An interview with leading climate scientist Bjorn Stevens

by Nic Lewis

This week, Die Zeit published an interview with Bjorn Stevens. Die Zeit is Germany’s largest weekly newspaper (distribution over one million), and has a highly educated readership.

Bjorn Stevens is Director at the Max Planck Institute for Meteorology, Germany’s main center for climate science research and modeling. He is well known for his work on sensitivity to climates, aerosols and especially clouds. Professor Stevens is an outstanding scientist and a key figure in the climate science establishment. He was co-lead co-ordinator of the World Climate Research Program’s Grand Challenge on Clouds, Cycles and Climate Sensitivity, and led the 2015 Ringberg Castle workshop, which launched the sensitive branch. climate sensitive.

The interviewer, Max Rauner, an experienced science journalist with a PhD in physics, mainly focuses on clouds, however Stevens also has interesting things to say about the scientists’ claims. climate according to alarm theory. An English translation of the interview appears below.

Read more


Climate Research: “Too Many Children’s Book Clouds”

How much fear are scientists allowed to instill in the climate debate? Cloud researcher Bjorn Stevens accuses his colleagues of being alarmists. We still know too little.

What do clouds do as the climate warms? This is what Bjorn Stevens, director at the Max Planck Meteorological Institute in Hamburg, is working on. His team simulated clouds in climate models. When it comes to cloud issues, the World Climate Report relies heavily on his expertise. At this point, however, Stevens wants to rewrite the world climate report.

DIE ZEIT: Professor Stevens, World Climate Report describes clouds as the biggest factor of uncertainty for climate projections. Why so?

Bjorn Stevens: See the cloud out there? In my field, most people think of a cloud as these small white objects in a blue sky.

ZEIT: Just like in children’s books.

Stevens: Yes, a pretty smooth cloud. As if you could draw a line around the edge of the cloud. But it’s an optical illusion, as anyone who has climbed the clouds in the mountains knows.

ZEIT: Because it’s foggy?

Stevens: Exactly. Cloud is a trickster. Even when the contours are sharp, the cloud structure is more like that of a puff pastry. However, many scientists use children’s book clouds as a guide because they are easier to simulate. This makes climate models less accurate.

ZEIT: How much water does this cloud contain?

Stevens: A cloud the size of an old building can hold only one liter of water.

ZEIT: That would fit in a pint of beer!

Stevens: If you distributed all the atmospheric condensate evenly around the globe, you would get a film of water only two tenths of a millimeter thick.

ZEIT: Why do clouds affect climate so much and flood entire countries?

Stevens: Flooding occurs because clouds can be very large, and the air circulating during storms constantly replenishes water. And they affect climate because they are made up of a large number of droplets that interact with sunlight and thermal radiation. A very large cloud has almost as many drops as the stars in the universe. And cloudy.

ZEIT: Are they warming or cooling the planet?

Stevens: Both. The energy balance of the earth has two parts: the first is the incoming sunlight and the second is the heat given off by the earth, i.e. infrared radiation. All clouds have a cooling effect by reducing the amount of sunlight reaching the earth’s surface. And all clouds have a warming effect by absorbing the earth’s infrared radiation so it doesn’t escape into space – the greenhouse effect. The balance sheet shows: Low-water clouds over the tropical ocean have the greatest cooling effect, and low-water ice clouds at high altitudes have the strongest warming effect. In general, the cooling effect is greater.

ZEIT: And how does this balance change with global warming? Scientists from the Potsdam Institute for Climate Impact Research (PIK) recently announce a worst case scenario . It also mentioned that by the end of this century, our planet could warm up to the point where all the clouds are practically evaporating and we will have to die.

Stevens: That’s impossible. Simply put, the atmosphere wants clouds because the air rises. It’s hard to escape the clouds.

ZEIT: Why do Potsdam climatologists claim otherwise?

Stevens: You will have to ask them that. I can only admire how colleagues there comb through literary majors for the most alarming stories. I find it unfortunate that these are subsequently presented without verification.

ZEIT: Is that the wrong scenario?

Stevens: Right. It is based on a work by our institute is taken out of context and more second sheet of paper that has a lot of shortcomings.

ZEIT: What is lacking?

Stevens: The dramatic behavior of the climate in this simulation is based on an oversimplification of the clouds, which has nothing to do with reality. If you look closely, the most alarming stories often fail to stand up to scientific scrutiny.

ZEIT: You too means tipping point forecast such as the melting of the Antarctic ice, the collapse of the Gulf Stream and the desertification of the Amazon rainforest?

Stevens: Yes, and most others. Of course, the world will change due to global warming, even more severe in some areas. But how, where and when is uncertain.

ZEIT: In German climate discourse, PIK often warns about tipping point , while your institute tends to minimize the danger of tipping points. Why so?

Stevens: Tippings are tempting and there’s a good chance they exist. But they are also a matter of definition. What do you think when you hear the word due point?

ZEIT: Self-reinforcing feedback cannot be changed.

Stevens: An irreversible rapid change, isn’t it? Like a falling pencil. He couldn’t back down on his own. But the cut-off points highlighted by my colleague Hans Joachim Schellnhuber and others at PIK are based on their privacy, much weaker definition . The cut-off points were reinterpreted to include less abrupt or even reversible climate changes. With this redefinition, they find tipping points everywhere. Then the alarm rang. My institute doesn’t lower the tipping points, we just place more value on clarity.

ZEIT: Are you jealous of the Potsdam Institute because its presence in the media ?

Stevens: Who doesn’t want to be interesting? Unfortunately, people prefer doomsday stories. I don’t understand much about that.

ZEIT: Are you saying that global warming is not a problem?

Stevens: That’s a big deal, in part because we know so little about its true impact. According to the IPCC, whether and where Biblical droughts and floods occurred? Not sure for almost all regions .

ZEIT: Stefan Rahmstorf from PIK compare yourself to a doctor who have discovered that smoking is dangerous and must now urge people to stop.

Stevens: As a scientist, I love to explain to people how the things I understand work. But what qualifies me to tell them how to behave? That must be the result of social discourse, which should be shaped by good journalism rather than by charismatic scientists. If people don’t learn to think for themselves, we’re lost anyway.

“Contributions of the clouds are still appreciated”

ZEIT: Let’s talk about the dangers of clouds again. Will clouds accelerate global warming?

Stevens: The interesting number here is climate sensitivity. It quantifies how much the earth will warm if the concentration of CO₂ in the atmosphere doubles…

ZEIT: … Compared with pre-industrialization CO₂ concentration. Will that remain in this century?

Stevens: If we continue as before, yes. In the final IPCC report , it is agreed that the average global temperature will then probably increase by between 2.5 and 4.0 degrees Celsius. According to the simulations, the higher temperatures are mainly due to changes in the clouds. We think this effect has been overestimated today.

ZEIT: Are the models faulty?

Stevens: Right. Too many children’s books, not enough real clouds. In the world climate research program, we have covered climate models. Models with the most extreme predictions failed and confidence in less catastrophic values ​​of climate sensitivity has increased . However, in my opinion, the contribution of clouds is still exaggerated.

ZEIT: How great is it?

Stevens: Based on our latest measurements and advances in theory, today I would say: no.

ZEIT: Zero?

Stevens: Yep, at least that’s my working hypothesis. Climate sensitivity then ended up being lower than the IPCC estimate, about 2.8 degrees. We should keep looking, but so far there is no evidence that clouds play a significant role.


The German original of the above interview, updated October 19, 2022 at 4:50 p.m., is available here.

As a matter of concern, if, as commented by Bjorn Stevens, one replaces the cloud response estimate given in the recent IPCC Sixth Assessment Report (AR6) with an estimate of zero. , an estimate based on AR6 responses that climate sensitivity will be reduced by 27%. Applied to the AR6 3.0°C central estimate of climate sensitivity, that reduction would change it to 2.2°C, the same as my recent feedback-based estimate paper estimate climate sensitivity using multiple lines of evidence.


News7g: Update the world's latest breaking news online of the day, breaking news, politics, society today, international mainstream news .Updated news 24/7: Entertainment, the World everyday world. Hot news, images, video clips that are updated quickly and reliably

Related Articles

Back to top button