Aerosols, like those created by the combustion of fossil fuels, cool the global climate directly and indirectly: the small suspended particles themselves reduce solar radiation and change the composition of clouds, which also increases radiation. Global warming caused by greenhouse gases has been significantly mitigated in this way in the past. By about a third since 1750, according to the Intergovernmental Panel on Climate Change (IPCC).
If the air quality improves, the cooling effect becomes less and less. Some climate researchers, such as Stefan Rahmstorf, assume that this year's temperature evolution – which was much more extreme than expected – also has to do with the decrease in cooling through aerosols. In particular, emissions from global shipping have decreased significantly in recent years due to a new agreement, namely by around 80 percent.
Clouds as radiation protection
To better reflect these possibly underestimated influences in climate models and predictions, it would be important to understand how and to what extent aerosols actually contribute to cooling. There are still some open questions about the interactions between polluting particles and cloud formation, according to researchers led by Tianle Yuan of the University of Maryland in the journal Science Advances.
Low clouds over the world's oceans reflect a large proportion of solar radiation back into space. The exact amount depends on three factors, according to the recently published study: the amount of water, the density of the droplets and the overall cloud cover. Aerosols can influence all three parameters. Among other things, the density of water droplets increases due to suspended particles, but at the same time the drops shrink. This causes the clouds to become brighter and more reflective. This phenomenon is known as the Twomey effect.
Complex interactions
Whether and how aerosols affect the absolute amount of water in clouds and the extent of cloud cover itself remains to be examined in detail. As Yuan and company explain, this is very difficult because there are many physical processes involved that can cancel each other out. Therefore, it is not yet entirely clear whether there are other indirect cooling effects caused by aerosols besides the Twomey effect. An evaluation of 18 years of satellite imagery should now bring a little more clarity to the complex relationships. The team compared cloud cover along global shipping lanes with nearby clouds that were not contaminated by debris.
On average, cloud cover along shipping lanes increased by nine percent due to aerosols, the study says. But there were very large regional differences. According to researchers, the interactions between aerosols, clouds and rain largely depend on the surrounding circumstances. For example, clouds become particularly strong due to aerosols when the air in the area is actually very clean. This confirms observations that previously cloudless areas are suddenly completely covered by clouds when ships pass by. Additionally, aerosols sometimes prevent rain from clouds, which also contributes to greater coverage.
More accurate models
In any case, the study results confirm that low clouds grow through aerosols, which, according to the study authors, plays a significant role in the cooling effect of the particles. Depending on conditions, this cooling effect is equivalent to between 51 and 396 percent of the Twomey effect. As the team concludes, the findings should help more accurately quantify aerosol-driven cooling in climate models and portray it with fewer uncertainties. This allows for better and more accurate predictions.