Almost every day, we can see clouds in the sky, but rarely we are aware of their wide-ranging impact on the climate. They can both raise and drop temperatures and have complex interactions with the particles in the air. These could even allow us to influence the clouds to fight climate change.
By Thomas Zauner
🛈 In a Nutshell
- Clouds form when invisible water vapor molecules in the atmosphere condense on tiny particles called aerosols.
- Aerosols stem from a wide variety of human and natural sources.
- Both clouds and aerosols themselves can have complex warming and cooling effects on the climate but overall, they result in net cooling.
- Artificial injections of aerosols could cool the climate further and help to fight some symptoms of climate change.
- There are still many open questions about the benefits and risks of geoengineering. Many scientists remain sceptical about how it could work and how to govern it globally.
They can look like scaly mackerels, soft cotton, or a mare’s tail. They have been affecting human lives since ancient times – producing rain, snow, and thunderstorms, being host to the gods, or spreading bliss on their ninth level. We are speaking of clouds.
Clouds are not only nice to look at, they also play a crucial role in the world’s climate because they can contribute to both the warming and cooling of the planet. They also interact with aerosols, tiny particles of solids and liquids in the air stemming from human or natural sources. Think of the enormous swaths of Sahara dust that can reach Europe or even the Americas.
At the University of Vienna, Paul Winkler and Blaž Gasparini are studying clouds, aerosols, and their interactions. Both are part of the Environment and Climate Research Hub which fosters interdisciplinary research in order to find solutions for our environmental and climate problems.
Cooling Off
Before we can explore how clouds and aerosols could help fight climate change, we need a bit of cloud physics. “In general, clouds are formed from invisible water vapor in the air which we experience as humidity,” explains Winkler. He is Professor of Experimental Aerosol Physics at the University of Vienna and deputy head of the research group Aerosol Physics and Environmental Physics. “The hotter the air, the more water vapor it can contain. Conversely, if air with a certain amount of water vapor cools down, at some point it cannot hold it anymore and the vapour becomes tiny floating droplets that form clouds.”
Simply put, when warm air rises in the atmosphere it cools down. Then its water vapor starts to condense into tiny water droplets. They scatter the sunlight and become visible as clouds. “In reality this is a very complex process,” Winkler adds. “Crucially, the vapor needs a tiny seed to start the condensation, an aerosol particle.”
A Myriad of Sources
Just a cubic centimetre of air can contain up to a million aerosol particles. Their sizes typically range from a millionth to a tenth of a millimetre. Aerosol particles can stem from natural sources like desert dust or sea spray being picked up by the wind, smoke from wildfires, chemicals released by plants that react in the atmosphere, or even volcanic eruptions. Nowadays, it is estimated that about 90 percent of all aerosols stem from natural sources. The rest comes from human sources like burning fossil fuels for energy, heating, and transportation, burning plants to clear land for agriculture, or sulphur dioxide released by industrial processes which reacts with water in the atmosphere to create aerosol particles.
“There is a myriad of sources for aerosols. In our lab, we study the effects of their shape, size, and composition on condensation,” Winkler says. He and his colleagues also collaborate with CERN on the CLOUD project studying how chemical reactions in the atmosphere create aerosol particles. The reaction components can stem from, for example, plankton, plants, volcanoes, or even cosmic rays—or many human sources like combustion or agriculture. Their findings also have implications for the climate.
Fewer Particles Cause Warming?
Aerosol particles in the atmosphere can have a direct and an indirect effect on the climate depending on their size and composition. The direct effect happens when they reflect sunlight back into space or absorb it gaining heat, cooling or warming the climate respectively. Globally, the direct effect of aerosols results in an overall cooling one. This also leads to concerns about reducing their numbers from human sources.
“We know that the many aerosol particles humans have been releasing since the beginning of the industrialisation have contributed to cooling the climate,” Winkler explains. “There now is a fear that improved aerosol regulations to protect human health and the environment would reduce their amount so much that we would lose their cooling effect.” However, new results show that this may not be the case.
“We have shown that the carbon-based compound isoprene is really effective in creating particles in the upper atmosphere. A lot of it is being released by plants in the Amazon. This shows that natural sources produced more aerosol particles than we previously thought. This could balance the reduction in the number of human-made ones,” Winkler adds.
Therefore, reducing the number of human-made particles would probably not have a warming effect on the climate because there are enough coming from natural sources to fill their role. However, this does not mean we can happily continue emitting aerosols, as they can have other negative effects on human health or the environment.
Cloud Seeds
Next to the direct effect of aerosol particles, there is an indirect effect on the climate based on their role as seeds for clouds. “More particles mean more seeds for droplets to form. The clouds then scatter light much more than the aerosol particles,” Winkler adds.
Surprisingly, clouds – especially high ones made from ice crystals instead of water droplets – can also contribute to warming the climate. Underneath them, they can trap the heat emitted from the ground. This is the case with the high and cold cirrus clouds Blaž Gasparini is studying. He is a senior scientist at the Department of Meteorology and Geophysics at the University of Vienna. He puts the warming effects of clouds into perspective, “While some clouds trap heat, the overall effect of all clouds is to cool the climate.”
Aerosols acting as seeds for droplets can then alter clouds and their effect on the climate. For example, you can see the trails behind big ships over the oceans on satellite images as their exhaust plumes contain lots of particles that act as cloud seeds. Clouds impacted by ship pollution therefore form more droplets reflecting more sunlight, appearing brighter from space. This unintended effect shows how humans could also willingly use aerosols and clouds to alter the climate.
Engineering the Climate
“Geoengineering means artificially altering the climate to combat its warming,” the cloud and climate scientist Gasparini explains. “There are many different ways being proposed on how to do this, but I think the important thing is that they alone are not a solution to climate change. They can only be a short-term painkiller that temporarily prevents the worst symptoms of climate change. We should not become dependent on them. The main solution is still drastically reducing the emission of greenhouse gases.”
Solar geoengineering can either reduce the amount of sunlight reaching the ground or let more radiation escape from the ground into space. Reflecting more sunlight could be achieved through creating aerosols by having dedicated ships spray sea water into the atmosphere as condensation seeds to form more highly reflective clouds. Another idea is to directly spray aerosol particles into the upper atmosphere. These would reflect the sunlight back into space. However, both these methods and their side effects are not yet fully understood.
Feeding High Clouds
Gasparini himself studies how to modify high-altitude cirrus clouds that trap heat radiation from the ground. “By seeding these clouds with the right kind of aerosol, we could have them form bigger ice particles,” the researcher explains. “Then, there would be fewer and heavier particles in the cloud making it more transparent and also sink faster. Both would allow for more radiation to escape through it into space. Yet, we still don’t know whether this method can cool the climate.”
Who should control the global thermostat?
There are many open questions remaining around solar geoengineering. It really could help us to fight climate change. Some companies already work on the commercialisation of these methods. Meanwhile, scientists are warning about the potential dangers of geoengineering: It could cause droughts, affect plants by reducing sunlight, deplete the vital ozone layer in the atmosphere, or decrease the efficiency of solar power.
The environmental physicist Winkler is sceptical towards geoengineering, “It is another intervention into many complex systems that we do not fully understand. Geoengineering holds the potential for many grave mistakes.”
“Who should control the global thermostat?” Gasparini asks, who is involved in a UK-funded research project on solar geoengineering. “It is important to study these methods so we can understand their impact and side effects. We do not want to just have others apply them without fully knowing what they could do,” he explains. “My biggest fear regarding geoengineering is human behaviour. We need good public communication about it, so it is not misinterpreted.”
Gasparini also wants more social scientists to engage with geoengineering in order to address the societal, political, and legal issues that cannot be covered by physicists and climate scientists alone. He emphasises the effort involved in interdisciplinary work: “It takes time to explain everything across disciplinary boundaries, but we must take that time.” The Environment and Climate Research Hub provides a space for these kinds of conversations. Both Winkler and Gasparini hope to gain new insights through the Hub’s platform for interdisciplinary exchange.
About the researchers
Paul Winkler is a professor at the University of Vienna and deputy head of the research group Aerosol Physics and Environmental Physics. In this role, he studies how different kinds of vapors condense from a gaseous form into particles under various conditions. He also investigates the dynamics and chemical processes of these tiny particles and how they can be characterised. Currently, he is involved in the CLOUD research project in collaboration with CERN which studies the formation of atmospheric aerosols under defined conditions in a measurement chamber. He is a member of the Environment and Climate Research Hub.
Blaž Gasparini is a senior scientist at the University of Vienna studying the impact of high clouds on climate, including their interactions with aerosols and their changes in a warmer climates. He is also involved in research about solar geoengineering to better understand its potential and risks. He is a member of the Environment and Climate Research Hub.
