No limits to human effects on clouds

The Press Room of the Weizmann Institute of Science (Israel): Understanding how clouds affect the climate has been a difficult proposition. What controls the makeup of the low clouds that cool the atmosphere or the high ones that trap heat underneath? How does human activity change patterns of cloud formation? The research of the Weizmann Institute’s Prof. Ilan Koren suggests we may be nudging cloud formation in the direction of added area and height. He and his team have analyzed a unique type of cloud formation; their findings, which appeared recently in Science indicate that in pre-industrial times, there was less cloud cover over areas of pristine ocean than is found there today.

Clouds need tiny particles called aerosols that rise in the atmosphere, in order to form. These aerosols – natural ones like sea salt or dust, or such human-made ones as soot – form nuclei around which the cloud droplets condense. In relatively clean environments, clouds can only grow as large as the amount of aerosols in the atmosphere allows: They will be the limiting factor in cloud formation.

The question is: Does the current load of aerosols in the atmosphere already exceed that limit, in which case adding extra particles should not greatly affect cloud form
ation; or do they continue to be a limiting factor as pollution rises, so that added aerosols would continue to influence the clouds? A model developed by Koren and his team showed that an increase in aerosols, even in relatively polluted conditions, should result in taller, larger clouds that rain more aggressively. ...

Koren, research student Guy Dagan and Dr. Orit Altaratz in the Earth and Planetary Sciences Department looked to an unlikely place to test their model: near the horse latitudes. These are subtropical regions far out in the oceans that were reviled in the past by sailors because the winds that carried their sails would die out there for weeks on end. Here was a lab for them to test the basic physics of their model: an atmospheric region controlled by well-defined meteorological conditions, which was sometimes pristine, sometimes containing low levels of aerosols. If the model was correct, transitions from one to the other should be dramatic. And they wanted to test their theory on the clouds that do form in this region – warm convective clouds that are fuelled by the ocean’s moisture.

...Koren: “We showed that convective clouds do not necessarily stop being aerosol-limited; under relatively polluted conditions the increase in aerosol loading will make the clouds taller, larger and their rain-rate stronger. As the area of this cloud cover grows, it reflects more of the shortwave radiation; but as the clouds get taller, their greenhouse effect becomes more significant, counteracting about half of their total cooling effect.”  

Convective clouds forming over the Amazon in a blanket of smoke. Image: Prof. Ilan Koren