Aerosols, clouds and climate

 

Clouds can cool or warm the underlying surface. They reflect some of the incoming shortwave solar radiation back to space (cooling). They also absorb infrared radiation and re-radiate it to all directions, with a wavelength (and thus energy) that depends on cloud temperature (and thus height). The latter is a quasi-greenhouse effect (warming).

 

The colder (=higher) the cloud, the less energy will be re-radiated to space, so the more the atmosphere will warm. (The same happens with greenhouse gases: the higher re-radiation into space takes place, the less energy is lost, and the more the atmosphere will warm. In that sense, the sky is the limit in terms of a warming effect of CO2: no matter if the absorption bands at the surface would be saturated, it would just move up the re-radiation to a higher altitude, thereby still adding to the warming effect.)

At night, there’s no solar radiation to reflect, and so clouds mainly warm the surface. However, the net effect of clouds is a cooling.

 

The basic response of liquid water clouds to an increased CCN population is expected to be a change in the droplet size spectrum: Assuming the liquid water content remains relatively unchanged, more cloud droplets will form, having a smaller average size. Absorption depends on particle volume, whereas reflection depends on its geometric cross-section, and thus the ratio of reflection over absorption of the cloud increases as the droplet size decreases. Dependent on the altitude of the cloud droplets, and the albedo of the underlying surface, this will usually result in cooling. A smaller average droplet size may increase cloud lifetime, via the suppression of rain. Cloud cover could be susceptible to CCN concentration, especially in remote areas. In reality, different feedback mechanisms involving micro-physics and radiative transfer make the picture more complicated. For example, the liquid water content of a cloud is highly variable and not much is known about the response of liquid water content to increasing CCN concentrations.

 

The effects mentioned here concern only “warm” clouds; much less is known about the response of ice clouds. If through human activities the concentration of ice nuclei has also decreased (e.g. black carbon), there could be a “glaciation indirect effect”: more ice nuclei cause a cloud to rain out quicker, and may thus lead to a decrease in cloud cover (i.e. opposite to the “warm” indirect effect). This is still very hypothetical, however.

 

From local measurements of ship tracks, relations were found between particle emissions, cloud droplet diameter and cloud albedo. Still, more simultaneous measurements of aerosol particles, cloud droplets and radiative properties are necessary to gain a more accurate understanding of climate change. There are strong indications that besides inorganic salts, some organic compounds may also contribute to the CCN activity of an aerosol. Ultimately though, size may matter more than chemistry for the cloud nucleating ability of aerosols.

Tags: aerosols, climate, clouds, CO2 saturation, radiation