As a climate scientist, Bjorn Stevens likes to attack boundaries. From his lab in Germany, he probes for the most likely high and low projections for how quickly the planet will warm thanks to climate change. It’s work he’s done with little public attention — he’s an expert in clouds, after all, which, though vital to predicting the speed of warming, can hardly compete with drought or storm surges for press coverage.
But earlier this year, that all changed. Letters from concerned teachers started rolling into his mailbox. They shared a single concern: Was he really saying that warming wasn’t going to be a big problem?
Mr. Stevens, a professor at the University of Hamburg and director of the atmospheric section at the Max Planck Institute for Meteorology, was befuddled by the question, but quickly got to the root of it. He had a paper coming out that took a new approach to estimating how much cooling is provided by reflective particles in the atmosphere. 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.”
Proposed more than a decade ago by Richard S. Lindzen, a climate scientist and prominent contrarian who retired from the Massachusetts Institute of Technology two years ago, the iris effect stated that, as the world warmed, the number of heat-trapping high clouds over the ocean would diminish, the sky opening up like an iris and cooling the atmosphere. Immediate problems appeared with his analysis; most prominently, reduced clouds would let in more sunlight, largely countering the effect. Yet Mr. Lindzen held to his idea and became a mainstay expert for politicians skeptical of global warming, arguing with certainty that climate change was overstated and not an immediate threat.
The new paper provides no evidence that clouds could limit warming to any great degree, as Mr. Lindzen purports. It does provide a plausible mechanism for the iris effect and then shows that, even if it existed, it would do little to limit warming. 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.”
The Role of Clouds
Climate science has plenty of uncertainty. Start with the “hiatus.” Over the past 15 years, surface temperatures haven’t risen as quickly as models predicted, though warming does continue. That could be because the ocean is soaking up more heat than usual; several small volcanic eruptions could be reflecting more sunlight than expected; air pollution in the developing world could be doing the same. Theories abound.
The hiatus has also prompted some scientists, including Mr. Stevens, to explore whether they were making more-systematic mistakes. The hunt for the planet’s warming rate remains the field’s biggest unsolved problem. “It’s our dark matter,” Mr. Stevens said. If they were setting the lower bound on it wrong, they had to figure it out.
Was there any way clouds could be implicated? Ephemeral and heterogeneous, prone to reflect sunlight or trap heat, clouds cover 70 percent of the planet at any moment. For three decades, they have made it impossible to reduce the uncertainty over the rate of global warming.
As temperatures rose, would cloud behavior change? Just a slight shift could cause dramatic results. Most current estimates predict, with huge error bars, that clouds could slightly exacerbate warming as greenhouse gases build in the air. But that’s far from a safe bet.
So was there any way that clouds could be suppressing warming and causing some part of the hiatus? Well, there was Mr. Lindzen’s iris effect, Mr. Stevens thought. What if they modeled it?
“We said, Let’s pretend Lindzen is right, just for fun, and there’s some evidence that he’s not completely out of touch,” he said. There is plenty of evidence that models don’t get clouds right, Mr. Stevens added. “Let’s take him at his word and see what happens.”
This was simple math, adding variables into an equation, not simulating the fine details of how a cloud forms. “This is very much a crude hammer,” said Stephen A. Klein, a research scientist at Lawrence Livermore National Laboratory who is unaffiliated with the study. The researchers whacked their climate model and then saw how slow its warming could go.
The model turned out to be resilient, dropping only several tenths of a degree. Mr. Stevens had attacked the lower bound for warming’s speed, and it held. That is one reason he believes climate sensitivity — the amount the planet will warm when carbon-dioxide levels double from preindustrial levels — is unlikely to be below 2 degrees Celsius, even though the U.N.'s recent climate-panel report put the lower bound at 1.5 degrees.
Their paper also proposes how the iris effect could work, something Mr. Lindzen had never done in detail. If rising temperatures cause storms to clump together more over the ocean, they say, then most of the oceanic air will be dry, and fewer flecks of water will escape to form high clouds. It’s a pattern seen recently in simple computer simulations.
In the real world? Not so much, but there is some evidence to back up their idea of how the iris effect could work. Sandrine Bony, a French scientist, has shown that when thunderstorms clump more over the ocean, the atmosphere dries and high clouds decrease. And there’s evidence that, when the tropics become warmer during El Niño, a short-term weather pattern in the Pacific Ocean, high clouds also decline; such fluctuations, however, make a poor proxy for long-term rising temperatures.
If their work does point toward reality, though, it won’t be good news, Mr. Pierrehumbert added. “The basic physics they’ve uncovered is not really a cause for comfort.”
At first, the iris effect might slightly mute temperatures, he said, but the same dynamic, once things got hot enough, would actually exacerbate warming.
An Airing of Ideas
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.”
Contrarian ideas have occasionally improved climate science. The world’s best cloud chamber, at CERN, a European particle-physics lab, would not have been built without a discarded theory that cosmic rays could be tied to climate change. But such examples are rare. 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.
Just as skeptics did with Mr. Stevens’s aerosol paper, the blogosphere will probably make a news blip with this new study. So it goes. 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.”
Though he did not say so explicitly, one gets the sense that Mr. Stevens hopes his recent papers will encourage his peers to feel a bit more free in what they say and publish; to shake off the chill that followed the leak of stolen emails from the University of East Anglia, in 2009, and repeated freedom-of-information probes.
“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. He tells his students to be careful in what they say and write. The notion that climate scientists could be free again to speak, in public, in full candor?
“That’s a naïve hope,” he said.
Paul Voosen is a senior reporter covering the sciences. Write him at paul.voosen@chronicle.com; follow him on Twitter @voooos; or see past work at voosen.me.