Hard Rain

A Weizmann Institute scientist and his colleagues caused a storm in the atmospheric research community a few years back when they suggested that tiny airborne particles known as aerosols may be among the main culprits in human-generated climate change. Aerosols affect cloud cover, and their impact on the local scale may be even greater than the greenhouse effect. 

 

Scientists have known for a while that such particles can have a number of different influences on cloud shape and formation. The problem is that some of these effects are warming and some are cooling; some seem to nudge the system toward greater rainfall and others toward less. Previously, because of limited methods for measuring the various effects, as well as the difficulty of creating an accurate model that could combine them all, the issue remained cloudy, if not downright foggy. Now Dr. Ilan Koren of the Environmental Sciences and Energy Research Department, working with Dr. Yoram Kaufman of the NASA/Goddard Space Flight Center, USA, has managed to weave together two opposing effects of atmospheric aerosols to provide a comprehensive picture of how they may be affecting our climate. 

 

Cloud formation begins with small amounts of such aerosols as sea salt and desert dust. The tiny particles serve as seeds around which water vapor in the air condenses, forming water droplets. When droplets are formed, heat is released, and this heat helps to drive the light droplets upward. As they rise, the small droplets collide, forming larger droplets. When the droplets reach a critical size, gravitation takes over and they fall from the cloud as rain. 

 

Koren's earlier studies had found evidence to suggest that the extra cloud-forming seeds planted in the atmosphere from man-made aerosol emissions (such as forest fires and burning fuel) lead to more but smaller water droplets, as the available water is divided among more seeds. Droplet collision becomes less efficient and rainfall is then suppressed. The lighter droplets are lifted farther up into the atmosphere, creating larger and taller clouds that persist for longer. Not only does this alter the global water cycle, but the increased cloud cover reflects more of the sun's radiation back into space, creating a local cooling effect on Earth.

 

To complicate matters, Koren, in another study, showed that certain types of aerosols – those containing black carbon (found, for instance, in airborne soot from burning coal) – can also decrease cloud cover, ultimately leading to a warming effect. This occurs because black carbon absorbs part of the sun's radiation, resulting in the atmosphere heating up and Earth’s surface cooling down, thereby preventing the conditions needed to form rain clouds. Fewer clouds mean weaker reflection of sunlight; less reflection of sunlight and absorption of radiation lead to warming.

 

Many policymakers and scientists have claimed, perhaps over-optimistically, that the effects of aerosols are mainly cooling, and that they may even help cancel out the greenhouse gases effect. Koren argues that it is the local effect that is worrisome: Clouds may retain their moisture over regions where they would normally precipitate, such as rainforests, or move to drop their rain over regions where it is not needed, such as oceans. Alternately, these effects could lead to the warming up of cold climate regions and the cooling down of hot ones. Such changes on top of global warming could have disastrous repercussions in the long run. 

 

Another question many have debated is: How can such tiny, localized particles affect weather systems thousands of kilometers away? The skeptics have claimed that, though aerosols undoubtedly play a role in cloud formation, it is negligible compared to that of key meteorological players such as temperature, pressure, the water vapor content of the air and wind strength.

 

To prove his theory, Koren needed a way to separate meteorolog-ical from aerosol influences. He and Kaufman used a network of ground sensors (AERONET) to measure the effect of aerosol concentration on cloud cover and the amount of radiation absorbed by aerosols at various locations across the globe and at different times of the year. Radiation absorption is relatively unaffected by meteorology, so if the skeptics were right and meteorology is the main influence, the correlation between aerosol absorption and cloud cover should have been hard to discern. But this was not the case. They observed the dual effect they had predicted: As the amount of aerosols increased, the amount of cloud cover increased; and as the amount of radiation absorption by aerosols increased, the amount of cloud cover decreased – for all locations, for all seasons. In light of this mathematical analysis, it becomes harder to deny that aerosols are, in fact, a major player in climate change. These results have recently been published in the journal Science.

 

“I would like to think that this study has finally cleared the air,” says Koren. “Hopefully policymakers will start to tackle the issue of climate change from a different perspective, taking into account not only the global impact of aerosols but local effects too.”    

 
Dr. Ilan Koren’s research is supported by the Samuel M. Soref and Helene K. Soref Foundation; and the Sussman  Family Center for the Study of Environmental Sciences.