An Interview with High Local weather Scientist Bjorn Stevens – Watts Up With That?

From Dr. Judith Curry’s Local weather And so forth.

by Nic Lewis

This week Die Zeit revealed an interview with Bjorn Stevens. Die Zeit is the most important German weekly newspaper (circulation effectively over a million), and has a extremely educated readership.  

Bjorn Stevens is Director on the Max Planck Institute for Meteorology, the principal German local weather science analysis and modeling centre. He’s very well-known for his work on local weather sensitivity, aerosols and, significantly, clouds. Professor Stevens is a superb scientist and a key determine within the local weather science institution. He’s joint lead co-ordinator of the World Local weather Analysis Programme’s Grand Problem on Clouds, Circulation and Local weather Sensitivity, and led the 2015 Ringberg Fort workshop that kicked off its local weather sensitivity arm.

The interviewer, Max Rauner, an skilled science journalist with a PhD in physics, centered primarily on clouds, nonetheless Stevens additionally had attention-grabbing issues to say about pronouncements by alarmist local weather scientists. An English translation of the interview seems under.Learn extra


Local weather analysis: “Too many youngsters’s e-book clouds”

How a lot worry are scientists allowed to unfold within the local weather debate? Cloud researcher Bjorn Stevens accuses his colleagues of being alarmist. He finds: We nonetheless know far too little.

What do the clouds do when the local weather warms up? That is what Bjorn Stevens, director on the Max Planck Institute for Meteorology in Hamburg, is researching. His analysis group simulates clouds in local weather fashions. On the subject of cloud points, the World Local weather Report depends closely on his experience. In the mean time, nonetheless, Stevens wish to rewrite the world local weather report.

DIE ZEIT: Professor Stevens, the world local weather report describes clouds as the best uncertainty issue for local weather forecasts. Why is that?

Bjorn Stevens: See that cloud on the market? In my area, most individuals consider a cloud as these compact white objects within the blue sky.

ZEIT: Identical to within the youngsters’s e-book.

Stevens: Sure, a reasonably fluffy cloud. As in the event you may draw a line across the fringe of the cloud. However that’s an optical phantasm, as anybody who’s climbed right into a cloud within the mountains is aware of.

ZEIT: As a result of it’s getting foggy?

Stevens: Precisely. Clouds are tricksters. Even when the contours are sharp, the cloud construction is extra like that of puff pastry. Nonetheless, many scientists use the kids’s e-book clouds as a information as a result of they’re simpler to simulate. This makes the local weather fashions much less correct.

ZEIT: How a lot water does this cloud include?

Stevens: A cloud the dimensions of an previous constructing can solely maintain a liter of water.

ZEIT: That would slot in a pint of beer!

Stevens: In the event you distributed all of the condensed water within the ambiance evenly across the globe, you’ll get a water movie that’s solely two tenths of a millimeter thick.

ZEIT: Why then do clouds have an effect on the local weather a lot and flood whole nations?

Stevens: Flooding happens as a result of clouds may be large and air circulation throughout storms continuously replenishes the water. They usually have an effect on the local weather as a result of they’re made up of an enormous variety of droplets that work together with daylight and thermal radiation. A really giant cloud has virtually as many droplets as there are stars within the universe. And there are various clouds.

ZEIT: Are they warming or cooling the planet?

Stevens: Each. The power steadiness of the earth has two elements: firstly, the incident daylight and secondly, the warmth given off by the earth, i.e. infrared radiation. All clouds have a cooling impact by lowering the quantity of daylight that reaches the earth’s floor. And all clouds have a warming impact by absorbing the earth’s infrared radiation so it doesn’t escape into area – the greenhouse impact. The steadiness sheet exhibits: Water-rich low clouds over the tropical ocean have the best cooling impact and low-water ice clouds at excessive altitudes have the strongest warming impact. General, the cooling impact is larger.

ZEIT: And the way does this steadiness change with international warming? Scientists from the Potsdam Institute for Local weather Influence Analysis (PIK) just lately revealed a worst-case situation . It additionally mentions that by the top of the century our planet may get so heat that each one the clouds are virtually evaporating and we’re doomed.

Stevens: That’s nonsense. Put merely, the ambiance needs to be cloudy as a result of air rises. It’s laborious to eliminate clouds.

ZEIT: Why do the Potsdam local weather researchers declare in any other case?

Stevens: You’ll should ask them that. I can solely admire how the colleagues there comb by means of the specialist literature for probably the most alarming tales. I discover it a pity that these are then offered uncritically.

ZEIT: So the situation is mistaken?

Stevens: Sure. It’s primarily based on a work by our institute taken out of context and on a second paper that has quite a few shortcomings.

ZEIT: What shortcomings?

Stevens: The dramatic habits of the local weather on this simulation was primarily based on an oversimplification of the clouds, which has nothing to do with actuality. In the event you look carefully, probably the most alarming tales usually don’t stand as much as scientific scrutiny.

ZEIT: Do you additionally imply tipping level forecasts such because the melting of the Antarctic ice sheet, the collapse of the Gulf Stream and the desertification of the Amazon rainforest?

Stevens: Sure, and most others. In fact, the world will change on account of international warming, much more dramatically in some areas. However how, the place and when is way from sure.

ZEIT: Within the German local weather discourse, the PIK normally warns of tipping factors , whereas your institute tends to downplay the hazard of tipping factors. Why is that?

Stevens: Tipping factors are fascinating, and there’s probability they exist. However they’re additionally a matter of definition. What do you consider if you hear the phrase tipping level?

ZEIT: Of a self-reinforcing suggestions that’s irreversible.

Stevens: An accelerating change that can’t be reversed, proper. Like a pencil falling down. He can’t fall again up by himself. However the tipping factors highlighted by my colleague Hans Joachim Schellnhuber and others at PIK are primarily based on their non-public, a lot weaker definition . Tipping factors are reinterpreted to incorporate much less abrupt and even reversible local weather adjustments. With this redefinition, they discover tipping factors all over the place. Then the alarm goes off. My institute doesn’t play down tipping factors, we simply place extra worth on readability.

ZEIT: Do you envy the Potsdam Institute for its media presence ?

Stevens: Who doesn’t need to be attention-grabbing? Sadly, individuals favor tales concerning the finish of the world. I don’t perceive a lot about that.

ZEIT: Are you saying that international warming just isn’t an issue?

Stevens: It’s an enormous drawback, partly as a result of we all know so little about its precise influence. In keeping with the IPCC , whether or not and the place biblical droughts and floods will happen is unsure for nearly all areas .

ZEIT: Stefan Rahmstorf from PIK compares himself to a physician who discovered that smoking is harmful and now has to name on individuals to cease.

Stevens: As a scientist, I like to clarify to individuals how issues I perceive work. However what qualifies me to inform them tips on how to behave? That should consequence from the social discourse, which needs to be formed extra by good journalism than by charismatic scientists. If individuals don’t study to assume for themselves, we’re misplaced anyway.

“The contribution of the clouds continues to be overrated”

ZEIT: Let’s speak concerning the hazard of clouds once more. Will Clouds Speed up International Warming?

Stevens: The attention-grabbing quantity right here is local weather sensitivity. It quantifies how a lot the earth will heat up if the CO₂ focus within the ambiance doubles…

ZEIT: …in comparison with the CO₂ focus earlier than industrialization. That will nonetheless be on this century?

Stevens: If we proceed as earlier than, sure. Within the final IPCC report , it was agreed that the worldwide common temperature would then in all probability rise by 2.5 to 4.0 levels Celsius. In keeping with the simulations, the upper temperatures are primarily brought on by a change within the clouds. We take into account this impact to be overestimated in the present day.

ZEIT: Had been the fashions defective?

Stevens: Sure. Too many youngsters’s e-book clouds, not sufficient actual clouds. On this planet local weather analysis program now we have tackled the local weather fashions. The fashions with probably the most excessive predictions have failed, and confidence within the much less catastrophic values of local weather sensitivity has elevated . For my part, nonetheless, the contribution of the clouds continues to be overstated.

ZEIT: How nice is it?

Stevens: Primarily based on our newest measurements and advances in idea, I’d say in the present day: zero.

ZEIT: Zero?

Stevens: Proper, a minimum of that’s my working speculation. The local weather sensitivity is then on the decrease finish of the IPCC estimate, round 2.8 levels. We must always preserve wanting, however to date there’s no proof that clouds play a significant function.


The unique German model of the above interview, up to date October 19, 2022 at 4:50 pm, is out there right here.

As a matter of curiosity, if, within the gentle of Bjorn Stevens’ feedback, one had been to switch the cloud suggestions estimate given within the latest IPCC Sixth Evaluation Report (AR6) with a zero estimate, the AR6 feedback-based estimate of local weather sensitivity would scale back by 27%. Utilized to the AR6 3.0°C central estimate of local weather sensitivity, that discount would change it to 2.2°C, similar to the feedback-based estimate in my latest paper that estimated local weather sensitivity utilizing a number of traces of proof.


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