American scientists have found new evidence that aerosols emitted by ships have an important cooling effect on the climate. This is because they increase the surface area of clouds, causing them to reflect more sunlight. The researchers wrote this down last week Science Advances
The idea that aerosols cool the climate by both brightening clouds and increasing their surface area has been around for decades. But how strong the two effects of aerosols on clouds are is still unclear and complex. Previous studies provided statistical evidence for the importance of the second effect: cloud broadening. But the now published study provided observational evidence for the importance of the second effect based on eighteen years of satellite data. Broadening the clouds can provide up to four times as much cooling as clearing the clouds, they concluded.
People and environment
Aerosols are tiny particles in the air. They can have either a natural or human source. People emit aerosols through, for example, the combustion of fossil fuels by ships.
On the one hand, aerosols are known as air pollution and have a negative impact on people and the environment. On the other hand, the particles have a cooling effect through their interaction with clouds.
“If you add more aerosols to the air, it affects the brightness and shape of the clouds and therefore the reflection of sunlight,” says Franziska Glassmeier of TU Delft. She is not involved in the study. A cloud consists of condensed water droplets. Those water droplets need aerosols to condense on. “When more aerosols are available to form water droplets, the cloud gets more, smaller droplets. The more drops, the brighter the cloud. And the brighter the cloud, the more reflection.
“And small water droplets rain down less quickly than large ones. This gives the cloud more time to grow in width. A wider cloud reflects more sunlight.”
The exact influence of these two effects on the climate is therefore still mysterious. Previous studies searched satellite images for statistical relationships between present aerosols and cloud properties in a particular region. “But this cannot say that the observed cloud properties are really caused by the aerosols, or that other meteorological effects play a role,” says Glassmeier.
Eighteen years of satellite data
That’s why the Americans looked at how aerosols from ships changed the clouds present on eighteen years of satellite data. “So these are kind of experiments in the real world,” says Glassmeier. The researchers compared the ‘polluted’ clouds with surrounding clouds.
What turned out? The effect of widening the clouds sometimes turned out to be less important than brightening the clouds, but sometimes the effect was four times as strong. “That shows that it is important to include both effects of aerosols on clouds in climate models,” says Glassmeier.
But Glassmeier is not yet convinced by the new analysis. “As with previous studies into the effects of aerosols on clouds, the American researchers’ method also has disadvantages. For example, the researchers only looked at a limited part of the world on the satellite data. For their analysis, they assume that aerosols and clouds behave the same way in other parts of the world. That’s a big assumption. The precision interaction between aerosols and clouds still remains uncertain.”
As important as the cooling effect of aerosols is, “we need to reduce air pollution because of its negative effects on health and the environment,” Glassmeier emphasizes. “To mitigate warming, we must stop greenhouse gas emissions.”
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