Lennart Bengtsson recently joined the Academic Council of the GWPF. This generated quite some attention on blogs and in the media. I interviewed him, but also Hans von Storch on Klimazwiebel, Axel Bojanowski had a story in Der Spiegel (English version), and there was an article in the Basler Zeitung.

Bengtsson emphasized that he has always been a “sceptic”. In the interview with me he said:

I have always been sort of a climate sceptic. I do not consider this in any way as negative but in fact as a natural attitude for a scientist. I have never been overly worried to express my opinion and have not really changed my opinion or attitude to science.

We all know that in climate discussions climate scientists are quick to say “we are all sceptics” so such a remark says little about Bengtsson’s exact viewpoints. The renowned Dutch science writer Simon Rozendaal then sent me a copy of his interview with Bengtsson published on 27 October 1990 (!) in the Dutch weekly Elsevier (for which Rozendaal still works as a science writer).

We can now confirm that Bengtsson was pretty “sceptic” in 1990. Here is the full translated Elsevier article:

A cool blanket of clouds

Climate expert Bengtsson puts the threat of the greenhouse effect in perspective

Next week, a large conference on the global climate will be held in Geneva. The most important topic of discussion: the greenhouse effect. Many hold the opinion that our planet is warming by the increase in carbon dioxide and that a climate disaster is looming. Maybe so, says Lennart Bengtsson, Europe’s most important climate scientist. Or maybe not. Bengtsson doesn’t actually know for sure. It could go either way.

Lennart Bengtsson is so far not daunted by the looming climate disaster. He frowns when looking at the tierische Ernst with which the rest of the world embraces the prediction that the planet warms due to the increase in gases like carbon dioxide (CO2). ‘It would become serious if the atmospheric CO2 concentration would decrease. Thanks to the greenhouse effect Earth is a habitable place. Were its concentration to decrease, then mean temperatures would plummet far below freezing. That really would be a catastrophe.’

The Sweed, who appears and talks like Max von Sydow, is director of the European Center for Medium Range Weather Forecast in Reading (United Kingdom), which supports eighteen European national weather centers like Dutch KNMI with computer models and simulations. Soon he will become director of the Max Planck institute in Hamburg and thereby will be in charge of Europe’s most important greenhouse effect computer model. ‘Until now the greenhouse effect research has concentrated in the United States, but Europe is advancing.’

There is something strange about the greenhouse effect. Many scientists babble and publish about it, but few really understand its ins and outs. Most of them treat assumptions as were they facts. Suppose that it would become two degrees warmer, how much higher would the Dutch dikes have to become? Or: suppose that we want to reduce CO2 emissions and still maintain economic growth for not so strong economies of Poland, Greece, and China, how much would the emissions of the wealthy Netherlands have to decrease? For the question whether the underlying assumptions are actually correct, one has to ask climate experts like Bengtsson.

He emphasizes that the greenhouse excitement is founded in computer simulations. And that computer generated models are not complete nonsense. ‘If for example such a model starts with a globally uniform temperature, then within a few months of simulation one would start to see the tropics warming and polar regions cooling. Remove the Amazon and after some time it reappears due to the torrential tropical rains. Such general characteristics of the global climate are part of the models.’

However, the models provide insufficient insight. ‘They are too coarse. While weather predictions nowadays have grid sizes of 100 by 100 kilometer, climate models work on a 500 by 500 km grid. In addition, models have problems with clouds. They are not able to predict what effect clouds have and they cannot distinguish between high and low clouds, yet we know that this differentiation has important consequences.’ Many other important aspects are lacking. Some of those cannot be incorporated simply because they are not well understood. ‘For a large part of the emitted carbon dioxide we do not know where it stays.’


Would there be no clouds, everything would be simple. ‘With a clear sky, increasing carbon dioxide or methane would lead to a reduction of heat radiation from the earth to the atmosphere. In addition, water vapor would amplify the so-called greenhouse effect. If temperatures increase, more water evaporates and water vapor is a powerful greenhouse gas.’

However, clouds do exist. It is these fluffy tufts that diminish much of the commotion surrounding the climate disaster. Clouds cool because they reflect sunlight. On the ground we notice this when we’re in a shadow. At the same time clouds warm because they prevent heat radiation from directly escaping to space: ground frost nearly always occurs under cloud free conditions. The simple question as to whether clouds cool or warm the Earth was until recently unanswered, and this says a lot about the current state of meteorology.

Among climate experts the opinion that clouds cool Earth is gaining ground, Bengtsson observes. ‘There are recent satellite observations, as reported in the scientific magazine Nature, showing that clouds reduce the greenhouse effect. In particular low level clouds are efficient cooling agents.’

Theoretically, the greenhouse effect could even cause a cooling rather than a warming of Earth. ‘The cooling effect of clouds is five times as strong as the temperature increase due to a doubling of atmospheric CO2.’ There is even an amplification of this feedback. Bengtsson: ‘If it gets warmer, clouds become whiter and thereby reflect more solar radiation.’

Such feedbacks are hardly part of the computer models that predict the warming, according to Bengtsson. ‘Almost no model is capable of dealing with the behavior of clouds. The models builders claim they do, but when we redo the calculations that turns out not to be true.’

There are other problematic issues. Were climate to really warm, snow and ice would have to melt. That would result in additional warming because white surfaces reflect more sunlight. ‘This additional warming is not present.’ Maybe the largest omission in knowledge about climate are the oceans.’ In most models it is assumed that the ocean is fifty meters deep, which is an average. But there are parts of the oceans that are several kilometers deep. Those would slow any potential warming. You could hide thousand years of warming in the ocean.’

The one small meteorological detail from the enormous amount of uncertainties, ambiguities and question marks that has become better understood is that an increase of CO2 and some other gases potentially has a warming effect. And that is what politics is focusing on right now. Bengtsson: ‘What happens in the Atlantic Ocean could have bigger consequences, but nevertheless all attention is focusing on the greenhouse effect.’


Bengtsson believes that climate experts should not pretend to be more knowledgeable than they really are. ‘In case of the greenhouse effect there is an interaction between media, politics and science. Every group pushes the other groups. Science is under pressure because everyone wants our advice. However, we cannot give the impression that a catastrophe is imminent. The greenhouse effect is a problem that is here to stay for hundreds of years. Climate experts should have the courage to state that we are not yet sure. What is wrong with making that statement clear and loudly?’

The excitement of the last weeks has moved everything into high gear. A United Nations committee (the IPCC) has released a report at the end of August which suggests that there is a broad scientific consensus about the existence of the greenhouse effect. This already has had political ramifications in many countries. For example, halfway October hundreds of Dutch politicians, experts, civil servants and industrialists have been discussing in Rotterdam themes from the 1960s like whether and how the Netherlands could lead the way (again). And early November there will be a global conference in Geneva about the global climate.

Bengtsson thinks that the IPCC has been particularly actuated for political reasons. ‘The IPCC prediction that with a doubling of CO2 in the atmosphere the temperature on Earth would rise by two degrees should be taken with a grain of salt.’

Due to the lack of understanding a thermometer remains crucial. And it is not pointing in the direction of a doomsday. ‘The temperature over the Northern Hemisphere has decreased since about 1950. In some countries the eighties were very warm, but there are countries where this is not the case. On Greenland there is little to be seen of the greenhouse effect. It has been very cold during the last couple of years.’

‘If you talk to the greenhouse mafia about these observations, they provide some answers, but those are not real. There is no proper support for the claim that the greenhouse effect should already be visible. It is sometimes stated that the Southern Hemisphere is warming. But there are so few observational sites over there that it is very difficult to draw any definitive conclusions about the temperature in the Southern Hemisphere.’

Bengtsson is not the only climate expert who thinks that much of the excitement about the greenhouse effect is undue. Many of his colleagues have been rather uneasy about what happened after they opened Pandora’s box. They have become afraid, now that politicians, camera crews, pressure groups and environmental departments worldwide have thrown themselves at the climate disaster, to openly state that what they have declared may have been a bit premature.

Bengtsson: ‘Many of us feel rather uncomfortable with much of what has been claimed about the greenhouse effect. No one had been talking about it because temperatures had been slightly on the decline during the last 30 years. Only after Jim Hansen of NASA had put the issue back on the agenda after the warm summer of 1988 has it become part of the political agenda. In itself there is no problem with that. Looking hundreds of years ahead the greenhouse effect could become a serious problem. Some policies are obviously a clever thing to do: save energy, become less dependent on oil, those are good ideas. But one cannot oversell the greenhouse effect. There are many environmental problems that are much more urgent like that of the sulphur dioxide in Eastern Europe.’

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