Scientists need to understand how microscopic water droplets and ice crystals form in clouds in order to predict what the climate will be like in the future.
LONGYEARBYEN: Winter clouds during the blue hour over Adventtoppen and Hiortfjellet just north of Longyearbyen on Svalbard. Unfortunately, there are very few measurements of the composition of clouds in the polar regions. This knowledge is important for creating more precise climate models. Photo: Yngve Vogt
One of the major challenges with current climate models is that they do not sufficiently take into account how the clouds change when the climate gets warmer.
Trude Storelvmo wants to do something about that. She is Professor of Meteorology at the Department of Geosciences at the University of Oslo and leads a number of research projects. They will increase understanding of what happens to the clouds when the climate changes. The new knowledge is important to be able to predict what the climate will be like in the future.
It is common knowledge that an increase in greenhouse gases warms the atmosphere. In contrast, it is less well known that the clouds also change when the atmosphere gets warmer.
- Our globe would have become much warmer if we had a world without clouds. We know that clouds have a strong cooling effect on the climate system, and we know that clouds respond to climate change. The question is how and by how much. We therefore want to find out to what extent the clouds intensify the warming.
Drops and ice crystals
When clouds form, it is very important to understand what is happening at the microscopic level. Here we are talking about dimensions as small as ten to twenty micrometres. A micrometre is one thousandth of a millimetre.
- We are particularly interested in those clouds which are relatively cold and which consist of both droplets and ice crystals. It is precisely there that dramatic changes occur when the atmosphere gets warmer. Then the composition of droplets and ice in the clouds changes. When there are more drops and less ice, the cooling effect of the clouds changes.
Unfortunately, the thermodynamic equilibrium between droplets and ice is quite unstable.
The researchers do not know enough about how the clouds change when there is a different composition of ice and droplets.
- When we have coexistence of drops and ice, thermodynamics says that the ice crystals should grow quickly and that the drops should evaporate, but in reality the physics is far more complex. There is no perfect mixture of ice crystals and droplets. The coexistence is chaotic and turbulent. And in certain micro-areas in the clouds there are more water droplets, and in other areas there is more ice.
Goes wrong in today's models
Trude Storelvmo is now working on understanding the clouds better on a micro level.
- The climate models are struggling to describe these clouds. They often calculate too much ice and too little liquid water in the clouds. This leads to the climate models overestimating the cooling effect from the clouds and misrepresenting how they influence the rate of warming. Unfortunately, we have found that the warming will be stronger than many climate models have concluded. It's not good news.
Trude Storelvmo wants to understand how the clouds affect the climate all over the world. To solve this Gordian knot, she is particularly interested in investigating clouds in the polar regions, meaning the Arctic and the Antarctic. This is where the biggest problems arise.
Lack of knowledge about clouds
Unfortunately, there are very few measurements of the composition of clouds in the polar regions. The researchers are therefore at the mercy of satellite measurements. The satellites move in orbits 20,000 kilometres above the ground. Then it is perhaps not so difficult to understand that the measurements become uncertain. The researchers have to make some assumptions.
Storelvmo therefore wants better measurements of the inner microscopic life of the clouds. Among other things, her research team is already in the process of revealing how the clouds look from manned aircraft based on Andøya at the northern end of Nordland. They will also measure the clouds in Antarctica. These measurements must be taken from the ground.
- Why do you have to travel to Antarctica to observe the clouds there? Aren't the clouds the same as in the Arctic?
- Much is the same, but some things are also different. Clouds behave differently over sea and land. Furthermore, it matters what kind of particles are in the clouds. These particles are called aerosols and are super important.
- As long as it is warmer than minus forty degrees, special particles are needed in the atmosphere to form ice crystals. They are very rare in the atmosphere. Drops do not freeze spontaneously until they come into contact with these particles or reach a temperature lower than forty degrees below zero. That is why there is so much liquid water in the atmosphere, even though it is bitterly cold.
There are fewer aerosols in Antarctica than in the Arctic.
- We can therefore not measure the clouds only in the Arctic and know what the clouds are like in the Antarctic.
It is precisely in the Southern Ocean that many clouds consist of a mixture of droplets and ice crystals that have a temperature of between zero and minus forty degrees. So Antarctica is an important part of the world for understanding how clouds form.
- We hunt for the aerosols that have the ability to form ice crystals. Only one in about ten thousand aerosols has this ability.
The most important aerosols for ice formation are microparticles from minerals and desert sand. Others are those that originate from marine organisms on the sea surface.
- Do the particles dampen the heating?
- The particles control a great deal of the lifetime of clouds and their ability to reflect sunlight. They have a cooling effect, but there are many open questions, such as what the accumulation of dust in dry areas will do to the clouds.
- What about man-made particles?
- Until recently, we thought they were not that important, but now we know that particles from certain types of industrial emission sources can form ice in the clouds. They can therefore also be important. We are in the starting pit of understanding what this has meant for climate development up to today. What will happen to the future climate depends on what kind of industrial emissions are involved and whether there will be more or fewer of these particles.
The Troll station in Antarctica
In order to gather more information about how clouds form over Antarctica, Storelvmo, in collaboration with, among others, the Norwegian Polar Institute, will use measurements from Troll - the Norwegian research station in Antarctica.
- Won't it be a limitation if you only measure the clouds in the Antarctic "summer"?
Apollon has deliberately put summer in an apostrophe, because the summer in Antarctica is still freezing cold.
- There is actually surprisingly little seasonal variation in the clouds down there.
The research station will also build up infrastructure so that they can carry out measurements all year round. These measurements must be made with both radar and lidar. And if you're wondering about the difference between radar and lidar: The wavelengths of lidar are shorter. Otherwise, lidar and radar are pretty much the same thing.
- The nice thing about these measurements: When we measure the clouds from satellite, we are unable to see all the way through the clouds. The signals are weakened when they enter the clouds. Then we lose a lot of information. It is therefore valuable to also examine the clouds from the ground.
Enormous calculations
In order to reach her goal, Trude Storelvmo has in her research projects connected with a number of researchers from a number of disciplines. Among her specialists are those who optimise the calculations on supercomputers. Without such machines, it is not possible to carry out the calculations.
- In the global climate model, we can choose a region, such as the Southern Ocean also known as the Antarctic Ocean, where we simulate what happens with a higher resolution. We want as high a resolution as possible.
A supercomputer used in High-Performance Computing (HPC) is at least ten thousand times more powerful than a regular PC in your office. This means that if your PC had taken a year to run a calculation, the same calculation would have taken 52 minutes to run on a supercomputer. But the climate models unfortunately require much more computing capacity than this. Depending on how high a resolution the researchers want, it can take up to a month to run the climate model on the supercomputer. If you had done the same calculations on your PC, they would have taken more than 800 years.
Economic consequences
Although ninety percent of their research is about how they can improve climate models by understanding more about the formation of clouds, Trude Storelvmo is also interested in the economic consequences of climate change.
- It started as a small curiosity-driven side project.
She got the idea when, a few years ago, she was a professor at Yale University in Connecticut, thirteen miles northeast of New York. There she worked closely with economists, including Professor William Nordhaus, who in 2018 received the Nobel Prize in Economics for precisely calculating the economic consequences of climate change.
The researchers at Yale have created a model that simulates how the economic situation changes in the various parts of the world when the temperature rises.
- The only climate-related parameter in the model is temperature changes. The model shows that a moderate increase in temperature will improve the economy in the northern latitudes, but that the economies in the global south will be affected even if the warming is relatively small.
Unfortunately, the uncertainty is great.
- Although the model is widely used, we believe that it is too simple.
Storelvmo saw a unique opportunity to connect the economic model at Yale with the Norwegian climate model NorESM to get a better picture of how climate change affects the economy.
- We have now created a model tool where we can eventually take several climate variables into account. This is about more than temperatures. It is just as important to take into account extreme events, such as extreme rainfall and extreme drought.
Trude Storelvmo says it has been challenging to connect the two models. This is recently the main focus of one of her PhD students, Jenny Bjordal.
- Our collaboration is out of the ordinary. No one else has so far done anything similar. We are in the process of publishing the first results. They will arrive during the year.
