Meteorology - Head in the Clouds

Dr. Stevens, with the help of climate models you simulate the formation of clouds. What role does global warming play in this process?

Bjorn Stevens: In order to regulate the surface temperature of the Earth and thus safeguard life-sustaining conditions, the energy balance of the planet must be in a state of permanent equilibrium. If the Earth’s surface receives more energy than it loses, it will warm up. If it gets too warm, Earth will radiate energy through its atmosphere into space. Clouds influence the amount of energy Earth receives from the sun as well as the ability of the Earth to reflect energy from its surface back into space. A changing cloud formation will alter this balance and, as a result, the temperature on the surface. At the same time, clouds themselves respond to changes in temperature. We must be aware of these interrelations so that we will be able to draw concrete conclusions about the climate.

Current climate models are not sufficient?
Conventional climate models represent an incomplete picture of reality. They are unable to precisely depict circulation systems in the atmosphere that are the result of clouds and storms; at best, they can provide a rough approximation. In fact, these models are of limited use when it comes to understanding how clouds and storms change, as well as understanding their effects on global warming.

You were one of the main authors of the fifth world climate report in 2013. The sixth report from 2021 is also based on the findings by your research team. How do clouds change as more and more carbon dioxide, methane and other greenhouse gases enter the atmosphere?
We are not entirely sure yet, but we have already excluded a number of possibilities. At the moment, it looks like this: the influence of clouds will change in such a way that global warming is not mitigated but instead will increase slightly. The exact impact of clouds on the reaction of Earth to changing greenhouse gases is very complicated. I have recently thought about the question of what it would mean if clouds were to not change at all – if they neither shrank nor grew in size.

As part of an international research project, you are studying the Passat clouds in the Caribbean. What are you hoping to find out?

These are very small clouds over the ocean that are capable of reflecting more solar radiation back into space than they are capable of absorbing radiation originating from Earth’s surface. Even minor changes in their number could drastically influence how intensely Earth will warm up as a result of increasing CO₂ concentrations. In order to find out more about these clouds and the factors which influence them, we are using Barbados as a base to study the Atlantic to the east of this island. To this end, we measure the clouds using probes on airplanes and ships. It seems clear even now that these clouds amplify Earth’s warming less than previously assumed. This is good news, but it should not lead us to believe that all is well.

With an appeal published in the journal “Nature Climate Change”, a team of researchers under your leadership pleaded for an alliance of major climate data centers to share their research findings. What else is needed to allow more precise predictions?

Unfortunately, people, through their actions, have a strong influence on Planet Earth. People have become a geological factor. To best understand increasing CO₂ concentrations and other global environmental changes, concentrated international efforts are needed. Only together will we be able to develop more and more precise climate models with the help of high-performance computers. This task is complex, and it should not be performed predominantly by young people, in particular PhD students, as is currently the case. In every respect, we simply need more and better resources. How can we expect to work together to mitigate the catastrophic consequences of climate change – if we are not capable of studying and understanding it collaboratively?

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