16 min., in German language (deutsches Transkript)
HK: Ulrike Lohmann, you’re a climate researcher and head of the Atmospheric Physics group in the Institute for Atmospheric and Climate Science at the ETH Zurich. What originally drew you to clouds?
UL: It actually happened quite by chance. I studied meteorology. You can take that as a degree course in Germany. At first, I was mainly interested in the dynamics of the atmosphere, and how low- and high-pressure areas develop. But then the supervisor I wanted to work with left Hamburg, and because I wanted to stay in Hamburg, I ended up with clouds. I should add, though, that I find clouds really interesting and have always been fascinated by them. It’s just that studying clouds isn’t an idea I would have come up with myself.
HK: Can you explain what actually causes clouds to form?
UL: Quite simply, they are created when air cools. The cooler air is, the less water vapour it can retain. At some point, the vapour condenses. In winter, when it’s cold outside, it condenses on the surfaces of windows. At lower temperatures, water vapour can’t be retained in the gas phase so much, so it condenses and forms droplets. In other words, for clouds to form, we need air that’s cooling down. Air cools when it rises. Then we need something for the vapour to condense on: either window panes or dust particles in the atmosphere.
HK: And why does air rise?
UL: Well, in summer, for example, the sun’s radiation warms the earth and causes the warm air to rise and expand. Alternatively, we can have a warm front, where warm air rises over cold air, or a cold front, where the cold air pushes itself under the warm air and forces the warm air upwards. In countries like Switzerland especially, mountains are a factor. When air passes over them, it also rises and cools. So there are three possibilities: mountains, fronts and solar radiation.
HK: And what are you and your group looking in to at the moment?
UL: I became interested in how clouds influence the climate system back in my Hamburg days. Do they make things warmer or cooler? What changes as a result of human activity? What can we expect from a warmer climate? I’ve always been concerned with how clouds are represented in the climate model. At some point I realized what we lack in order to understand clouds: measurements. That’s when I started making my own laboratory measurements in Halifax. We wanted to find out which aerosols make suitable ice-creating particles – the specks of dust that can initiate cloud formation by forming ice crystals. That’s what we do in the laboratory. We also go out into the field and try to understand how ice forms in clouds. We get our data from a range of sources: laboratory measurements of aerosols that form ice, field tests to see what’s going on in the actual atmosphere, and studies in the climate model.
HK: We’ve heard that there is still a lot of uncertainty in climate models when it comes to cloud formation.
UL: The climate models only have a limited resolution: traditionally, grid boxes measuring 100 kilometres by 100. But most clouds are much smaller, so they simply can’t be represented in detail. A cumulus cloud might measure one kilometre by one kilometre. That’s too small for the model. And all the processes happening within the cloud are on the micrometre scale, smaller than the width of a human hair. That always has to be shown in simplified form, and simplification means uncertainty.
Clouds form when a parcel of air rises, expands and cools because the pressure decreases. At some point, the water vapour can no longer be held in the atmosphere, so it condenses to form cloud droplets. That is well understood. In the model, the higher the resolution, the better we can represent the ascending air currents. That’s why weather forecasts now have a resolution of around 1 km or even better, because it allows the ascending and descending air currents to be calculated accurately. Then you need the aerosol particles for the water vapour to condense on, and they are something we don’t fully understand. We know what the sources are, but we don’t necessarily know how suitable they are for cloud droplets to condense on or for making ice crystals. Ice formation is an area where a lot is still unresearched, because on average only one aerosol in 100,000 or a million can act as an ice-creating particle. It’s like looking for a needle in a haystack. Which aerosol particles they are, and what their distinctive features are, is something we don’t really know. What we do know, though, is that the formation of ice in clouds is key to causing precipitation. Most of the rain that we get here in the middle latitudes comes from the ice phase in the cloud. So understanding how ice forms is central.
HK: We were surprised when your colleague Reto Knutti told us a couple of years ago that scientists don’t monitor the colour of the sky because it’s not a relevant parameter. As artists, that naturally bugged us. Since then, we’ve been trying to develop measuring devices to fill the gap and observe the sky’s colour. We’d like to do that on the roofs of museums. So far. we’ve managed to observe the sky photographically, with cameras. In future, we’d like to do it spectrally as well. The timelapse photographs we showed you clearly reveal the sky as it’s influenced by human beings. Especially when there are aeroplanes: the condensation trails often create entire blankets of clouds which you can see on many days. How do you react when you see films like that?
UL: I like the films as such, because I find it fascinating to see how clouds develop, regardless of whether they are natural or human-made. The condensation trails make our influence on the atmosphere clearly visible. Otherwise, the CO2 concentration isn’t something we can see. We influence the weather and the radiation budget. We create additional warming that wouldn’t be there without the condensation trails.
HK: Can you envisage pictures like that being used for scientific purposes?
UL: Yes, absolutely. As you’ve just said, you can see the chronological sequence, and that makes it easier to track whether the cirrus cloud I can see formed naturally or came from a condensation trail.
It’s true that the sky’s colour as such isn’t a measurable parameter that’s much use to us. But when we have powerful volcanic eruptions rising 15 or 30 kilometres into the stratosphere carrying a lot of aerosol particles with them, the result is that much more sunlight gets reflected. The aerosols reflect the sunlight, so the sky looks whitish and veiled. That’s when changes in the sky’s colour can be truly interesting. It’s also one reason why the sky over cities tends to look hazy. They’re often so polluted that we hardly see blue sky any more. By the way, you’d be more likely to get lovely blue images of the sky if you put the cameras on a mountain rather than where the museums are. In fact, the colour of the sky does tell us something about how clean or polluted the atmosphere is.
HK: We propose that museums collect the colours of the sky or the sky itself because now it is human-made.
UL: Ah, I see. That’s where you’re coming from!
HK: We’re interested in that human intervention. I think the fact that we are changing the sky is what really bothered us when we spoke to Reto Knutti. And that perhaps brings us on to something else. Human-made clouds were officially incorporated into the Cloud Atlas in 2017. How do those clouds influence the climate?
UL: They have a warming effect. Some clouds make things cooler and others make them warmer. A good illustration is that when you have a cloudless night, it’s colder in the morning, and when you’ve had cloud cover, it’s relatively warm. That’s down to the warming effect of clouds. The further up they are, the stronger that effect is. Those condensation trails that are at the same altitudes as the cirrus clouds are actually the highest clouds we have in the troposphere, the lowest layer of the atmosphere. They act rather like greenhouse gases, regardless of whether it’s day or night. During the day, though, clouds reflect sunlight. The further down in the atmosphere they are, the more they do so. In the lower troposphere, temperature decreases as you go higher. You’ll know that from hiking in the mountains: the further up you climb, the colder it gets. And as I’ve mentioned, the warmer it is, the more water vapour the atmosphere can hold, and the more of that vapour can condense. That’s why the clouds that form in warm temperatures are much, much denser. Seen from below they are grey. When we have such lovely, low clouds, we can hardly see the sun, which tells us that the majority of the sunlight is being reflected. It’s about the interaction between how much sunlight is reflected from the clouds and how much of the long-wave radiation is retained. And that interaction depends on the layer at which the clouds form and whether it’s day or night, and also what the surface beneath is like: whether it’s a snow-covered landscape or an ocean. A number of factors are involved.
HK: When we were talking to Reto Knutti and Atsumu Ohmura, it became clear that they disagreed about how the colour of the sky over Zurich will change, and whether it will become bluer or whiter. What’s your prediction?
UL: The sky when it’s cloudless will become bluer over the long term. The more aerosols there are, the hazier the sky. We’re making the air cleaner, and that will intensify the blueness. We know that low clouds are on the decline, while high clouds – the condensation trails and the cirrus – are tending to increase. In all, the trend is for clouds to accentuate climate change. The overall cloud cover of both types is still difficult to predict. Low clouds are decreasing in the tropics, but that’s not where we are or will be in the foreseeable future. High clouds are growing everywhere. Overall, cloud cover will increase, meaning the sky is generally whiter, but it’ll still be a bit bluer when clouds are absent.
HK: The influence of human beings is everywhere. Is that also changing the science?
UL: Yes, definitely. If we want to find out what our contribution is, we really need to know how things were in the past. What did the clouds look like when there were even fewer aerosol particles? We’ve already made research trips to the Arctic to help us find out. It’s easy to work there, because it’s clean. But even that is changing. The ice is melting there too, of course. There are almost no pristine areas left. Even the Arctic is being influenced by humans, during the seasons when the air comes from the middle latitudes. It’s difficult to take atmospheric measurements that exclude the human influence, certainly since CO2 has become an issue: it’s increased everywhere. We can see that from the curve that records the CO2 concentration at Mauna Loa on Hawaii, which was chosen as a location precisely because the air is clean. So yes, there’s actually nowhere left on Earth that isn’t affected.
A lot of work is being done that factors out the influence of human beings, and the climate sciences have sophisticated methods of doing that. It allows us to conduct simulations that exclude CO2 due to human activity along with other greenhouse gases, as well as our aerosol pollution. That means we can calculate how likely an extreme event such as the Ahr Valley floods in Germany would be in the absence of climate change. But every measurement we take now contains a significant component of our own human-made climate change.
HK: Why do you think we as a society find it so difficult to respond adequately to the science that’s actually out there?
UL: There are many sides to that question. Firstly, we don’t see the danger. When the hole in the ozone layer was in the news, things were different. Sunburn is visible, and when someone goes to the doctor and is told they have skin cancer, that’s a real diagnosis that has very clear causes. Atmospheric CO2 isn’t visible. It accumulates gradually. Of course, we notice the extreme summers and extreme events, we know there are droughts, forest fires and floods all over the place. But in most cases they are far away. And the further away they are, the less we feel affected by them.
Plus, all the solutions are uncomfortable one way or another. They have uncomfortable consequences for politicians, the economy or our own behaviour. We’re not getting something for nothing. Substituting CFCs to avoid further damage to the ozone layer was easy. There were alternatives. Anything can be changed where there is a technical solution. When it’s inconvenient politically, economically and in terms of our own actions, it’s much more difficult. That’s the main problem, in my view. There are these various levels that go together, and no single one can solve things on its own. It would be simpler if politicians said “from now on there’s a carbon tax, end of story”. Or business said “OK, we’ll just produce much, much less so we use less energy”. That’s not an option either. As long as we are approaching things from an economic perspective, we’re going about it the wrong way. Yes, we really need a massive change.
Ulrike Lohmann investigates the role of clouds and aerosol particles in the climate system. The atmospheric physics group she heads at ETH Zurich is trying to understand cloud microphysics, in particular the formation of ice in clouds. The researchers conduct experiments in the laboratory and take part in field campaigns. This enables them to improve the representation of clouds in climate models and carry out numerical simulations of the impact of clouds and aerosol particles on past, present and future climates.
The interview with Christina Hemauer and Roman Keller took place on 22 January 2024 in Ulrike Lohmann’s office at ETH Zurich.