implications of aerosol forcing for climate sensitivity. The paper:

Rethinking the lower bound on aerosol radiative forcing

Bjorn Stevens

Abstract. Based on research showing that in the case of a strong aerosol forcing, this forcing establishes itself early in the historical record, a simple model is constructed to explore the implications of a strongly negative aerosol forcing on the early (pre 1950) part of the instrumental record. This model, which contains terms representing both aerosol-radiation and aerosol-cloud interactions well represents the known time history of aerosol radiative forcing, as well as the effect of the natural state on the strength of aerosol forcing. Model parameters, randomly drawn to represent uncertainty in understanding, demonstrates that a forcing more negative than −1.0 W m−2 is implausible, as it implies that none of the approximately 0.3 K temperature rise between 1850 and 1950 can be attributed to northern-hemispheric forcing. The individual terms of the model are interpreted in light of comprehensive modeling, constraints from observations, and physical understanding, to provide further support for the less negative ( −1.0 W m−2 ) lower bound. These findings suggest that aerosol radiative forcing is less negative and more certain than is commonly believed.

The implications of Steven’s result, when combined with climate sensitivity estimates, was summarized in my recent testimony:

However, the reduced estimates of aerosol cooling lead inescapably to reductions in the estimated upper bound of climate sensitivity.

The alleged ‘denier’ wing of the media wrote articles typified by this Breitbart article: New climate paper gives global warming alarmists ‘one helluva beating.’

Bjorn Stevens responded with a letter posted on his website, entitled: No, My Study Is Not a “Death Blow” to Global Warming Hysteria

Paper #2

A new paper by Stevens was published yesterday:

Missing iris effect as possible cause of muted hydrological change and high sensitivity in climate models

Thorsten Mauritsen and Bjorn Stevens

Abstract. Equilibrium climate sensitivity to a doubling of CO2 falls between 2.0 and 4.6 K in current climate models, and they suggest a weak increase in global mean precipitation. Inferences from the observational record, however, place climate sensitivity near the lower end of this range and indicate that models underestimate some of the changes in the hydrological cycle. These discrepancies raise the possibility that important feedbacks are missing from the models. A controversial hypothesis suggests that the dry and clear regions of the tropical atmosphere expand in a warming climate and thereby allow more infrared radiation to escape to space. This so-called iris effect could constitute a negative feedback that is not included in climate models. We find that inclusion of such an effect in a climate model moves the simulated responses of both temperature and the hydrological cycle to rising atmospheric greenhouse gas concentrations closer to observations. Alternative suggestions for shortcomings of models — such as aerosol cooling, volcanic eruptions or insufficient ocean heat uptake — may explain a slow observed transient warming relative to models, but not the observed enhancement of the hydrological cycle. We propose that, if precipitating convective clouds are more likely to cluster into larger clouds as temperatures rise, this process could constitute a plausible physical mechanism for an iris eﬀect.

The media hasn’t had much of a chance to respond to this one yet, but for reference see this backstory on Lindzen’s adaptive iris effect in the Wikipedia (includes links to the relevant publications). NASA has a pretty good article (2002) explaining the iris effect.

The rejection of Lindzen’s iris hypothesis by ‘important’ scientists (see BishopHill for some quotes) has long been used to discredit anything Lindzen has to say on climate.

In the past year, I’ve encountered two other talks that supported the iris hypothesis (one that I can’t remember and another one by Kerry Emanuel). There’s also a paper by Sandrine Bony. I need to dig into this subject (it’s right up my scientific alley), and hope to do a technical post on this sometime soon.

Chronicle of Higher Education

But back to the cross-fire; how are the ‘consensus police’ taking this? Paul Voosen (one of my favorite science journalists) has a good article In Search of Limits, a Scientist Pushes Bounds. Excerpts:

Climate contrarians got hold of it, and conservative websites like the Daily Caller pumped up its results to argue that it was a “death blow” to global-warming “hysteria.” Mr. Stevens had been dragooned into the climate war.

It was a fight Mr. Stevens didn’t want, but he wasn’t afraid of it. It’s a truism today that scientists who study the slowest possible speed at which the planet will warm will have their work adopted and misused by contrarians. It’s the type of attention that leaves researchers wary, Mr. Stevens said.

“There is a certain hesitance to work on topics that could be used by others to call into question those things we think we know,” he said. “We would have a much more vigorous debate if we weren’t worried about our words being misused.”

Mr. Stevens’s willingness to enter the fray will come further into view on Monday, with the publication of a new paper, written with Thorsten Mauritsen, in Nature Geoscience that gives ever-so-slight credence to a favorite theory of climate contrarians called the “iris effect.”

It’s meant as a conversational stimulant, Mr. Stevens said. That’s not necessarily how it’s being taken, he added. “People are nervous about the result being misinterpreted.”

Some scientists, though they welcome Mr. Stevens’s contribution, wish the paper had been written in a different way. “This paper is designed to make a larger story out of a relatively small result,” said Chris Bretherton, a professor of atmospheric sciences and applied mathematics at the University of Washington.

“I thought this was not well written and quite misleading,” added Kevin E. Trenberth, a senior scientist at the National Center for Atmospheric Research. There was no need to sound the “iris” trumpet, given how far it was from Mr. Lindzen’s original ideas, he added. “They even put it in the damn title.”

Contrarians often paint climate science as a clubby community of conspirators. This is a fine example of how researchers will investigate any idea if it has merit, said Raymond T. Pierrehumbert, a geoscience professor at the University of Chicago.

“It’s a good thing that people take the ideas seriously and think hard about them,” he said, “because even wrong ideas can be stimulating.”

For Mr. Lindzen, the new paper serves as a bit of told-you-so in the twilight of his career. He’s unable to get his new work past peer review, he says. Other researchers would say that’s because he ignores appropriate criticisms of his ideas, but Mr. Lindzen compared it to being a Soviet scientist trying to publish about relativity during Stalinism.

“What Bjorn is doing is going as far as you can and still get published,” Mr. Lindzen said.

For all the headaches he’s caused other climate scientists, Mr. Lindzen did play a role in pointing out the need to understand high-cloud effects better. But then he became his own worst enemy, Mr. Pierrehumbert said, turning “iris so disreputable by making outrageous claims.”

Mr. Trenberth has spent a lot of time exposing flaws in Mr. Lindzen’s work; there’s other research he could have done. “You may learn some things in the process, but in some ways it’s a waste of time,” Mr. Trenberth said.

Sure, Mr. Pierrehumbert said, it’s a good idea to avoid sloppy phrasing that can be easily misquoted, but communicating the work to other scientists is most important. Distortion is “just the cost of doing business,” he said. “There’s really no way to actually keep any kind of work from being misused.”

“My job,” Mr. Stevens said, “isn’t to convince the public more” about the reality of climate change. “I have a naïve faith the truth will win out.”

As someone who had his correspondence leaked and his words used against his research, Mr. Trenberth is not so sure. The notion that climate scientists could be free again to speak, in public, in full candor?

“That’s a naïve hope,” he said.

JC comment: Bjorn Stevens hits the bullseye with the bolded statement above. The scientists griping about all this are advocates, trying to convince the public of the ‘reality’ of climate change so that they will support emissions reductions policies. As Stevens rightly points out, this is not the job of climate scientists.

Uncertainty monster

In the early noughties, Bjorn Stevens and I attended a lot of the same meetings, mostly related to the GEWEX Cloud Systems Study group. I hadn’t seen him in awhile, but we both attended the 2011 Santa Fe Conference on Global and Regional Climate (discussed in this previous blog post). Excerpt:

On Tuesday night, a conference dinner was held. I gave a keynote presentation entitled “The uncertainty monster at the climate science-policy interface.” The talk is on youtube. My talk was interrupted by an irate audience member (who is an AR5 author).

The ‘irate audience member’ was Bjorn Stevens. He STRENUOUSLY objected to the following two slides, which my words included ‘many scientists’:

I will rewatch this when I have time (just about to leave for the airport again). So Stevens is not an IPCC/UNFCCC ideologue, but he seems in denial that some of his colleagues are.

P.S. I am giving a new uncertainty monster presentation tomorrow (to a group of economists), this will be the subject of tomorrow’s post

JC reflections

Kudos to Bjorn Stevens for working on these important and challenging problems and for understanding that it is not his job to convince the public that they need to reduce fossil fuel emissions.

The ‘consensus’ disease, and the naive belief of too many scientists that we need to speak ‘consensus’ to power, is slowing down scientific progress. Lennart Benngtson’s recent saga is another case in point (see previous blog post).

JC warning to Bjorn Stevens: In my quest to objectively evaluate the IPCC’s attribution argument and stand up for research integrity post Climategate, I was not ‘pulled’ away from the establishment community by ‘deniers’; rather I was ‘pushed’ away by scientists who were IPCC ideologues and advocates. Watch out.