The response of low clouds to aerosols is more complex than expected

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New work highlights the complexity of the response of low clouds to pollution particles. The study, published on April 26 in the journal Atmospheric chemistry and physics, is the first to assess the temporality of the aerosol-cloud relationship on the basis of observational data.

Human activities emit two broad categories of compounds into the atmosphere. Greenhouse gases like carbon dioxide (CO2) or methane (CH4) and small solid or liquid particles called aerosols. However, if the former induce a warming of the climate, the latter participate on the contrary in cooling it by increasing, directly or indirectly, the reflecting power of the atmosphere.

However, there is no symmetry between the two influences. Indeed, greenhouse gases have a lifespan ranging from a few decades to several centuries while the aerosols are eliminated in a few days. In other words, their effect would disappear almost instantly if anthropogenic emissions stopped abruptly. The cumulative dynamics of CO2 and you CH4 thus explains why it is the warming effect which prevails very largely.

Aerosols and climate, an interaction still difficult to quantify

The aerosols still mask part of the warming. For decades, researchers have tried to precisely quantify the magnitude of this effect. That is, to assess what fraction of the rise in temperature is actually made invisible. This work is however made difficult by the spatio-temporal heterogeneity of forcing linked to aerosols and the multiplicity of processes involved.

More precisely, if the direct impact is rather well understood, the same is not true of the indirect impact which involves an interaction of particles with clouds. In particular, those which do not precipitate. Without going into details, we can remember that aerosols increase the brightness of low clouds. A mechanism very well illustrated by the contrails of ships. Thus, the amount of solar radiation reflected back to space is increased, hence the cooling principle.

Illustration of the impact of aerosols emitted by a ship on low clouds. The droplet concentration is shown at the top. The corresponding satellite image as well as the trajectory of the vessel are shown below. Associated with the wind field, they allow researchers to follow the evolution of the drag over time. Credits: Imperial College London.

In a new study, researchers have attempted to clarify the indirect impact by studying the speed at which low clouds respond to the injection of anthropogenic aerosols. To do this, they analyzed the response of marine stratus and stratocumulus after passing ships. Also, scientists have used the trails left by them as a sort of stopwatch for aerosol-cloud interactions. Incidentally, the marine environment makes it much easier to isolate the forcing of the particles emitted because the environment is not very polluted.

The response of the clouds: a complex temporality

The detailed analysis of satellite imagery and the trajectory of the ships revealed an important element. Following the passage of ships, the number of water droplets increases very quickly. Consequently, the cloud becomes brighter on a course corresponding to the path taken by the boat. But the changes don’t stop there. Other changes continue to occur, and this up to more than 20 hours after the passage of ships !

“Short-term changes have been relatively well studied, but how the response evolves over longer time scales is less well known and has mostly been studied with computer models only,” reports Edward Gryspeerdt, lead author of the paper. “This is important for the climate because we often rely on short-term changes to better understand how pollution affects clouds. But our results show that cloud water response could be underestimatedif the total impact of aerosols over time is not taken into account ”.

The authors also showed that, in some cases, the simple addition of aerosols is enough to trigger the appearance of low clouds. A fact that comes from the great purity of the marine atmosphere, which sometimes contains too few condensation nuclei to allow the nucleation of water droplets.

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