Aerosols are very small particles that are present everywhere in the atmosphere. They can be anything from a few nanometres to a few micrometres in size and come from different sources – some natural such as when the winds bring dust particles from deserts, or when they sweep over the ocean and small water droplets are carried along, which then form salt crystals when they dry out. However, a major source of particles is of course combustion from industry and other human activity.
Particles in the air
The reason you cannot see the cloud when you exhale in the clean air of the Arctic is that the water vapour in the exhaled air cannot form cloud droplets if it cannot attach to and grow around an aerosol particle. In contrast, if there are many particles in the atmosphere the formation of the cloud is affected. In a cloud with a high concentration of particles, the cloud droplets are smaller and reflect more sunlight because they form a larger surface than in a cloud with fewer and larger droplets. This has a cooling effect on the climate.
“Since industrialisation we have not only emitted lots of greenhouse gases, which increase global warming”, says Moa Sporre. “Combustion has also led to significantly more particles in the air.”
Moreover, this may have had a masking effect on the temperature increase. It may sound like it is a positive thing to have many aerosol particles in the atmosphere; however, as is often the case, it is not so simple. Many of the particles that emerge from combustion are dangerous to breathe in and air pollution is an increasing public health problem around the world.
“However, as we try to clean our emissions there is a risk that we will see additional increases in the temperature because we are removing part of the cooling effect”, says Moa Sporre. “And if we were to stop all of our emissions today the greenhouse gases would continue to warm the climate for decades to come, while the aerosol particles would disappear in a week.”
The climate of tomorrow
In order to predict what the future climate will be like, we need to know how significant the masking effect of aerosol particles has been – something of considerable uncertainty in climate research. One reason is that there are many different kinds of particles with different properties with regard to how they absorb or reflect sunlight and how they interact with water and affect cloud formation.
This is one of the major focus areas in MERGE (ModElling the Regional and Global Earth system), which is a strategic platform for climate research in which Moa Sporre is involved.
Clouds play a major role for the climate. Small changes in the properties of clouds has a major impact on how much solar radiation is reflected and absorbed. Therefore, it is important that they are represented in the right way in the climate models. This is also complicated.
“It is a problem of scale”, says Moa Sporre. “In climate modelling, the Earth is divided into grid boxes, both horizontally and vertically, and it is very difficult to create an average value for the clouds’ impact on the climate in the grid box as different clouds have different properties and their impact depends on where they are located. We are trying to analyse events that take place on a very small scale in models that use scales that are very large.”
Aerosol research increasingly important
In the past decade, another area of aerosol research has received more attention, namely the organic gases emitted by the Earth’s plants. These include the gases you smell when you walk in a forest or when you bring a Christmas tree into your home. These substances that oxidise and condense into new particles have not received significant attention previously, but they are important because they have been part of the atmosphere since the beginning.
“However, we do not know at which levels these particles were present in the atmosphere before industrialisation”, says Moa Sporre. “And, due to combustion, the chemical conditions in the atmosphere have changed since then. We do not know if the substances behave in the same way.”
One theory is that if, with a warmer climate and more carbon dioxide in the atmosphere, there was to be increased vegetation on Earth, the particles from the plants could contribute a cooling effect on the climate. A lot of research is taking place in this area.
“To understand the anthropogenic impact on the planet, we must first understand how the climate works in itself”, concludes Moa Sporre.
About Moa Sporre at portal.research.lu.se